Highlights of RSPT 2414 Mechanical Ventilation: Unit 1 By Elizabeth Kelley Buzbee AAS, RRT- NPS, RCP.

Highlights of RSPT 2414 Mechanical Ventilation: Unit 1

By Elizabeth Kelley Buzbee AAS, RRT-

NPS, RCP

Normal ventilation

• With normal compliance of 100 ml/cmH20 pressure

• and normal RAW of .5 to 2.5 cm H20/L/second the WOB is easy because the driving pressure is low.

Persons need help breathing when:

• The driving pressure might be excessive • or the patient may lack the ventilatory

muscles • The patient may lack the ventilatory drive

Define respiratory failure

• Inability to oxygenate the tissues (Pa02 less than 60 mmHg) and /or to remove C02 (PaC02 more than 50 mmHg)

• in persons with chronic hypercapnia, the person is ok until his C02 rises to the point that he has partially compensated respiratory acidosis—he has de-compensated

Classify Respiratory failure• Acute hypoxemic respiratory failure- if refractory hypoxemia

02 will not help– Decreased Pi02 such as with smoke inhalation or high altitude– Diffusion problems such as increased alveolar-capillary or decreased

surface area due to atelectasis• Acute hypercapnia respiratory failure- uncompensated

respiratory acidosis– Associated with decreased ventilatory drive or decreased alveolar

ventilation– Patient may be hypoxic only because the PA02 is decreased due to

increased PaC02—may or may not need increased Fi02 once ventilation starts

• Chronic hypercapnia respiratory failure- partially compensated respiratory acidosis– Exacerabation of existing problem such as COPD, neuromuscular

disorder or morbid obesity

Differentiate between the V/Q mismatch and a shunt or shunt-like effect:

• V/Q mismatch :– Acute respiratory failure that will respond to supplementary 02

TX– When there is low V/Q, we have low ventilation with good

perfusion– When there is high V/Q, we have good ventilation with poor

perfusion • Shunt: Acute respiratory failure with refractory hypoxemia

– Supplementary 02 will not help– Physiological shunt of 10% in WLN—more is pathological shunt

02 indices to determine if patient is in refractory hypoxemia

– Use the Pa02:Fi02 as Pa02:Fi02 to determine if we can correct hypoxemia

– Use the a/A ratio to determine if patient is above .14 to .17

– Use rule of 50: Fi02 more than 50% with Pa02 less than 50 mmHg

02 indices to determine if patient’s hypercapnia is the only reason he is

hypoxemic• If the P[A-a]D02 is not elevated (10 mmHg for

young and 25 mmHg for elderly), the hypoxemia may only be due to the rise in alveolar C02 replacing the alveolar 02. Once his alveolar ventilation is increased by our putting him on mechanical ventilation, his Pa02 is corrected

Conditions that result in increased WOB due to a need for driving pressures higher

than they can handle

• increased RAW

• decreased lung compliance• Persons at risk for muscle fatigue– Persons with severe muscle fatigue need to rest on

mechanical ventilation for 24 – 48 hours

• VC of less than 20 ml/kg IBW requires some ventilator support. • VC of less than 25 ml/kg IBW is associated with decreased ability to

cough effectively.• [PI max] inspiratory max pressure measures weakness of inspiratory

chest wall muscles and diaphragm. a need for mechanical ventilation is seen with a[PI max] of less -20 cmH20

• [PE max] expiratory max pressure measures weakness of the abdominal muscles. a need for mechanical ventilation is seen with a [PE max] less than + 40 cmH20

• be aware that facial weakness can result in false values for these two figures– if the patient cannot seal properly—needless to say, that alone tells

us we have problems with patient being able to protect his airway

Lung function studies that demonstrate situations that result in ineffective ventilator muscle action

Situations that result in increased VD ventilation will make a person need mechanical ventilation

• anatomical VD conducting airways. Comprises about 30% of the VT of the body.

• is equal to 1 ml / pound of IBW• Is always present, but can be reduced by tracheostomy

which bypasses upper airways• VD/VT ratio will change, as the patient’s VT varies but the VD

will stay the same• alveolar VD when an alveoli gets ventilation but no perfusion, it is

considered alveolar VD • as CO drops or there are problems with pulmonary blood

flow the alveolar VD will rise above baseline• physiological VD

• is the sum of the anatomical VD + the alveolar VD

Problems with VD/VT

• physiological VD is the sum of the anatomical VD + the alveolar VD

– the normal VD /VT is about .3 or 30%. It is not uncommon for mechanically ventilated persons to have VD /VT of .6 and higher.

– if physiological VD is excessive, we can increase the VT to get the alveolar ventilation back to an effective level

– Failure to get the VD /VT below .6 will prevent successful weaning of a patient from mechanical ventilation.

Clinical signs and symptoms of respiratory failure in the adult patient.

– inadequate alveolar ventilation: • hypercapnia above 55 torr & pH below 7.20

– inadequate lung expansion: • VT less than 5 ml/kg IBW, VC less than 10 ml/kg IBW requires full

ventilator support, and RR over 35 bpm – poor muscle strength:

• MIP less than -20 cmH20, VC less than 10 ml/kg and MVV less than 2x VE

– increased WOB: • VE more than 10 LPM & VD/VT more than .6

– hypoxemic respiratory failure:• P(A-a)D02 on 100% more than 350 mmHg • Pa02/Fi02 less than 200.

ABG associated with respiratory failure.

• Acute respiratory acidosis with moderate/severe hypoxemia

• Partially compensated respiratory acidosis with moderate / severe hypoxemia. Chronic patient is no longer compensating effectively.

• Panic values on ABG: • PaC02 above 55 torr & pH below 7.20• Serial ABG in which the PaC02 rises each time

Bedside measurements of increased WOB

• Calculation of the RAW

– If RAW increased, WOB increased

– High RAW – high driving pressure needed

• Calculation of the lung Compliance [CL]– If CL decreased, WOB increased

– Low CL high driving pressure needed

Mechanical ventilation:

• a machine that can perform bulk transfer of gas into the lung for a patient who cannot perform this task effectively enough to exchange gases.

• The ventilator works during inspiration, while exhalation is usually passive.

Phases of ventilation

• Inspiratory phase: inspiration in which gas enters the lung. – The TI is a function of the flow rate, the VT and the

patient’s RAW

• Expiratory phase: the portion of the breath that is concerned with the passive flow of gas out of the lung. – The TE will be a function of the TI, and to a great part

to the patient’s RAW

I:E ratio:

• comparisons of the TI to the TE.

• Normal I:E ratio during spontaneous breathing is 1:1.5, but to minimize some of the hazards of mechanical ventilation, with positive pressure ventilation, this ratio needs to be 1:2 or more.

• A patient with significant air-trapping may require much longer 1:E ratio such as 1:3 or 1:4.

Cycle time:

• cycle time = TI + TE

• cycle time = 60 seconds/BPM

Airway pressures• PIP- highest pressure during the inspiratory phase– at the

end of inspiration. – This is P1 of the RAW formula

• P plateau: during a breath hold, this is the second pressure during inspiration.– On a graphic, it looks like a flat plateau. – This pressure is the P2 of the RAW formula and the Δ P of the

static compliance formula • Baseline pressure: After the positive pressure breath is

given, the airway pressure returns to the baseline, which may be zero or a positive number if there is PEEP or CPAP.

Airway pressure

• PAW: the “mean airway pressure” is the average airway pressure.– It is a function of the inspiratory time (Ti), – the PIP, – the baseline pressure – and the I:E ratio.

PAW = [PIP ( I )] + [PEEP (E)][I + E]

Different types of ventilation

• Positive pressure ventilation• Negative pressure ventilation• Invasive mechanical ventilation• Non-invasive mechanical ventilation:

Ventilator modes: • Full-support mechanical ventilation: Most fatigued patients need

to be rested for 24-48 hours, but a serious complication of full-support is that after a few days, the patient’s respiratory muscles start to atrophy quickly.

• • Partial-support mechanical ventilation: SIMV or IMV are examples

of partial-support mechanical ventilation. Frequently patients are started on full support and are moved to partial support after the mandatory rest period.

• Spontaneous modes: When a patient is past the point of needing

full or even partial support, we can challenge the patient with the machine acting only as a monitoring device with/without alarms and mechanical intervention in case of apnea or hypoventilation. – Patients on spontaneous modes of ventilation must have an intact

ventilatory drive, and must be able to maintain their PaC02 with little or no help from the machine.

flow• Wave forms/graphics: electronic devices convert

airway pressures, volumes or flows into a graphic• Peak flows /flow rates: All modern positive

pressure ventilators have peak flow rates. If you select the flow/time wave form you can see the flow pattern: – Constant flow– Descending ramp– Sine wave

VT tidal volume

• To adjust the VE for the PaC02, we can alter the RR or the VT.

• Set VT: the VT the RCP selects that may or may not be the same as the delivered VT

• Return VT: the delivered VT that is measured at the exhalation point

• Corrected VT: the VT that is corrected for volume that is lost in the tubing as it swells during positive pressure