Circle system low flow anesthesia

CIRCLE SYSTEM (LOW FLOW)

By Dr. Geeta

choudhary

CIRCLE SYSTEM CAN BE:

closed (fresh gas inflow exactly equal to patient uptake, complete rebreathing after carbon dioxide absorbed, and pop-off closed)

semi-closed (some rebreathing occurs, FGF and pop-off settings at intermediate values), or

semi-open (no rebreathing, high fresh gas flow)

CIRCLE SYSTEM CONT.

Circle systems Most commonly used Adult and child appropriate sizes Can be semiopen, semiclosed, or closed

dependent solely on fresh gas flow (FGF) Uses chemical neutralization of CO2 Conservation of moisture and body heat Low FGF’s saves money

CONT.

Circle system Allows for mechanical ventilation of the

lungs using the attached ventilator Allows for adjustment of ventilatory

pressure Allows for semiopen, semiclosed, and

closed systems based solely on FGF Is easily scavenged to avoid pollution of

OR environment

CONT.

COMPONENTS

Absorber : An absorber assembly consists of an absorber two ports for connection to breathing tubes a fresh gas inlet. Other components that may be mounted inspiratory and expiratory unidirectional valves, an adjustable pressure limiting (APL) valve, and

a bag mount.

Absorber with two canisters in series, a dust/moisture trap at the bottom and a drain at the side. The lever at the right is used to tighten and loosen the canisters. Note that the date the absorbent was last changed is marked on the lower canister.

CANISTERS

The absorbent is held in canisters. The side walls are transparent so that

the absorbent color can be monitored. A canister with tinted side walls may

make it difficult to detect color changes in the absorbent.

A screen at the bottom of each canister holds the absorbent in place.

Absorber with a single canister. It is loosened and tightened by twisting.

Absorber with a single canister it is loosen and tightened by twisting.

Absorber with single disposable canister A:with the canister in place. B :with canister removed. The two valve at the top prevent loss of gas when the canister is removed

INDICATOR

Color When Fresh WhenExhausted Phenolphthalein White Pink

Ethyl violet White Purple Clayton yellow Red Yellow Ethyl orange Orange Yellow

Mimosa Z Red White

REACTIONS BETWEEN ABSORBENTS AND ANESTHETIC AGENTS

Haloalkene Formation Halothane degradation most often

occurs during closed-circuit anesthesia and produces the haloalkene 2-bromo-2-chloro-1, 1-difluoroethene (BCDFE).

Compound A Formation Sevoflurane decomposes in the

presence of some carbon dioxide absorbents to compound A

SEVERAL FACTORS INFLUENCE THE AMOUNT OF COMPOUND A IN THE BREATHING SYSTEM.

Fresh Gas Flow: more with low fresh gas flow Absorbent Composition: greatest with

absorbents containing potassium or sodium hydroxide.

Absorbent Temperature : low temp cause decrease comp A formation

Concentration of Sevoflurane Anesthetic Length Water Content :

CARBON MONOXIDE FORMATION

Carbon monoxide is produced when desflurane, enflurane, or isoflurane is passed through dry absorbent containing a strong alkali (potassium or sodium hydroxide)

When sevoflurane is degraded by absorbent, carbon monoxide is formed if the temperature exceeds 80°C

FACTORS ASSOCIATED WITH CARBON MONOXIDE FORMATION

Absorbent Composition Absorbent Desiccation Anesthetic Agent - The highest carbon monoxide levels

have been seen with desflurane followed by enflurane then isoflurane Temperature Inside the Absorber Fresh Gas Flow Carbon Dioxide Absorption

THE APSF HAS PROVIDED A NUMBER OF RECOMMENDATIONS

that an anesthesia department should take to prevent absorbent desiccation if the department continues to use strong alkali absorbents with volatile anesthetic agents. These, including

All gas flows should be turned OFF after each case

Vaporizers should be turned OFF when not in use

CONT.

The absorbent should be changed routinely, at least once a week, preferably on a Monday morning, and whenever fresh gas has been flowing for an extensive or indeterminate period of time.

The canister should be labeled with the filling date. Checking this date should be part of the daily

machine checklist. If a double-chamber absorber is used, the

absorbent in both canisters should be changed at the same time.

CONT.

Canisters on an anesthesia machine that is commonly not used for a long period of time should not be filled with absorbent that contains strong alkali or should be filled with fresh absorbent before each use.

The integrity of the absorbent packaging should be verified prior to use.

The practice of supplying oxygen for administration to a patient who is not receiving general anesthesia through the circle system should be strongly discouraged

CONT.

Using fresh gas to dry breathing system components should be discouraged

The temperature in the canister should be monitored and the absorbent changed if excessive heat is detected.

Consideration should be given to removing absorbent from canisters in induction rooms and to using high fresh gas flows to eliminate rebreathing.

WHEN AND HOW TO CHANGE THE ABSORBENT

Inspired Carbon Dioxide-Most reliable method

Indicator Color Change Heat in the Canister If excessive dust is present

UNIDIRECTIONAL VALVES

Two unidirectional (flutter, one-way, check, directional, dome, flap, nonreturn, inspiratory, and expiratory)

valves are used in each circle system to ensure that gases flow toward the patient in one breathing tube and away in the other.

They are usually part of the absorber assembly

Unidirectional valve. Left: Reversing the gas flow causes the disc to contact its seat, stopping further retrograde flow. Right: Gas flowing into the valve raises the disc from its seat and then passes through the valve. The guide (cage) prevents lateral or vertical displacement of the disc. The transparent dome allows observation of disc movement.

Horizontal unidirectional valves. Note the cages that prevent the discs from being displaced.

INSPIRATORY AND EXPIRATORY PORTS

The inspiratory port- has a 22-mm male connector downstream of the inspiratory unidirectional valve through which gases pass toward the patient during inspiration.

The expiratory port- has a 22-mm male connector upstream of the unidirectional valve through which gases pass during exhalation.

These ports are usually mounted on the absorber

Y-PIECE

is a three-way tubular connector with two 22-mm male ports for connection to the

breathing tubes a 15-mm female patient connector for a tracheal

tube or supraglottic airway device. The patient connection port usually has a coaxial

22-mm male fitting to allow direct connection between the Y-piece and a face mask.

A septum may be placed in the Y-piece to decrease the dead space.

The breathing tubes attach to the inspiratory and expiratory ports

FRESH GAS INLET

The fresh gas inlet may be connected to the common gas outlet on the anesthesia machine by flexible tubing.

the fresh gas inlet port, or nipple, has an inside diameter of at least 4.0 mm and that the fresh gas delivery tube has an inside diameter of at least 6.4 mm

ADJUSTABLE PRESSURE-LIMITING VALVE

During spontaneous breathing, the valve is left fully open and gas flows through the valve during exhalation.

When manually assisted or controlled ventilation is used, the APL valve should be closed enough that the desired inspiratory pressure can be achieved.

When this pressure is reached, the valve opens and excess gas is vented to the scavenging system during inspiration.

PRESSURE GAUGE

Many circle systems have an analog pressure gauge (manometer) attached to the exhalation pathway.

The gauge is usually the diaphragm type. Changes in pressure in the breathing system

are transmitted to the space between two diaphragms, causing them to move inward or outward. Movements of one diaphragm are transmitted to the pointer, which moves over a calibrated scale.

Diaphragm-activated pressure gauge. Two thin metal diaphragms are sealed together, with a space between them. This space is connected to the breathing system. Variations in pressure in the breathing system are transmitted to the diaphragms, which bulge outward or inward. A series of levers is activated, moving the pointer, which records the pressure.

RESERVOIR BAG

The bag is usually attached to a 22-mm male bag port (bag mount or extension).

It may also be placed at the end of a length of corrugated tubing or

a metal tube leading from the bag mount providing some freedom of movement for the anesthesia provider.

VENTILATOR

Ventilator is also part of circle system.

BAG/VENTILATOR SELECTOR SWITCH

A bag/ventilator selector switch provides a convenient method to shift

rapidly between manual or spontaneous respiration and automatic ventilation without removing the bag or the ventilator hose from its mount.

the selector switch is essentially a three-way stopcock.

One port connects to the breathing system.

The second is attached to the bag mount.

The third attaches to the ventilator hose.

The handle or knob that is used to select the position indicates the position in which the switch is set.

Bag/ventilator selector switch. In the Bag position, the reservoir bag and APL valve are connected to the breathing system. In the Ventilator position, the APL valve and bag are excluded from the breathing system.

Respiratory Gas Monitor Sensor or Connector

Both mainstream and sidestream devices can be used with the circle system.

Airway Pressure Monitor Sensor The sensor can be inserted into the

circle system by using an adaptor, or it may be incorporated into the absorber assembly.

ADVANTAGES OF CIRCLE SYSTEM

Cost reduction (use less agent and O2) Increased tracheal warmth and humidity Decreased exposure of OR personnel to

waste gases Decreased pollution of the environment

REMEMBER that the degree of rebreathing in an anesthesia circuit is increased as the fresh gas flow (FGF) supplied to the circuit is decreased

DISADVANTAGES OF CIRCLE SYSTEM

Greater size, less portability Increased complexity

Higher risk of disconnection or malfunction Increased resistance (of valves during

spontaneous ventilation) Dissuading use in Pediatrics (unless a circle

pedi system used) Difficult prediction of inspired gas

concentration during low fresh gas flow

LOW FLOW ANESTHESIA

Any technique that utilises a fresh gas flow (FGF) that is less than the alveolar ventilation can be classified as „Low flow anaesthesia‟.

a technique wherein at least 50% of the expired gases had been returned to the lungs after carbon dioxide absorption.

CONT.

Baker had classified the FGF used in anaesthetic practice into the following categories:

Metabolic flow : about 250 ml /min Minimal flow : 250-500 ml/min. Low flow : 500- 1000 ml/min. Medium flow : 1 - 2 l/min. For most practical considerations, utilisation

of a fresh gas flow less than 2 litres/min may be considered as low flow anaesthesia.

CONT.

there exists a need for a system that provided the advantages of the completely closed circuit and at the same time, reduced the dangers associated with it.

Low flow anaesthesia fulfilled these requirements.

CONT.

Low flow anaesthesia involves utilising a fresh gas flow which is higher than the metabolic

flows but which is considerably lesser than the conventional flows.

The larger than metabolic flows provides for considerably greater margin of safety and satisfactory maintenance of gas composition in the inspired mixture.

THE NEED FOR LOW FLOW ANAESTHESIA

Completely closed circuit anaesthesia is based upon the reasoning that anaesthesia can be safely maintained if the gases which are taken up by the body alone are replaced into the circuit taking care to remove the expired carbon dioxide with sodalime. No gas escapes out of the circuit and would provide for maximal efficiency for the utilisation of the fresh gas flows.

EQUIPMENT

The minimum requirement for conduct of low flow anaesthesia is effective absorption of

CO2 from the expired gas, so that the CO2 free gas can be reutilised for alveolar ventilation.

The circle system should have the basic configuration with

two unidirectional valves on either side of the sodalime canister, fresh gas entry, reservoir bag,

pop off valve, and corrugated tubes and „Y‟ piece to connect to the patient.

CIRCLE SYSTEM

CAN: Canister of CO2 absorber

RB: Reservoir bag

SPECIFIC SAFETY FEATURES OF ANESTHETIC TECHNIQUES WITH REDUCED FGF

1. Improved equipment maintenance2. The long time constant: 3. Improved knowledge of the theory and

practice of inhalational anesthesia.

HOW TO ADJUST FGF AT DIFFERENT PHASES OF LFA

Premedication, pre-oxygenation and induction of sleep are performed according to the usual practice. Concerning adjustment of FGF anesthesia can be divided into 3 phases:1. Initial HIGH flow2. Low flow3. Recovery

1.INITIAL HIGH FLOW PHASE At the beginning of anesthesia high FGF of 5-6 LPM is necessary to wash out nitrogen (N2) from the patients body tissues. High initial flow facilitates the filling of the breathing system with the desired gas composition which in turn influences patient uptake and distribution of the anesthetic agents.

Reproduced with permission from “Low Flow Anesthesia with Draeger machines” by Prof.J.A.Baum

Reproduced with permission from “Low Flow Anesthesia with Draeger machines” by Prof.J.A.Baum

2. LOW FLOW PHASE

After the high flow phase of 5-15 min, or when the target gas concentrations has been achieved FGF can be reduced at the desired low flow level. The lower the FGF the greater the difference between the vaporizer setting and inspired concentration of the anesthetic agent in the breathing circuit will be.With low FGF, time to reach the desired concentration in the inspiratory gas will be prolonged.Hence, monitoring of oxygen and anesthetic agent concentration is essential and necessary in LFA.

IF THE FLOW PROVIDED IS TOO SMALL FOR THE PATIENT’S NEEDS THE BELLOW WILL GRADUALLY GO DOWN, DOWN DOWN...

Reproduced with permission from “Low Flow Anesthesia with Draeger machines” by Prof.J.A.Baum

THE PRACTICE OF LOW FLOW ANAESTHESIA

under the following three categories: 1. Initiation of Low flow anaesthesia 2. Maintenance of Low flow anaesthesia 3. Termination of Low flow anaesthesia.

INITIATION OF LOW FLOW ANAESTHESIA.

Primary aim at the start of low flow anaesthesia is to achieve an alveolar concentration of the anaesthetic agent that is adequate for producing surgical anaesthesia (approximately 1.3MAC).

The factors that can influence the build up of alveolar concentration should all be

considered while trying to reach the desired alveolar concentration.

CONT.

These factors can broadly be classified into three groups

1) Factors governing the inhaled tension of the anaesthetic,

2)Factors responsible for rise in alveolar tension,

3) Factors responsible for uptake from the lungs

thus reducing the alveolar tension.

FACTORS AFFECTING THE BUILD UP OF ALVEOLAR TENSION

Factors governing the inhaled tension of the anaesthetic,

1 BREATHING CIRCUIT VOLUME 2. RUBBER GAS SOLUBILITY 3. SET INSPIRED CONCENTRATION

CONT.

FACTORS AFFECTING THE RISE IN ALVEOLAR TENSION

1. CONCENTRATION EFFECT 2. ALVEOLAR VENTILATION UPTAKE BY THE BLOOD 1. CARDIAC OUTPUT 2. BLOOD GAS SOLUBILITY 3. ALV – VENOUS GRADIENT

METHODS TO ACHIEVE DESIRED GAS AND AGENT CONCENTRATION

Use of high flows for a short time Prefilled circuit Use of large doses of anaesthetic

agents. Injection techniques :The exact dose to

be used is calculated thus: Priming dose (ml vapour) = Desired

concentration x {( FRC + Circuit volume) +( Cardiac output x BG Coeff.)}

THE MAINTENANCE OF LOW FLOW ANAESTHESIA

This phase is characterised by 1. Need for a steady state anaesthesia

often meaning a steady alveolar concentration of

respiratory gases. 2. Minimal uptake of the anaesthetic

agents by the body. 3. Need to prevent hypoxic gas

mixtures.

MANAGEMENT OF THE OXYGEN AND NITROUS OXIDE FLOW DURING THE MAINTENANCE PHASE

A high flow of 10 lit/min at the start, for a period of 3 minutes,

is followed by a flow of 400 ml of O2 and 600 ml of N2O for the initial 20 minutes and

a flow of 500 ml of O2 and 500 ml of N2O thereafter.

This has been shown to maintain the oxygen concentration between 33 and 40 % at all times.

The Gothenburg Technique : Initially high flows, oxygen at 1.5 l/min and nitrous

oxide at 3.5 l/min had to be used for a period of six minutes after the induction of anaesthesia and this constitutes the loading phase.

This is followed by the maintenance phase in which the oxygen flow is reduced to about 4ml/kg

and nitrous oxide flow adjusted to maintain a constant oxygen concentration in the circuit.

The usual desired oxygen concentration is about 40%.

MANAGEMENT OF THE POTENT ANAESTHETIC AGENTS DURING MAINTENANCE PHASE

Weir and Kennedy recommend infusion of halothane (in liquid ml/hr) at the following rates

for a 50 kg adult at different time intervals. 0-5 min 27 ml/hr 5-30 min 5.71 ml/hr 30-60 min 3.33ml/hr 60-120 min 2.36 ml/hr These infusion rates had been derived from the

Lowe's theory of the uptake of anaesthetic agent.

They had approximated isoflurane infusion (in liquid ml/hr) based on the Lowe's formula as follows:

0 - 5 min. 14 + 0.4X wt. ml/hr. 5 - 30 min. 0.2 X initial rate. 30-60 min. 0.12Xinitial rate. 60-120min. 0.08X initial rate. For halothane infusion, they had suggested that the

above said rates be multiplied by 0.8 and for enflurane, multiplied by 1.6. These rates had

been suggested to produce 1.3 MAC without the use of nitrous oxide. The infusion rates had to be halved if nitrous oxide is used.

TERMINATION OF LOW FLOW ANAESTHESIA

There are only two recognised methods of termination of the closed circuit. They are as follows:

1 )Towards the end of the anaesthesia, the circuit is opened and a high flow of gas is used to flush out the anaesthetic agents which accelerates the washout of the anaesthetic agents.

This has the obvious advantage of simplicity but would result in wastage of gases.

2 )The second method is the use of activated charcoal Activated charcoal when heated to 220oC adsorbs the potent vapours almost completely.

ADVANTAGES OF THE LFA1. QUALITY OF PATIENT CARE2. ECONOMIC BENEFITS3. ENVIRONMENTAL BENEFITS4. REDUCE OPERATING ROOM POLUTION5. ESTIMATION OF ANESTHETIC AGENT

UPTAKE AND OXYGEN CONSUMPTION6. BUFFERED CHANGES IN INSPIRED

CONC.7. HEAT AND HUMIDITY CONSERVATION8. LESS DANGER OF BAROTRAUMA

DISADVANTAGES OF LFA:

More Attention Required Inability to Quickly Alter Inspired ConcentrationsDanger of HypercarbiaAccumulation of Undesirable Gases in the SystemUncertainty about Inspired ConcentrationsFaster Absorbent Exhaustion

THE END