Transcript
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Chapter 12
Anesthesia VentilatorsA ven ti la tor (brea thi ng mac hine ) is an auto ma tic dev ic e des ign ed to prov ide oraugment patient venti lat ion. Newer anesthesia venti lators are an integral part of the
anesthesia workstat ion. They are designed with more features and v enti latory
modes than earl ier models and have the abil i ty to venti late more dif f icult pat ients
and to allow ventilation to be tailored to the patient's needs.
Tradit ional anesthesia venti lators could not provide as high inspiratory pressures o r
f lows as their i ntensive care unit (ICU) counterparts (1,2,3,4). As a result , some
ICU venti lators needed to be adapted for use during surgery in order to care for
patients who were dif f icult to venti late. I f posit ive end-expiratory pressure (PEEP)
were needed, often the anesthesia provider had to add a PEEP valve to the
anesthesia breathing system. Some of these valv es were imprecise, not v ariable,
and could be misconnected (Chapter 7). On some older ventilators, the user had to
manually enable the low pressure alarm when the ventilator was turned ON. Also, it
may have been necessary to close the adjus table pressure l imit ing (APL) valve
and/or turn the bag/ventilator switch when turning on the ventilator. Another
drawback of older venti lators was that separate models or dif ferent bellows
assemblies were required for adult and pediatric patients. The delivered t idal
volume was affected by fresh gas f low and breathing system compliance. Finally,
older venti lators offered only volume control v enti lat ion.
The demand for performance equivalent to ICU venti lators has led to a number of
improvements in anesthesia venti lators. High inspiratory pressures and f lows can
be delivered. Newer anesthesia venti lators have an integral PEEP valve, and many
have several venti latory modes. Another improvement is improved f lexibil i ty so that
the venti lator can deliver volumes for a wide range of patients from the smallest
child to the largest adult . The new venti lators are designed to overcome the effects
of f resh gas, breathing system compliance and gas compression on t idal volume.
Turning the venti lator ON involves fewer steps and automatically enables the low
airway pressure alarm.
Venti lators used in anesthesia are c overed by international and U.S. s tandards
(5,6 ,7).
This chapter wil l cover a number of venti lators available at the t ime of this writ ing.
I t is impossible to provide all of the details that need to be mastered to safely use a
part icular venti lator. Software updates and upgrades occur frequently. I t is
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important that the user manual be studied before using a venti lator that is
unfamiliar to the anesthesia provider.
Definitions
Barotrauma : Injury result ing from high airway pressure.
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Compliance: Ratio of a change in volume to a change in pressure. I t is a
measure of distensibil i ty and is usually expressed in mil l i l i ters per centimeter
of water (L or mL/cm H 2O). Most commonly, compliance is used in reference
to the lungs and chest wall. Breathing system components, especially
breathing tubes and the reservoir bag, also have compliance.
Continuous Posit ive Airway Pressure (CPAP): Airway pressure maintained
above ambient. This term is commonly used in reference to spontaneous
venti lat ion.
Exhaust Valve : Valve in a venti lator with a bellows that when open allowsdriving gas to exit the bellows housing.
Expiratory Flow Time : Time between the beginning and end of expiratory
flow.
Expiratory Pause Time : Time from the end of expiratory f low to the start of
inspiratory flow.
Expiratory Phase Time : Time between the start of expiratory f low and the
start of inspiratory f low. I t is the sum of the expiratory f low and expiratory
pause times.
Fresh Gas Compensation : A means to prevent the fresh gas f low from
affect ing the t idal volume by measuring the actual t idal v olume and using this
information to change the volume of gas delivered by the v enti lator.
Fresh Gas Decoupling: A means to prevent the fresh gas f low from affect ing
the t idal volume by isolat ing the fresh g as f low so that it doesn't enter the
breathing system during inspiration.
Inspiratory Flow Time : Period between the beginning and end of inspiratory
flow.
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Inspiratory Pause Time : That portion of the inspiratory phase time during
which the lungs are held inf lated at a f ixed pressure or volume (i.e., the t ime
during which the inspiratory phase has zero f low). I t i s also c alled the
inspiratory hold, inf lat ion hold, and inspiratory plateau . The inspiratory pause
time may be expressed as a percentage of the inspiratory phase t ime.
Inspiratory Phase Time : Time between the start of inspiratory flow and the
beginning of expiratory f low. I t is the sum of the inspiratory f low and
inspiratory pause times.
Inspiratory: Expiratory Phase Time Ratio (I:E ratio): Ratio of the inspiratory
phase time to the expiratory phase time.
Inspiratory Flow Rate: Rate at which gas f lows to the patient expressed as
volume per unit of t ime.
Inverse Ratio Venti lat ion : Venti lat ion in which the inspiratory phase t ime is
longer than the expiratory phase time.
Minute Volume : Sum of all t idal volumes within one minute.
Peak Pressure : Maximum pressure during the inspiratory phase t ime.
Plateau Pressure : Resting pressure during the inspiratory pause. Airway
pressure usually fal ls when there is an inspiratory pause. This lower
pressure is called the pl atea u pres sure .
Posit ive End-expiratory Pressure (PEEP): Airway pressure above ambient at
the end of exhalat ion. This term is commonly used in reference to controlled
venti lat ion.
Resistance: Ratio of the change in driving pressure to the change in f low
rate. I t is commonly expressed as centimeters of water per l i ter per second
(cm H2O/L/second).
Sigh : Deliberate increase in t idal v olume for one or more breaths.
Solenoid: A component that controls pneumatic f low by means of an
electronic signal. Spil l Valve : The valve in an anesthesia venti lator that allows excess gases i n
the breathing system to be sent to the scavenging system after the bellows
or piston has become fully f i l l ed during exhalat ion.
Tidal Volume : Volume of gas entering or leaving the p atient during the
inspiratory or expiratory phase t ime.
Venti latory (Respiratory) Rate or Frequency: Number of respiratory cycles
per minute.
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Volutrauma : Injury due to overdistention of the lungs.
Work of Breathing: Energy expended by the patient and/or ventilator to move
gas in and out of the lungs. I t is expressed as the rat io of work to volume
moved, commonly as joules per liter. It includes the work needed to
overcome the elast ic and f low-resist ive forces of the both the respiratory
system and apparatus.
Relationship of the Ventilator to the Breathing System
A ven ti la tor repl ac es the res erv oir bag in the bre ath ing sys tem. I t ma y be
connected to the breathing system by a bag/venti lator selector valve (Chapter 9).
On some newer workstat ions, turning the bag/venti lator selector switch to theventi lator posit ion or a mode select ion switch turns ON the venti lator. On other
venti lators, there is an ON-OFF switch.
Most anesthesia venti lators have a bellows in a box (bag in a bott le, double circuit)
design (Fig. 12.1). The bellows is housed in a pressure chamber, and the inside of
the bellows is co nnected to the breathing system. The bellows acts as an interface
between the breathing system and the venti lator driving gas, just as the reservoir
bag acts as an interface between the breathing system and the anesthesia
provider's hand. I t separates
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breathing system gas from driving gas. The pressure of the anesthesia provider's
hand is replaced by the driving gas pressure that compresses the bellows.
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View Figure
Figure 12.1.Functioning of the bellows-in-box ventilator.A:Beginning of inspiration. Driving gas begins to bedelivered into the space between the bellows and itshousing. The exhaust valve (which connects the driving gas
pathway with atmosphere) is closed. The spill valve (whichvents excess breathing system gases to the scavenging
system) is also closed. B:Middle of inspiration. As drivinggas continues to flow into the space around the bellows, its
pressure increases, exerting a force that causes the bellowsto be compressed. This pushes the gas inside the bellows
toward the breathing system. The exhaust and spill valvesremain closed. If the pressure of the driving gas exceeds the
opening pressure of the safety relief valve, the valve willopen and vent driving gas to atmosphere. C:End of
inspiration. The bellows is fully compressed. The exhaustand spill valves remain closed. D:Beginning of expiration.
Breathing system (exhaled and fresh) gases flow into thebellows, which begins to expand. The expanding bellowsdisplaces driving gas from the interior of the housing. Theexhaust valve opens, and driving gas flows through it toatmosphere. The spill valve remains closed. E:Middle ofexpiration. The bellows is nearly fully expanded. Drivinggas continues to flow to atmosphere. The spill valveremains closed. F:End of expiration. Continued flow of gasinto the bellows after it is fully expanded creates a positive
pressure that causes the spill valve at the base of the bellowsto open. Breathing system gases are vented through the spillvalve into the scavenging system.
During inspirat ion, driving gas is delivered into the space between the bellows and
its housing. This causes the bellows to be compressed so that gas f lows into the
breathing system. At the same t ime, the spil l v alve (which vents excess gases to
the scavenging system) and exhaust valve (which vents driving gas) are closed.
During exhalat ion, the bellows re-expands as breathing system gases and fresh gas
flow into it . Driving gas is v ented to atmosphere through the exhaust valve. After
the bellows is ful ly expanded, excess gas from the breathing system is vented to
the scavenging system through the spil l valve.
Instead of a bellows in a box, some venti lators have an electrically driven piston.
By eliminating the need for a drive gas circuit (an addit ional source of compressible
volume), a stable f low delivery can be provided. In piston venti lator systems that
are presently available, the reservoir bag is not isolated from the breathing system
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during the exhalat ion phase of automatic venti lat ion and acts to modulate pressure
increases in the system. During inspirat ion, when the piston forces gases into the
breathing system, the bag is isolated from the breathing system and collects the
fresh gas f low entering the breathing system. On some venti lators, the bag can be
seen to expand and contract with respirat ion even though the piston is ac tually
venti lat ing the patient. A problem with piston venti lators may be air entrainment
with a disconnection
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(8,9). In this case, the machine may not alarm and the patient wil l c ontinued to be
venti lated, but air wil l be entrained, result ing in lower concentrat ions of oxygen and
anesthetic agents.
Factors That Affect the Delivered Tidal Volume
F r e sh Gas F low
With older v enti lators, the delivered t idal and minute volumes changed when the
fresh gas f low, I :E rat io, or respiratory rate was altered despite the bellows
excursion remaining unchanged. I f the fresh gas f low increased, the t idal and
minute volumes increased (10 ,11 ,12,13 ). I f the fresh gas f low decreased, the t idal
and minute volumes decreased. Since fresh gas was added to the inspired t idal
volume only during inspirat ion, venti lator sett ings that prolonged the inspiratory
time (and thereby increased the I:E ratio) would cause an increased tidal volume.
Lower I :E rat ios decrease the t idal v olume. As respiratory rate i ncreased, the
increase in t idal v olume from fresh gas f low was less, a lthough the effect on minute
volume remained the same. Slowing the respiratory rate had the opposite effect.
Manufacturers have re-engineered their venti lators to eliminate the fresh gas effect
on the inspired volume. One method is to measure the inspired fresh gas f low and
compensate for it by altering the bellows excursion (fresh gas compensation).Another metho d is to pre ven t the f resh gas f rom en te ring the brea thing sys tem
during inspirat ion by using a valve that diverts the fresh gas into a reservoir bag
during inspirat ion (fresh gas decoupling).
Com p l i an c e an d Com p r e s s i o n Vo l ume s
Decreases in compliance in the breathing system can be accompanied by
decreases in t idal volume as more of the inspiratory f low is expended by expanding
the components. Gas compression losses depend on the volume of the breathing
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system and the pressure d uring inspirat ion. Advanced t echnology now all ows the
venti lator to compensate for changes in breathing system compliance by altering
the volume delivered. Breathing system compliance is determined during the
checkout procedure before use. For accurate compliance compensation, the
breathing system must be in the configurat ion that is to be used when the checkout
procedure is performed. Changes in the circuit configuration (such as lengthening
the breathing tubes or adding components) wil l c ause the compensation to be
inaccurate (14).
Other venti lators measure inspired volumes at the patient connection and adjust the
venti lator excursions accordingly.
Lea k sA le ak aro und the trac hea l tub e or sup rag lot ti c dev ic e wi l l cau se a decrease in tida l
volume that is not taken into account by the ventilator. Sidestream gas monitors
may decrease the volume delivered to the patient.
Components
D r iv i n g Gas Sup p l y
Most currently available anesthesia v enti lators are pneumatically p owered but
electrically controlled. The driving (drive, power) gas is either oxygen, air, or a
mixture of air and oxygen. I t is usually less expensive to power the venti lator with
air. Some venti lators can switch between driving gases so that if there is a loss of
pressure in the primary driving gas supply, the other gas can be used.
Some venti lators use a device called an injector (Venturi mechanism) to increase
the driving gas f low. An injector is shown in Figure 12.2. As the gas flow meets a
restrict ion, its lateral pressure drops (Bernoull i principle). When the lateral
pressure drops below atmospheric, air wil l be entrained. The result is an increase
in the total gas f low leaving the injector, and a decreased consumption of driving
gas.
A sign if icant fl ow of gas is neces sary to driv e a bellows (15 ,16 ,17). The amount will
vary, depending on the venti lator and the sett ings. The use of a gas cylinder to
power a venti lator may quickly deplete the gas supply.
Con t r o l s
The venti lator controls regulate the f low, volume, t iming, and pressure of the
bellows compression or piston movement.
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A l a rm s
The venti lator and workstat ion standards (6,7) group alarms into three categories:
high, medium, and low priority, depending on whether the condition requires
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immediate ac t ion, prompt act ion, or operator awareness but not necessari ly act ion
(Chapter 26).
View Figure
Figure 12.2.Injector (Venturi). Gas flows through theconstricted area at a high velocity. The pressure around it
drops below atmospheric, and air is entrained. The net result
is an increase in total gas flow leaving the outlet of theinjector.
The venti lator standard (6) mandates an alarm that indicates that the pressure in
the breathing system has exceeded a set limit (high-pressure alarm). On modern
venti lators, this threshold is adjustable by the user, usually with a default around
50 cm H2O. There must be an alarm to indicate that the pressure in the breathing
system has not reached a minimum value within a certain time period (low airway
pressure alarm).
P r es s u r e -l im i t in g Me c h an i smA pressure -limiting mechan ism (p res sure -l im it in g val ve, ma x im um limi te d pressure
mechanism, driving gas pressure relief valve, pressure l imitat ion mechanism,
maximum working pressure control, pressure l imit controller, inspiratory pressure
limit, adjustable pressure relief valve, high pressure safety relief valve,
overpressure release) is designed to l imit the inspiratory pressure. The anesthesia
workstat ion standard (7) mandates that this be adjustable. An adjustable
mechanism carries the hazard of operator error. If set too low, insufficient pressure
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for venti lat ion may be generated; if set too high, excessive airway pressure may
occur. Setting the pressure limit 10 cm H 2O above the peak pressure achieved with
the desired t idal volume and f low rate wil l avoid most barotrauma (18 ).
Pressure-l imiting devices work in one of two ways. When the maximum pressure is
reached, one type holds the pressure at that level unti l the start of exhalat ion, at
which t ime the pressure decreases. The other type terminates inspirat ion when the
pressure l imit is reached so that the pressure drops immediately.
Be l l o w s A s s emb l y
Bellows
The bellows is an accordionlike device that is attached at either the top or bottom
of the bellows assembly. Latex-free bellows are available. There are two types of
bellows, distinguished by their motion during exhalation: ascending (standing,
upright, f loating) and descending (hanging, inverted). Ventilators with descending
bellows were common until the mid 1980s. After that, most new ventilators had
ascending bellows, but descending bellows are used by a number of more recent
venti lators.
With an ascending bellows (Figs. 12.36, 12.44), the bellows is attached at the base
of the assembly, and the bellows is compressed downward during inspirat ion.
During exhalat ion, the bellows expands upward. These venti lators impose a slight
resistance at the end of exhalat ion, at which t ime the pressure in the bellows rises
enough (2 to 4 cm H2O) to open the spil l valve. The t idal volume may be set direct ly
by adjust ing the inspiratory t ime and f low or b y a p late that l imits upward excursion
of the bellows. With a disconnection or leak in the breathing system, the bellows
wil l collapse to the bottom or fai l to expand fully. The venti lator may continue to
deliver small t idal volumes (19).
To deliver the entire t idal volume, the bellows mus t descend to the proper level or,
depending on the venti lator, be fully compressed at the end of the inspiratory
phase. I f the inspiratory f low is insuff icient to ful ly compress the bellows or achieve
the desired t idal volume, a l ower t idal volume wil l be delivered.
With a descending bellows (Fig. 12.50), the bellows is attached at its top and is
compressed upward during i nspirat ion. There is usually a weight in the dependent
port ion of the bellows that facil i tates downward re-expansion during exhalat ion. As
the weight descends, it can cause a small negative pressure in the bellows and
breathing system. With a leak or disconnection in the breathing system, the weight
in the bellows wil l cause the bellows to expand, and room air wil l enter the
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breathing system. All or part of the next inspirat ion wil l then be lost into the room.
Newer ventilators with hanging bellows employ sophisticated software to detect
disconnections or leaks (20 ,21). The software analyzes sensor outputs and tr iggers
appropriate alarms. A negative pressure relief valve prevents the patient from being
exposed to negative pressure.
Housing
The bellows is surrounded by a clear plast ic c ylinder (canister, bellows chamber or
cylinder, pressure dome) that allows the bellows movement to be observed. A scale
on the side of the housing provides a rough approximation of the t idal volume being
delivered. The housing for piston venti lators usually has a scale that can be
observed.
Exhau s t Va lv e
The exhaust valve (exhalat ion valve, v enti lator relief valve, compressed gas
exhaust, bellows control valve) communicates with the i nside of the bellows
housing on pneumatically powered v enti lators. I t is closed during inspirat ion.
During exhalat ion, it opens to allow driving gas inside the housing to be exhausted
to atmosphere. With a piston venti lator, there is no need for an exhaust valve.
Sp i l l Va l ve
Because the APL valve is isolated from the breathing system during venti lator
operation, a spil l valve (vent valve, dump valve, overf low valve, expired gas outlet,
expiratory valve or port, safety dump valve, pop-off valve, relief v alve, f lapper
valve, pressure relief valve, overspil l
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valve, gas evacuation outlet valve, exhaust gas valve, gas evacuation or evacuator
valve, expiratory pressure relief valve) is used to direct excess respired gases into
the scavenging system. This valve is c losed during inspirat ion. During exhalat ion, it
remains closed unti l the bellows or piston is ful ly expanded, then opens to vent
excess breathing system gases. The scavenging transfer tubing connects the
exhalat ion port of the spil l v alve to the scavenging system interface (Chapter 13).
With an ascending bellows, the spil l v alve has a minimum opening pressure of 2 to
4 cm H2O (22). This enables the bellows to f i l l during exhalat ion. This amount of
PEEP is applied to the breathing system. I t is not applied with a piston or a hanging
bellows venti lator.
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With a pis ton venti lator, excess gas is vented through a spil l valv e, which may not
in the v enti lator, or through an electronically controlled APL valve, which acts as a
spil l valve.
Ven t i l at o r Ho s e Con n e c t i o n
The venti lator standard (6) requires that the fitt ing on the tubing connecting the
venti lator to the breathing system be a s tandard 22-mm male conical f i t t ing. A f i l ter
may be used on the tubing to lessen transmission of pathogens and part icles. In
most newer venti lators, a separate hose is not present, and the connections
between the venti lator and the breathing system are internal. This reduces the
likelihood of misconnections, disconnections, or kinked hoses (23 ).
Pos i t i v e End - ex p i r a t o r y P r e s s u r e Va lv e
PEEP valves are discussed in Chapter 7. Modern venti lators have integral
electrically operated PEEP valves. Some venti lators apply PEEP to the entire
system, while others apply i t only to the expiratory hose (23).
A sta nd ing causes a small amo unt (2 to 4 cm H2O) of PEEP. There is no unset
PEEP with hanging bellows or piston-driven ventilators.
Ventilation Modes
Anesthes ia ven ti la to rs offe r one or mo re ven ti la ti on modes (18 ). Many offer dual
modes to gain the advantages of both. Venti lator sett ings must be c arefully
individualized in each mode to avoid hypoventi lat ion, hyperventi lat ion, volutrauma,
or barotrauma. I t is important when switching from one mode to another to ensure
that the t idal volume, peak pressure, and alarm sett ings are appropriate.
A ven ti la tor can del iver gas by genera ting f low or pressure. With f low genera tors ,
the f low pattern can be constant (square wave) or nonconstant (accelerat ive or
decelerat ive). Pressure generators produce a c onstant or nonconstant pressure.
Inspiratory f low rate varies according to the preset p ressure and the patient 's
resistance and compliance.
The characterist ics of inspirat ion and exhalat ion related t o the venti lator sett ings,
compliance, and resistance are ref lected in the pressure and f low-volume loops.
These are discussed in detail in Chapter 23.
Features of some commonly used v enti latory modes are shown in Table 12.1 . The
terminology used to describe the way a v enti lator operates has not been universally
agreed on, and some manufacturers have coined new terms for their venti lators.
Vo l ume Con t r o l
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The most commonly used mode in the operating room is v olume control (volume-
controlled or volume) venti lat ion, in which a preset t idal v olume is delivered. The
tidal or minute volume and respiratory rate are set by the anesthesia provider and
delivered by the venti lator, independent of p atient effort. I t is t ime init iated, volume
limited, and cycled by volume or t ime.
Flow rate is f ixed at a c onstant value during inspirat ion. I f the inspiratory f low is too
low to provide the set tidal volume, the bellows or piston will not complete its
excursion. I f the f low is set at a faster rate than is needed to provide the t idal
volume, there wil l be an
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inspiratory pause. An excessively high peak inspiratory pressure may result f rom
sett ing the inspiratory f low rate too high (24). The inspiratory phase may be
terminated before the t idal volume has been delive red if the peak ai rway pressure
reaches the set pressure l imit.
TABLE 12.1 Ventilatory Modes
Mode I nitiation Limit Cycle
Volume control ventilation Time Volume Volume/Time
Pressure control ventilation Time Pressure Time
Intermittent mandatoryventilation
Time Volume Volume/Time
Synchronized intermittentmandatory ventilation
Time/Pressure Volume Volume/Time
Pressure support ventilation Pressure/Flow Pressure Flow/Time
Typically, a volume control waveform shows steadily increasing pressure during
inspirat ion. Changes in c ompliance or resistance are ref lected in changes in peak
inspiratory pressure and the difference between peak and plateau pressure ( 25).
For a g iven set t idal volume, the pressure in the breathing system is determined by
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the resistance and compliance of the breathing system and the patient. Plateau
pressure is a ref lect ion of compliance. Peak pressure is also inf luenced by
resistance. The pressure-volume and f low-volume loops associated with volume
control venti lat ion are seen in Figures 23.22and 23.23.
Adding PEEP dec rea ses the t id al volu me del ivered, wi th th e eff ec t gre ate r wi th
small t idal volumes (26 ,27). On newer venti lators with integral PEEP, venti lat ion
may be better maintained (4).
I f closed system suctioning is performed during volume control v enti lat ion, there
wil l be a signif icant r ise in airway pressure when the ca theter is inserted and low
airway pressure during suctioning (28 ,29,30).
P r es s u r e Con t r o lPressure control (pressure-l imited, pressure-controlled, pressure-preset control,
lung protect ive, or p ressure) venti lat ion is available on many anesthesia venti lators
(2,31,32,33). With this mode, the operator sets the inspiratory pressure at a level
above PEEP. The venti lator quickly increases the pressure to the set l evel at the
start of inspirat ion and maintains this pressure unti l exhalat ion begins.
Inspiratory gas f low is highest at the beginning of inspirat ion, then decreases.
Increased resistance may change the shape of the flow-versus-time waveform to a
f latter, more square-shaped pattern as t idal volume delivery shif ts into the latter
part of the inspirat ion (25). This allows the venti lator to preserve t idal volume with
increased resistance until resistance becomes severe. The pressure-volume and
flow-volume loops show special characterist ics seen with pressure-controlled
venti lat ion (Figs. 23.29, 23.30).
When pressure control venti lat ion is used, t idal v olume is determined by the rise
t ime and set pressure. Tidal volume is not set or constant but f luctuates with
changes in resistance and compliance and with p atient-venti lator asynchrony (25).
If resistance increases or compliance decreases, the tidal volume will decrease. It
has been postulated that a decrease in t idal volume with pressure control
venti lat ion would detect a part ial ly occluded tracheal tube, but it was f ound that
t idal volume was not decreased unti l the occlusion was nearly complete (25 ).
Unlike most ICU v enti lators, an anesthesia venti lator in the pressure control mode
operates with a preset I :E rat io, so increasing the respiratory rate shortens
inspiratory t ime and lowers t idal volume (2). An increase in PEEP causes a
reduction in t idal volume. Tidal volume is not affected by fresh gas f low because
excess gas is v ented through the spil l v alve.
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On some venti lators, the inspiratory f low is adjustable (Fig. 12.48). There may also
be a sett ing that controls the inspiratory r ise t ime. For patients with good
compliance, inspiratory f lo w should be high to ensure that the inspiratory pressure
is rapidly attained. Limit ing the maximum inspiratory f low is useful to avoid
overshooting the target pressure, especially when compliance is low.
In patients with lung injury or during single-lung venti lat ion, pressure control
venti lat ion may improve oxygenation and produce greater t idal volumes than
volume control venti lat ion because of the decelerat ing f low pa ttern that delivers gas
to the alveoli early during inspirat ion (31). I t is often used with supraglott ic devices
and patients with narrow or part ial ly obstructed tracheal tubes to provide venti lat ion
at relat ively low pressures (34 ,35 ). I t may be useful i f there is an airway leak (e.g.,
uncuffed tube, supraglott ic airway device, bronchopleural f istula). However, i f there
is a large leak, the cycling pressure l imit may not be reached, causing a prolonged
inspirat ion (18 ).
During closed system suctioning (Chapter 3), pressure control venti lat ion results in
less int r insic PEEP during catheter insert ion and less subatmospheric pressure
during suctioning than during volume control venti lat ion (28 ,29 ).
In t e rm i t t e n t Manda t o r y
With intermittent mandatory venti lat ion (IMV), the venti lator delivers mechanical
(mandatory, automatic) breaths at a preset rate and permits s pontaneous,
unassisted breaths of a c ontrollable inspiratory gas mixture between mechanical
breaths. The venti lator has a secondary source of gas f low for spontaneous
breaths. This ut i l izes either continuous gas f low within the c ircuit or a demand
valve that opens to allow gas to f low from a reservoir. Continuous gas f low at a rate
greater than peak inspiratory f low involves no addit ional work of breathing but
requires a large volume of f resh gas. The demand valve system, although more
eff icient in f resh gas use , can impose signif icant work of breathing on the patient.
This mode is often used for weaning patients from mechanical venti lat ion. The IMV
rate is gradually
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reduced, allowing increased t ime for the patient 's spontaneous breaths.
Syn c h r o n i z ed In t e rm i t t en t Manda t o r y
Synchronized intermittent mandatory venti lat ion (SIMV) s ynchronizes venti lator-
delivered breaths with the patient's spontaneous breaths. If patient inspiratory
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activity is detected, the venti lator synchronizes its mandatory breaths so that the
set respiratory frequency is achieved. Posit ive pressure (mandatory) breaths may
occur at irregular intervals.
The time between the end of each mandatory breath and the beginning of the next
is subdivided into a spontaneous breathing t ime and a tr igger t ime. During the
trigger t ime, the venti lator checks whether the airway pressure has dropped a
minimum amount below the pressure measured at the end of the expiratory phase.
If a drop is not sensed, the venti lator delivers a breath. The tr igger window may be
adjustable (Fig. 12.49).
A ma nd ato ry ti da l vol um e an d a minim um mechanical ven ti la ti on ra te must be
selected. This determines the minimum minute ventilation. When setting the
venti lator rate, the patient 's spontaneous rate must be considered. I f the SIMV rate
is set too high, the patient may become apneic. Sett ing an I:E rat io is not required
in SIMV. The I:E rat io wil l change as the patient 's respiratory rate and rhythm
changes.
SIMV is used to facil i tate emergence from anesthesia as the patient transit ions
from controlled to spontaneous venti lat ion. I t ensures a minimal amount of
venti lat ion while freeing the anesthesia provider from periodically venti lat ing the
patient by hand. It reduces the incidence of patient-ventilator disharmony where the
patient tr ies to f ight the venti lator and the need for sedation or narcosis for t he
patient to tolerate mechanical venti lat ion. During anesthesia, SIMV may be used to
provide backup mechanical ventilation for spontaneously breathing patients. SIMV
can be c ombined with pressure support venti lat ion (PSV).
Manda t o r y M i n u t e
Mandatory minute ventilation (MMV) is a method of mechanical ventilation in which
the amount of venti latory support is automatically adjusted to f luctuations in
spontaneous venti lat ion so that a preset minute venti lat ion is delivered. The
venti lator circuitry monitors spontaneous expired volume and, if i t fal ls below a
predetermined level, provides the dif ference between the selected and actual
minute volume.
P r es s u r e Sup p o r t
PSV (pressure-assisted or assisted spontaneous venti lat ion) has been a feature of
ICU venti lators for years and is now on many anesthesia venti lators (36 ,37,38,39).
I t is designed to augment the patient 's spontaneous breathing by applying posit ive
pressure to the airway in response to patient-init iated breaths. A disadvantage of
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this mode of venti lat ion is that if the patient fai ls to make any respiratory effort, no
pressure-supported breaths wil l be init iated. To avoid this potential ly disastrous
situation, most venti lators have a backup or apneic SIMV rate in case that the
patient 's spontaneous respirat ion ceases (assist/control venti lat ion).
A suppo rted brea th ma y be pressure or f low ini t ia ted. Flo w t ri ggeri ng imp oses les s
inspiratory workload than pressure tr iggering and is used more frequently (40 ).
When the user-selected f low or s ub-baseline pressure caused by a spontaneous
breath is reached, f low from the venti lator begins and the set pressure is quickly
reached. The venti lator then modulates the f low to maintain that pressure. The f low
decreases unti l i t fal ls below a predetermined fract ion of the init ial rate (usually 5%
or 25%) or a f ixed f l ow (usually 5 L/minute) or after a specif ic durat ion as a backup
(41). At this p oint, f low is terminated and exhalat ion begins. Because the PSV level
is reached early in inspirat ion and is maintained throughout the inspiratory phase,
the pressure waveform has a square, flat-topped shape. PEEP may be added if
needed.
The anesthesia provider must set the tr igger sensit ivity and the inspiratory pressure
(usually from 5 to 10 cm H 2O). The tr iggering sensit ivity should be set so that it wil l
respond to inspiratory effort without auto-cycling in response to art ifactual changes
in airway pressures. The init ial inspiratory f low is usually nonadjustable but can be
changed on some venti lators by adjust ing the inspiratory r ise t ime (Fig. 12.48). The
optimal init ial inspiratory f low is highest in patients with low compliance, high
resistance, and most act ive v enti latory drive. On some venti lators, the tr igger
window can be changed (Fig. 12.49).
Tidal volume is de termined by the pressure support level, lung characterist ics, and
patient effort. The desired t idal volume should be calculated and the pressure
support level adjusted so that the desired volume is delivered. I f the exhaled
volume is inadequate, the inspiratory pressure should b e increased or inspiratory
rise t ime decreased (if adjustable). PEEP may cause an increase in t idal volume
(42). Very high inspiratory f low (due to a high set pressure) may dec rease t idal
volume by prematurely terminating inspirat ion (37). As the patient's effort
increases, the level of inspiratory pressure can be reduced. Undesired
hyperventi lat ion can be treated by adjust ing the tr igger sensit ivity, pressure level,
or trigger window or, if these seem adequate, additional sedation.
PSV can be used to reduce the patient 's work of spontaneous breathing (38 ,43 ,44).
In addit ion, it c an increase the functional residual capacity. I t may be useful for
preoxygenating obese patients by improving the eff iciency
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of spontaneous venti lat ion and during weaning from mechanical venti lat ion. I t can
be useful with a supraglott ic airway device to keep the airway pressure lower than
the supraglott ic device leak pressure (38,42,45). If there is a leak around the
device, PSV will be able to compensate for the leak to some extent, as the airway
pressure is maintained irrespective of the volume.
An advan tage of PSV is th e synchro ny between th e pat ient and the ven ti lator. The
patient controls rate, volume, and inspiratory t ime. This may increase patient
comfort. Breath stacking and f ight ing the ven ti lator are decreased. Even patients
who are init ial ly tachy-pneic may be s uccessfully managed in this mode, as the
pressure support can be set suff icient ly high to augment t idal v olume and hence
reduce the respiratory rate. Peak and mean airway pressures are lower than with
volume control venti lat ion, reducing the risk of barotrauma (42 ).
Too high an i nspiratory f low may cause patient discomfort (33). PSV wil l deliver a
variable minute volume in a patient with a changing respiratory drive. Inappropriate
venti lator tr iggering can occur with PSV (40 ,46,47 ). This may be caused by a leak
or a decrease in airway pressure caused by cardiac contract ions.
With closed system suctioning, PSV results in a lower airway pressure during
catheter insert ion and higher end-expiratory pressure during s uctioning than either
volume control or pressure control venti lat ion (28,48 ).
Specific Ventilators
Drage r A V2+
The AV2+ is the successor to the AVE and AV2 ven-t i lators.
Description
The AV2+ is shown in Figures 12.3and 12.4. I t has an ascending bellows. Ti dal
volume is adjusted by using the kn ob above the bellows assembly, which raises or
lowers a plate at the top of the bellows. A scale on the bellows housing provides a
rough indication of the tidal volume delivered.
Most venti lator controls are located across the top of the venti lator. To the left of
the t idal volume control and above the bellows is the inspiratory pressure l imit
control. On the left above the pressure limit and tidal volume controls is the
frequency control with a digital
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readout to the left. To the right of the frequency control are the control and display
for the I:E ratio. In order to set an inverse ratio, an extended range button below
the display and control must be depressed.
View Figure
Figure 12.3.Drager AV2+ ventilator. I:E,inspiratory:expiratory.
View Figure
Figure 12.4.Drager AV2+ ventilator. (Courtesy of Drager
Medical.)
To the right of the I:E rat io control are the inspiratory f low control and gauge. The
scale on the gauge is divided into low, medium, and high f low.
To the right is t he venti lator ON/OFF switch. A green l ight next to the switch
indicates that the ventilator is turned ON. The ventilator may be turned ON at this
switch or by turning the Manual/Automatic switch on the absorber to the automatic
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posit ion. The venti lator control switch can only be turned ON with the
Manual/Automatic switch in the Automatic posit ion. I f the ON/OFF switch is turned
ON with the bag/vent selector switch in the bag posit ion, a fault l ight to the left of
the ON/OFF switch wil l b e i l luminated.
The spil l valve (Fig. 12.5) is at the base of the bellows assembly. A pilot line from
the canister connects through the top of the spil l valve to a balloon diaphragm.
Pressure in the canister causes the balloon diaphragm to be i nf lated, closing the
opening to the scavenging system. The ball in the spil l valve ensures that a certain
pressure must be present to allow gas f low through the spil l valve, even if the
balloon diaphragm is deflated. This results in approximately 2 cm H 2O PEEP in the
breathing system.
The internal construct ion is shown in Figures 12.6 to 12.9. The inspiratory pressure
regulator reduces the gas from approximately 50 psig to the value indicated on the
inspiratory f low gauge. A solenoid in the oxygen l ine l inks the pneumatic and
electronic port ions of the venti lator. When the solenoid is energized, it al lows gas
to f low through the control valve in the oxygen l ine to the Venturi mechanism.
A smal l-diameter tub e carr ies oxygen from th e ins pi rato ry f lo w regulator to the top
of the auto-ranging valve, which controls the amount of ambient air entrained at any
given inspiratory sett ing. The auto-ranging valve contains a diaphragm that is
depressed when pressure is applied. The plunger moves downward, controlling the
opening through which ambient air is entrained during inspirat ion.
View Figure
Figure 12.5.Spill valve on Drager AV2+ ventilator.(Courtesy of Drager Medical.)
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The control valve allows gas to f low through it when pressure is applied. The
Venturi receives oxygen from the control valve and air f rom the auto-ranging valve
and combines them to form the drive gas that pushes the bellows downward during
inspirat ion.
The pilot actuator, which controls the opening of the exhaust valve, operates in
response to oxygen pressure that enters at the top. When sufficient pressure is
applied, the valve moves downward against the spring, which closes the valve when
no pressure is applied. During inspiration, the pressure of the driving gas inside the
bellows housing pushes the bellows downward. When no pressure is applied to the
pilot actuator, the exhaust valve opens, and the driving gas f lows to atmosphere
through the exhaust valve.
Controls
The venti lator can deliver t idal v olumes f rom 20 to 1500 mL. Respiratory rate can
be set from 1 to 99 breaths per minute (bpm). Inspiratory f low can be set between
10 and 100 L/minute. The I:E rat io can be set from 1:4.5 to 4:1. The inspiratory
pressure l imit range is 15 to 120 cm H 2O.
Alarms
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The AV2+ alarms are associated with the anesthesia machine and are not part of
the ventilator. Alarm messages are displayed on the anesthesia machine monitoring
screen. Warnings are accompanied by a three-pulse pattern that is ini t ial ly
repeated ev ery few seconds in a series o f descending volume and then constantly
at ful l v olume unti l the alarm condit ion is resolved. Cautions are accompanied by a
three-pulse tone pattern that is repeated every 30 s econds. Advisories ut i l ize a
single tone or no sound, depending on the advisory. The highest priority currently
act ive alarm condit ion is annunciated. Audio signals for lower-priority alarm
condit ions are temporari ly suppressed to minimize confusion caused by
simultaneous alarms.
Ventilation ModesVolume control is the only v enti latory mode on this venti lator. The v enti lator is t ime
cycled and v olume preset.
I n s p i r a t i o n
During inspirat ion (Fig. 12.6), the controller energizes the solenoid and pressurizes
the control valve, causing it to open. This allows oxygen from the pressure
regulator to f low to the bellows assembly. A small port ion of the oxygen is diverted
to the pilot actuator. This causes the pilot actuator to move downward, sealing the
exhaust valve and preventing drive gas from escaping to atmosphere. A smallport ion of the regulated oxygen also f lows to the auto-ranging valve and opens i t in
proport ion to the sett ing on the inspiratory f low regulator.
Oxygen f lows through the Venturi, entraining room air. The drive gas, consist ing of
oxygen and entrained air, then pressurizes the space between the bellows and the
canister. This causes the bellows to b e compressed and gases inside the bellows
flow to the breathing system.
The spil l v alve prevents gases from entering the scavenging system during
inspirat ion. Drive gas f lows through the pilot l ine and inf lates a balloon diaphragm
that blocks the outlet between the inside of the bellows and the scavenging system.
This valve remains closed unti l the bellows has reached its l imit of expansion
during exhalation.
In s p i r a t o r y P a u s e
During the inspiratory pause (Fig. 12.7), the controller continues to energize the
solenoid. As long as oxygen flows to the bellows assembly, pressure on the pilot
actuator is maintained, and the exhaust valve remains closed. Since the bellows is
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completely compressed, no addit ional gas can enter the bellows housing, and no
more air is entrained by the Venturi. Excess oxygen is vented to atmosphere
through the air entrainment port. The pilot line and the balloon diaphragm
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in the sp il l valve remain pressurized, so gas f low to the scavenging system remains
blocked.
View Figure
Figure 12.6Drager AV2+ ventilator. Inspiration. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)
E x h a l a t i o n
During exhalat ion (Fig. 12.8), the controller de-energizes the solenoid, which s tops
the f low of ox ygen through it . The oxygen in the tubing between the solenoid and
the supply valve is vented to atmosphere via a small exhaust tube at the top of the
solenoid. Once this oxygen is v ented, the control valve closes and stops the f low of
oxygen to the Venturi. This also allows the pilot actuator to depressurize. With the
pilot actuator depressurized, the spring forces the plunger upward, opening the
exhaust port. Exhaled gases push the bellows up ward. Drive gas vents to
atmosphere through the exhaust port. As the pressure in the canister dec reases,
the pressure within the pilot l ine for the spil l v alve also decreases, and the balloon
diaphragm deflates. The ball check v alve below the balloon diaphragm presents
more resistance to the f low of ex haled gas than does the bellows, so exhaled gases
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continue to f i l l the bellows. Expiratory f low ends when the bellows reaches the plate
at the top.
View Figure
Figure 12.7.Drager AV2+ ventilator. Inspiratory pause.I : E, inspiratory : expiratory. (Redrawn courtesy of DragerMedical.)
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View Figure
Figure 12.8.Drager AV2+ ventilator. Exhalation. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)
E x p i r a t o r y P a u s e
Af ter the bellows ha s reached maximum expans io n (Fig. 12.9), the expiratory pause
time begins. The solenoid and the supply valve remain closed. The pressure in the
pilot l ine to the spil l valve decreases to atmospheric, and the balloon diaphragm
deflates. When pressure from gas in the bellows exceeds the resistance created by
the weight of the ball in the spil l v alve, the ball is l i f ted, and gases can f low into the
scavenging sys tem.
Special Features
A saf ety re l ief val ve vents dri ve ga s to atm os ph ere if th e dri ve gas pressure
exceeds 120 cm H2O.
In the event of mains power failure, a fully charged battery wil l power the v enti lator
for approximately 20 minutes. There are yellow indicators to signify that there is
alternating current (AC) power failure and that the battery power is low. I f the
machine has switched to
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battery power, a three-pulse tone sounds every 30 seconds. A battery test button is
present on the machine. A green battery test indicator signifies that the battery
power is s at isfactory. Battery messages are displayed on the monitor screen.
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View Figure
Figure 12.9.Drager AV2+ ventilator. End exhalation. I:E,inspiratory:expiratory. (Redrawn courtesy of DragerMedical.)
Hazards
The problems discussed below were reported with the predecessor AV-E venti lator.
Since the two venti lators are similar i n construct ion, there is a possibil i ty that
similar problems could occur with the AV2+ venti lator.
A cas e has bee n rep ort ed in wh ich the mu f fl er plac ed ov er the driv in g gas exhaus t
became saturated with water and obstructed the flow from the bellows chamber
(49). This resulted in high airway pressures as gas continued to f low into the
venti lator.
In another reported case, the control valve malfunctioned, result ing in continuous
driving gas f low to the bellows (50). High airway pressures resulted.
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Prolongation of the inspiratory phase owing to i nsuff icient parts lubricat ion has
been reported (51).
Ventilatory irregularit ies resulting from improper seating between the bellows and
its mount have been reported (52).
There has been a report of the spill valve becoming incompetent, resulting in
hypoventi lat ion (53 ). In other reported cases, the pilot l ine connecting the bellows
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chamber to the spil l valve became kinked so that it was occluded (54 ,55). If the
pilot l ine becomes occluded during inspirat ion, hypoventi lat ion wil l result because
the spil l valve wil l be open. I f the occlusion occurs during exhalation, gas wil l be
unable to exit the circuit , and the pressure inside the ci rcuit wil l increase.
PEEP can result under certain circumstances. This was reported when some hoses
were draped over the spil l valve, part ial ly obstruct ing the pilot l ine (56 ,57).
Cleaning and Sterilization
Cleaning and disinfect ion or s teri l izat ion are c omplicated matters b eyond the scope
of this text. They are explained in detail in the operator's manual.
D rage r D i v an
The Divan venti lator (digital venti lator for anesthesia) is a component of the North
Ame ric an Dra ge r 6000 series ma chi nes .
Description
The venti lator control panel (Fig. 12.10) is situated at the front of the anesthesia
machine below the desktop. To alter a function, the key f or that function is pressed.
Changes are made by using the rotary knob at the right of the control panel and the
change confirmed by pressing the knob. I f the al tered value or mode is not
confirmed within 10 seconds, the venti lator returns to the previous value.
At the lef t s id e of th e con tro l panel is a Ma nu al / Sponta ne ou s key. Af ter pressin g
this key and confirming the sett ing, the patient can breathe spontaneously or be
venti lated manually by adjust ing the APL v alve.
Below the Manual/Spontaneous key is the Volume Mode key. When it is pressed
and its function confirmed, the venti lator goes into the volume control mode. Below
the volume mode key is the SIMV key. To the right of this key is the key for
pressure control venti lat ion (Pres Mode).
To the right of the Manual/Spontaneous key is a window with a bar graph that
indicates piston movement and displays the percent of the set t idal volume (0%
represents f ull exhalat ion, while 100% indicates inspirat ion to the set t idal volume).
Below the bar graph window is a numeric display window that displays the values
for the keys below it . At the left is the sett ing for maximum allowable pressure
(Pmax) in the volume and SIMV modes or preset airway pressure (Pset) in the
pressure control mode. In the middle is the sett ing for t idal volume in the SIMV and
volume control modes. At the right is the respiratory rate sett ing. To alter a
parameter, the key under it is pressed and the rotary knob rotated to increase or
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decrease the sett ing. The new value is displayed in the window above the key.
When the proper
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value is displayed, it is confirmed by pushing the rotary knob.
View Figure
Figure 12.10.Control panel of Divan ventilator. (Redrawncourtesy of Drager Medical.)
To the right of the numeric display is an alphanumeric display. This prompts the
operator to take certain act ions during the checkout procedure and updates the
progress of the checkout. After a change in v enti latory mode or parameter is made,
there wil l be a prompt to confirm the change. Other messages report ing status and
faults are displayed here.
Under the alphanumeric display are addit ional keys. The I:E key sets the I:E rat io.
The % I.P./Flow key sets the rat io of i nspiratory pause t ime to inspirat ion phase
time in the volume control and SIMV modes and the inspiratory flow rate in the
pressure control mode. The PEEP key is used to set the PEEP in all modes. The
SIMV Rate key sets the minimum ventilatory rate in the SIMV mode.
At the ri gh t s ide of the control pa ne l is a s ta ndby key. In th is mo de, driv e gas us e is
minimized, and inspection or repairs can be performed.
Above th e s tandb y key is a te st key. Th is causes the venti lato r to measure sys tem
compliance and leakage. This can only be init iated when the venti lator is in the
standby mode.
Below the tabletop is an electrically powered piston (Fig. 12.11). The manufacturer
offers an optional top cover with a transparent window that allows th e user to see
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piston movement. I f there is inadequate gas in the breathing system to allow the
piston to retract ful ly, the piston wil l stop and alert the anesthesia provider. This
wil l prevent a negative pressure from being exerted (23).
A hea te r is incorpo rated in to th e absorb er he ad to min imize mo istu re condensati on.
The internal construct ion of the venti lator and breathing system is shown in Figures
12.12to 12.17. V1 is the fresh gas decoupling valve. V2 is the surplus gas valve.
V3 is the valv e that controls gas f low to the APL and gas relief valves. Gas is
aspirated from near the patient port, analyzed, and returned to the circuit
downstream of the expiratory unidirect ional v alve. An ultrasonic f low sensor
(Chapter 23) and PEEP valve are located in the expi ratory l imb.
ControlsThe venti lator cannot be set to values result ing in an inspiratory f low greater than
75 L/minute, a minute volume greater than 25 L/minute, or an expiratory t ime of
less than 400 ms.
PEEP can be set from 0 to 20 (default 0) cm H 2O. PEEP is not available in the
Man/Spont or SIMV modes.
The peak airway pressure (Pmax/Pset) can be set from 10 to 80 (default 25) cm
H2O in the volume control and SIMV modes and 10 to 70 (default 10) cm H 2O in the
pressure control mode. When the maximum allowable pressure is reached, f low is
adjusted so that the pressure remains constant through the end of inspirat ion. In
this situat ion, the full t i dal volume may not be delivered.
Figure 12.11.Piston ventilator of Divan ventilator.
(Courtesy of Drager Medical.)
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View Figure
The minimum difference between Pmax and PEEP is 5 cm H 2O. If the pressure
increases by more than 5 cm H2O above Pmax, inspiration is immediately stopped
and expirat ion begins.
The % inspiratory pause/flow (% I.P./Flow) sets the length of the inspiratory pause
in the v olume control and SIMV modes or the inspiratory f low in the pressure
control mode. The pause range is 0% to 60% (default 10%). During pressure control
venti lat ion, the inspiratory f low rate can be set from 5 to 75 (default 50) L/minute.
The available t idal volumes are 10 to 19 mL, 20 to 100 mL, and 110 to 1400
(default 600) mL.
The respiratory rate range is 6 to 80 (default 12) bpm in the v olume and pressure
control modes and 3 to 8 0 (default 12) bpm in the SIMV mode.
The range of available I :E rat ios is 1:3 to 2:1 (default 1:2). I f an inverse I:E rat io is
set and co nfirmed, a message is displayed.
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View Figure
Figure 12.12.Divan ventilator. Spontaneous inspiration.APL, adjustable pressure limiting; PEEP, positive end-expiratory pressure. (Redrawn courtesy of Drager Medical.)
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Alarms
Alarm limits , wh ich dep end on th e ven ti la to ry mode and the pati en t , are presen te din an alarm window on the machine monitor screen. If an alarm limit is exceeded or
not reached, the alarm limits menu is displayed, and the value is highlighted.
Related alarms are combined. A message that indicates the computer analysis of
the problem wil l appear.
The alarm silence key i n the alarm window allows the alarms to be audio paused
(silenced) for 60 seconds if pressed once and for 120 seconds if pressed twice.
Alarms can be sus pended by press ing a key on the bo ttom of the scree n.
Warnings are announced by a three-tone sequence of h igh, high, low. The tones
are also in a sequence of dif ferent volumes with the f irst and fourth sequence being
at ful l v olume. Alarm messages are displayed on a f lashing red background with
white text. The f lashing stops when the alarm silence button is depressed. Flashing
resumes when the audio pause (si lence) period has ended.
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View Figure
Figure 12.13.Divan ventilator. Spontaneous exhalation.APL, adjustable pressure limiting; PEEP, positive end-expiratory pressure. (Redrawn courtesy of Drager Medical.)
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Cautions are displayed on a f lashing yellow background in black text. The
messages are announced by a three-tone burst of low, low, high. Announcements
occur every 30 seconds.
Adv is ori es are displ aye d on a wh ite backgro und wi th blac k tex t tha t does not f la sh.
A sing le ton e ma y sou nd.
An al arm log can be ac ces sed. It wi l l al lo w the cl in ic ian to observ e all a la rm ev en ts
that have occurred during the case. I t c an store up to 500 events.
Ventilation Modes
The desired mode (Volume or Pressure Control or SIMV) is selected by pushing the
key for that mode and confirming that choice.
In the pressure control mode, inspiratory f low rate can be set independent of airway
pressure (Pset). However, Pset may not be achieved if the inspiratory f low rate is
too low. In this case, an alarm message wil l be displayed.
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View Figure
Figure 12.14.Divan ventilator. Inspiration during manualventilation. APL, adjustable pressure limiting; PEEP,
positive end-expiratory pressure. (Redrawn courtesy ofDrager Medical.)
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During SIMV, the time between each mandatory respiration and the beginning of
the next is subdivided into a spontaneous breathing t ime (Tspont) and a tr igger
t ime (Ttrigger). During the tr igger t ime, the system checks whether the airway
pressure has dropped at least 0.5 cm H2O below the pressure measured at the end
of expirat ion. I f this has not occurred, the venti lator delivers a breath.
S p o n t a n eo u s B r e at h i n g
To allow spontaneous breathing, the MANUAL/SPONTANEOUS key is pressed and
the APL valve set to SPONT. Valves V1 and V3 are open, while V2 is closed. When
the patient inspires (Fig. 12.12), the inspiratory valve opens, and gas f lows f rom
the reservoir bag. During exhalat ion (Fig. 12.13), the expiratory valve opens, and
exhaled gases pass through the absorber and into the bag. During late exhalat ion,
the pressure rises,
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and excess gas f lows through V3 and the gas relief valve to the scavenging system.
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View Figure
Figure 12.15.Divan ventilator. Inspiration duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)
Manu a l
For manual ventilation, the Manual/Spontaneous key is pressed, and the APL valve
is set to MAN. The pressure during inspirat ion wil l be l imited by the APL valve
sett ing. When the pressure l imit is reached, excess gas wil l f low through V3 and the
APL valv e to th e scav eng ing system through the gas rel ie f val ve (Fig. 12.14).During exhalation, exhaled gases flow through the absorber into the reservoir bag.
Mech a n i c a l
When mechanical venti lat ion is s elected, the APL valve is closed. The bag
functions as a reservoir for fresh gas.
During inspirat ion (Fig. 12.15), valves V1, V2, and V3 are closed. Piston movement
produces gas f low through the inspiratory valve to the patient port. Fresh gas
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continues to enter the reservoir bag but does not affect the t idal volume, because
valve V1 is closed (fresh gas decoupling).
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View Figure
Figure 12.16.Divan ventilator. Mid exhalation duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)
When exhalat ion begins, the expiratory valve opens, allowing exhaled gases to f low
through the absorber and into the retracting piston and to the reservoir bag through
V1, which opens. Valves V2 and V3 remain closed. Fresh gas f lowing in to the
system mixes with some of the exhaled gases in the piston v enti lator. Mid
exhalat ion is depicted in Figure 12.16. The piston retracts, allowing the cylinder to
fill with gas from the reservoir bag and fresh gas. During the later part of exhalation
(Fig. 12.17), V2 opens, and gases are vented to the scavenging system through the
gas relief (spil l) valve.
Special Features
The Divan venti lator decouples fresh gas f low from t idal volume. Fresh gas entering
the circuit during inspirat ion
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is isolated from the patient circuit and accumulates in the reservoir bag. I f the
oxygen f lush is act ivated during inspirat ion, the gas wil l not be added to the t idal
volume but wil l enter the reservoir bag (23). The reservoir bag will inflate and
deflate during mechanical v enti lat ion.
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View Figure
Figure 12.17.Divan ventilator. Late exhalation duringmechanical ventilation. APL, adjustable pressure limiting;PEEP, positive end-expiratory pressure. (Redrawn courtesyof Drager Medical.)
This venti lator compensates for breathing system compliance and gas c ompression
so that the patient receives the set t idal volume. Information that makes t idal
volume compensation possible is gathered during the automated checkout. The Y-
piece must be occluded and fresh gas f low s et at a minimum to perform these
measurements.
Small-diameter breathing tubes are recommended for pediatric patients where tidal
volumes are less than 200 mL. After switching to the pediatric tubes, the leak and
compliance test should be performed before the patient is connected to the
venti lator. I f a low-compliance circuit such as a pediatric circuit were added without
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conducting a compliance test, the v enti lator could deliver excessive volumes. To
prevent this from occurring when a t idal volume of less than 200 mL is selected, the
venti lator wil l use the measured circuit compliance only if i t is 0.8 mL/cm H2O or
less (58 ). I f the measured circuit compliance is higher, a default value of 0.6 mL/cm
H2O is used.
The breathing system and pis ton assembly are designed to minimize circuit v olume
and the t ime that it takes the system to respond to changes in fresh gas
composit ion. A low-f low wizard helps the clinician to assess the fresh gas surplus.
I t provides graphical information of fresh gas surplus, a message report, and a help
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key. The message area gives recommendations for use with low f lows, including
bag size and venti lator sett ings.
When the anesthesia machine is turned ON, an automated checkout process that
requires about 5 minutes is set in motion. Other than a few prompts reminding the
anesthesia provider to s et a p ressure at the APL valve and to occlude the Y-piece
on the breathing system, this checkout is ful ly automatic. The checkout allows the
computer to determine information about gas compression, l eaks, and compliance
of the breathing system.
If the v enti lator detects an internal fault that might affect patient safety during
mechanical venti lat ion, it init iates a safe s tate in which venti lat ion can be continued
in the Manual/Spontaneous mode. When the v enti lator enters the safe state, the
clinician is alerted by a display reading Equipment Fault , and an audible tone
sounds. The ventilator now performs as if it were in the manual/spontaneous mode.
The venti lator override button is on the machine near the absorber head. I t i s
provided in the event there is an unforeseen condit ion that the software does not
recognize. Activat ing this override removes power from the v enti lator and allows
manual or spontaneous v enti lat ion.
The Narkomed 6000 has battery backup that will power the machine and ventilator
for at least 30 minutes. An alarm indicates when the battery has only another 10
minutes. After the batteries are exhausted, the machine can continue to be used
with manual venti lat ion o r spontaneous breathing.
Respitone is an option on the 6400 anesthesia machine. I t is a venti lat ion sound
composed of two distinct tones. One tone annunciates when the pressure waveform
crosses the apnea threshold during inhalation. Another tone annunciates on the
rising edge of a carbon d ioxide waveform corresponding to exhalat ion.
Evaluation
A compari son between a Divan and an AV 2+ venti lato r was ma de duri ng s imul ated
venti lat ion of pediatric patients (59 ). The Divan offered advantages in the low t idal
volume range during v olume control venti lat ion.
The Divan and an ICU venti lator were compared by using both an infant lung model
and infants with congenital heart disease (60 ,61 ). Both v enti lators provided
adequate venti lat ion in the volume control mode.
In comparison with an ICU v enti lator and an anesthesia venti lator with a gas-
powered bellows during pressure control venti lat ion, the Divan maintained t idal
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volume with increasing respiratory rates better than the other anesthesia venti lator
but not as well as the ICU venti lator (2).
HazardsA high ne ga tive pressure applie d to the airway can ex cee d the abi l ity of the
venti lator's negative pressure relief valve, causing the piston to lock (62 ,63 ). The
problem can be remedied by opening the ventilator cover and removing the piston
to break the negative seal.
The venti lator override button is in a rather inconspicuous place (64).
A cas e of powe r supp ly failu re th at interr up te d ven ti lati on ha s been re po rte d
(64,65). The l inkage to the backup batteries prevented them from kicking in.
Cleaning and DisinfectionMost of the v enti lator parts, including those exposed to breathing gases, can be
steam autoclaved. See the operation manual for specific disassembly and
steri l izing instructions.
D rage r Fab i u s GS
Description
On the Fabius machine, the venti latory module, which includes a piston, is located
behind a door on the left side of the machine (Fig. 12.18). The piston is inside ametal case that wil l swing out when the door is opened (Fig. 12.19). A window
allows the operator to view piston movement.
The piston assembly is shown in Figures 12.20and 12.21. Electrical power is used
to raise and lower the piston. The motor is n ear the bottom of the cylinder that
holds the piston. There are two roll i ng diaphragms that seal the p iston and prevent
mixing of ambient and respired gases. The upper diaphragm is attached at the top
and f its over the upper end of the piston. The lower part of upper diaphragm rolls
upward and downward as the piston moves upward and downward (Fig. 12.20). The
lower diaphragm is connected between the piston wall and the inside of the
cylinder. As the piston moves downward, the space above the upper diaphragm
increases, allowing exhaled gases to enter that space. There are high pressure and
negative pressure relief valves on the top of the piston, connecting with the space
for respired gases.
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View Figure
Figure 12.18.Drager Fabius GS ventilator. A windowallows the operator to view piston movement.
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The display screen is shown in Figure 12.22(see page 338). At the left side are
keys that determine the venti latory mode (volume control, pressure control,
manual/spontaneous). A rotary mouse is at the bottom right of the screen. Once a
parameter is selected, the value is altered by turning the rotary mouse and is
confirmed by depressing it . The Standby key is to the right of the rotary mouse. To
the right of the rotary mouse and above the Standby key is the Mains Power l ight-
emitting diode (LED), which, when lit, confirms that the machine is connected to a
functioning electrical system.
To the right of the screen are three keys. The bottom one is the Home key. I t
causes the main screen to be displayed. The Setup key is above the Home key.
When pressed, the displayed window enables the operator to view and change
venti lat ion and to review sett ings.
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Above th e Setu p key is the Al arms key. Whe n pressed , al arm limits are shown on
the right side of the screen. To the right of the Alarms key is the Alarm Silence key.
Pushing this causes act ive alarms to be audio paused for 2 minutes.
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View Figure
Figure 12.19.Drager Fabius GS ventilator. When the dooris opened, the piston ventilator, which is inside a metal case,will swing out.
View Figure
Figure 12.20.Piston assembly. As the piston movesdownward, the upper diaphragm moves downward with it,creating a space for respired gases.
To the right of the Alarms key are two LED lamps that indicate the urgency of the
alarm message. A status bar near the top of the screen displays the venti latory
mode being used. I t also d isplays alarm silence status, battery power level , and the
time.
To the left side of the screen are virtual f lowmeters for air, oxygen, and nitrous
oxide. To the right of the f lowmeters in the upper third of the screen is an alarm
window. This displays up to four of the highest priority
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alarms. To the right of this window, the inspired oxygen concentrat ion and alarm
limits for oxygen concentrat ion are displayed. The respiratory volume monitor
window is the middle window to the right of the flowmeter window. It displays
respiratory rate and tidal and minute volumes. Below the respiratory volume window
is the breathing pressure monitor window. I t displays PEEP values and peak and
mean inspiratory pressures. Below the flowmeters and the breathing pressure
window is the breathing pressure waveform window. Below this are six windows
associated with venti lator parameters. Below these windows are keys fo r the
associated parameters.
View Figure
Figure 12.21.As the piston moves upward, gases are forcedout of the space at the top.
Controls
The range for the maximum venti lat ion pressure (PMA X ) is 10 to 70 (default 40) cm
H2O. Other controls are discussed under the individual venti lat ion modes.With volume control venti lat ion frequency can be set from 4 to 60 (default 12) bpm.
The t ime rat io between the inspiratory and expiratory t ime phases (Ti:Te) range is
4:1 to 1:4 (default 1:2). The inspiratory pause can be s et from 0% to 5 0% (default
10%). PEEP can be set from 0 to 20 (default 0) cm H 2O. The range for tidal volume
is 20 to 1400 (default 600) mL.
In the pressure control mode, the inspiratory pressure (P INSP ) can be set from 5 to
60 (default 15) cm H2O,
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and the inspiratory f low can be set from 10 to 75 (default 30) L/minute. PEEP can
be set from 2 to 20 (default 0) cm H2O. Venti lat ion frequency can be s et from 4 to
60 (default 12) bpm.
View Figure
Figure 12.22.Display screen for the Drager Fabius GSventilator.
Alarms
Alarms are auto ma tical ly ena bled when the venti lator is swi tche d to a ven ti la ti on
mode. Alarm messages are displayed in the alarm box in the center of the top of
the data screen. The text displays are followed by exclamation marks (!). There are
three marks (! ! !) for warnings, two (! !) for caution, and one (!) for advisories. The
LEDs to the right of the alarm silence key indicate the urgency of the alarm
condit ion. A warning is signaled by a blinking red LED. A caution is expressed by a
blinking yellow LED. An advisory is indicated by a continuous yellow LED. Warning
tones are continuous. Caution tones enunciate every 30 sec onds. An advisory has
a single or no t one.
Ventilation Modes
The Fabius offers volume control and pressure control venti lat ion. PSV can be
added. When the venti lat ion mode is changed, the function is displayed across the
bottom of the data screen and above the appropriate key.
S t a n d b y
In the Standby mode, the ventilator stops, and the monitoring and alarms are
turned OFF. I f gas f low is detected, a Gas Sti l l Flowing message appears in the
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alarm window. If the machine is in the Standby mode for 5 minutes and there is no
user input, the machine goes into the Sleep mode, and a screen saver appears.
Man u a l / S p o n t a n e o u s
In Man/Spon mode, the piston in the venti lator is moved to its topmost posit ion to
minimize system compliance. The APL bypass valve is closed, direct ing excess gas
through the APL valve.
For spontaneous v enti lat ion, the APL valve is put in the SPONT posit ion, in which it
is ful ly open. During inspirat ion (Fig. 12.23), gas from the bag f lows through the
fresh gas decoupling valve and the inspiratory unidirect ional valve to the Y-piece.
During exhalat ion (Fig. 12.24), exhaled gases f low through the expiratory
unidirect ional valve and the absorber. The reservoir bag f i l ls with a c ombination offresh gas and gas that has passed through the absorber. Excess gas exits through
the APL valve.
During manual venti lat ion, the APL valve is set to the MAN posit ion. The opening
pressure can be adjusted from 5 to 70 cm H2O. As the bag is compressed (Fig.
12.25, see page 341), the gas in the bag f lows through the fresh gas decoupling
valve, the inspi ratory unidirect ional valve, and the Y-piece. Some gas f lows
retrograde through the absorber and the APL valve, which is adjusted to provide
the proper pressure. During exhalat ion, exhaled gases f low through the expiratory
unidirect ional valve and the absorber. The reservoir bag f i l ls with a c ombination of
fresh gas and gas that has passed through the absorber.
Mech a n i c a l
When the Fabius is in automatic mode, the APL bypass v alve is held open. Fresh
gas decoupling is accomplished by using a decoupling valve between the fresh gas
inlet and the breathing system. The reservoir bag wil l inf late and deflate during
mechanical venti lat ion. I f the oxygen f lush is act ivated during inspirat ion, the gas
wil l not be added to the t idal volume but wil l enter the reservoir bag (23 ).
During inspirat ion (Fig. 12.26, see page 342), the pressure generated by the piston
closes the fresh gas decoupling valve. Fresh gas f lows retrograde through the
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absorber and enters the reservoir bag. The piston pushes gas through the
inspiratory unidirect ional valve and the inspiratory hose to the Y-piece. I f the
pressure exceeds the pressure l imit, the Pmax valve opens.
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View Figure
Figure 12.23.Drager Fabius GS ventilator. Inspirationduring spontaneous breathing. PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.
During exhalat ion (Fig. 12.27, see page 343), exhaled gas f lows through the
expiratory unidirect ional valve and into the reservoir bag, where fresh gas has been
collect ing during inspirat ion. The piston retracts, drawing in gas. Excess gas f lows
through the APL bypass valve and the exhaust valve to the scavenging system.
Special Features
A high pre ssure an d a negat iv e pressure relief val ve are lo cated at the to p of th e
venti lator piston (Figs. 12.20, 12.21). The high pressure relief valve opens at 75 5
cm H2O. The negative pressure safety relief valve lets in air at -2 to -5 cm H2O.
If a fault in the venti lator is not corrected and the anesthesia provider cannot switch
to manual venti lat ion by using the Man/Spont mode, manual venti lat ion is st i l l
possible. To do this, the ON/OFF system power s witch on the rear panel is switched
OFF, then ON.
The Fabius GS has battery backup that wil l power the machine and venti lator for at
least 45 minutes if the batteries are fully charged. I f the power fails, the mains LED
will go out, a message wil l appear, and a battery symbol wil l appear in the status
bar. After the battery is exhausted, the patient can be v enti lated in the
manual/spontaneous mode.
Breathing system compliance is determined during the checkout procedure. For
accurate compliance compensation, the breathing system must be in the
configurat ion in which it is to be used for the patient when the checkout procedure
is performed.
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The Fabius GS is equipped with fresh gas decoupling. During mechanical
inspirat ion, the fresh gas decoupling valve closes. This directs the fresh gas to the
reservoir bag, thereby stopping it from being added to the inspired t idal volume.
During exhalat ion, the decoupling valve
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opens, allowing exhaled gas and the accumulated fresh gas to f i l l the piston.
View Figure
Figure 12.24.Drager Fabius GS ventilator. Exhalationduring spontaneous breathing. PEEP, positive end-expiratory pressure; APL, adjustable pressure limiting.
Hazards
Ai r can be entr aine d du ri ng mecha nical ven ti lat io n if the re is a disconne cti on or
leak or the fresh gas f low is d irected to the wrong circuit (9,66,67 ). This could lead
to patient awareness and hypoxia. Such a problem should be discovered during the
checkout procedure but could occur later.
Venti lator fai lure result ing from worn parts of the motor that drives the bellows has
been reported (68,69). A ventilator failure warning was posted and manual
venti lat ion was possible after the venti lator was placed in the s tandby mode.
Cleaning and Sterilization
External parts of the ventilator may be cleaned with detergents and disinfectants.
The parts that are exposed to respiratory gases can be removed from the v enti lator.
The venti lator diaphragm, cover, and hoses can be steam autoclaved.
D r ag e r Apo l l o
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Description
The main screen of the Drager Apollo machine is shown i n Figure 12.28(see page
344). At the left on the bottom are virtual f lowmeters. Above this is gas-monitoring
information. To the right of this is the carbon dioxide waveform and pressure- and
flow-volume loops. Below the pressure-volume loop are bar graphs f or t idal volume
and airway pressure. To the right of the carbon dioxide waveform and f low-volume
loop are the values for venti latory parameters. Below this are the pipeline and
cylinder pressures. At the right are soft keys for various other functions.
Venti latory functions are controlled by using two sets of ke ys below the bottom of
the screen (Fig. 12.29, see page 345). The keys in the bottom row are used to
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set the ventilatory mode (manual/spontaneous, volume mode, pressure mode, or
pressure support). To s elect a mode, the key is pressed, and the knob to the right
is pressed to confirm the change. The right key is used to select the auxil iary
common gas outlet, which is an op tional feature.
View Figure
Figure 12.25.Drager Fabius GS ventilator. Inspiration
during manual ventilation. PEEP, positive end-expiratory
pressure; APL, adjustable pressure limiting.
Above th is row o f keys is an oth er row that is us ed to se t the ven til ati on para me ters .
To alter the sett ing, the key is pressed, and the knob is rotated to increase or
decrease the value shown above the soft key unti l the desired value is reached.
Then, the knob is pressed to confirm that sett ing.
To the right of the knob (not shown in Fig. 12.29) is the standby key, which is used
to switch between operating and s tandby modes.
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The internal construct ion of the Apollo venti lator is shown in Figures 12.30to 12.35
(see pages 345,346,347 ,348 ,349,350 ,35 1). Fresh gas enters the breathing system
and passes through the fresh gas decoupler valve. The venti lator, which has an
electrically driven piston, is connected to the inspiratory side of the circuit
downstream of the fresh gas decoupler valve. A f low sensor just downstream of the
unidirect ional valve monitors the inspiratory f low.
On the exhalat ion side of the circuit another f low sensor, a pressure gauge and a
PEEP valve are located upstream of the expiratory unidirect ional valve. The
reservoir bag and APL valve as well as an APL bypass valve leading to the exhaust
valve from the venti lator are between the expiratory unidirect ional valve and the
absorber.
Controls
The range for pressure l imitat ion (PM AX ) is 10 to 70 (default 40) cm H2O, with a
minimum of PEEP +10 cm H2O. The range for t idal volume is 20 to 1400 (default
600) mL. With PSV, the tidal volume range is 10 to 1400 mL. Respiratory frequency
can be set from 3 to 80 (default 12) bpm in volume control and pressure control
top related