18

Anesthesia Breathing Circuits

Vivekanand Kulkarni, MD, PhD • Romy Yun, MD

The anesthesia breathing circuit creates a local environment surrounding the patient within which respiratory gas exchange occurs. The circuit is vital for ventilation of the patient undergoing anesthesia and provides a conduit through which anesthetic gases can be administered.

Incorporation of unidirectional valves and a CO2 absorber into an anesthesia circuit allows rebreathing of some exhaled gases and constitutes rebreathing systems.

1. Overview 1. Most modern circuits incorporate 22 mm corrugated breathing tubes with one

end (elbow) linked to the patient’s airway via a mask/laryngeal mask airway (LMA)/endotracheal tube (ETT) and a fresh gas inlet (FGI) at the other.

2. A spring-loaded pop-off valve/adjustable pressure-limiting (APL) valve and a reservoir breathing bag (RBB) are also included.

3. With most circuits, there is a general increase in dead space so that Vd/Vt increases from 0.33 to 0.46 if intubated and to 0.66 with a mask.

4. The resistance to breathing depends on the presence of valves and the diameter of the tubing.

5. The positions of the elements of these circuits determine their functional characteristics.

6. When there is no CO2 absorber present, they constitute nonrebreathing systems, which were classified by Mapleson into A, B, C, D, E, and F.

7. When unidirectional valves and a CO2 absorber are included, the resistance to breathing is greater, and these circuits constitute rebreathing Systems.

2. Nonrebreathing systems—Mapleson circuits (Fig. 18-1)

3.

Figure 18-1 Mapleson Circuits s minute ventilation. MV…

Circuits classified as nonrebreathing systems are not intended to allow rebreathing of exhaled gases, but rebreathing can still occur.

1. Nonrebreathing systems work by using fresh gas flow (FGF) to push the patient’s exhaled gas down the expiratory limb of the circuit.

1. The exhaled gases collect in the RBB, and some escapes through an expiratory (APL) valve.

1. The remaining exhaled gas is diluted by FGF2. The patient draws his next breath through the expiratory limb and RBB.

1. Some rebreathing of the previous breath may occur depending upon the degree of dilution (from FGF rate, patient’s tidal volume (TV), and length of expiratory pause).

2. Mapleson in 1954 classified the circuits into five groups depending on the position of the FGI, RBB, and APL in the circuit.

3. The F system, as described by Willis, was later added to the original group of five.

4. Gas is expelled through the APL. 1. In the A, B, and C classes, expired gas leaves the circuit close to the

patient.2. The D, E, and F are T-piece systems, and expired gas leaves the circuit

away from the patient.5. Since there is no CO2 absorber, there is always potential for rebreathing some

amount of exhaled gases using Mapleson circuits. 1. The amount of rebreathing depends on the following:

1. The circuit elements and their position.2. Minute ventilation (MV)3. The pattern of breathing (TV, respiratory rate, peak inspiratory

flow rate, Expiratory pause duration, and I:E ratio)4. The FGF

1. The FGF needed to prevent significant rebreathing depends also on the mode of ventilation.

1. Spontaneous2. Controlled

2. The efficiency with respect to FGF needed to prevent rebreathing is 1. During Spontaneous Ventilation A > D,F,E > C,B2. During Controlled Ventilation D,F,E > B,C > (Table 18-1)

Table 18-1 The Advantages and…

6. Mapleson circuits 1. Mapleson A—Magill circuit

1. It has a single breathing tube with the FGI and RBB close to the machine, while the APL is close to the patient connector or elbow.

2. A drawback of Mapleson A is that waste gas is released close to the patient, which makes scavenging difficult.

3. Spontaneous ventilation (Fig. 18-2) 1. During expiration, the corrugated breathing tube is filled

first by dead space gas, which is CO2 free, followed by Alveolar gas from the patient end of the circuit and simultaneously by FGF from the FGI.

2. The flow from the FGI continues during the expiratory pause.

3. If the FGF is high enough, pressure in the circuit rises, the APL opens and expels CO2 containing expired Alveolar gas.

4. During the next inspiration, the FGF and some dead space gas will be inhaled.

5. When the FGF is above 70% of MV during Spontaneous Ventilation, rebreathing is prevented as long as breathing is regular.

Figure 18-2 Mapleson A (Magill Circuit…

4. Controlled ventilation 1. The inspiratory force is provided by the anesthesiologist

who squeezes the RBB after partially closing the APL.2. During inspiration, therefore, some fresh gas is vented via

the APL.3. During expiration, expired gas fills the RBB. This is mixed

with the FGF during the expiratory pause, so that rebreathing CO2 containing gas is inevitable.

4. Now the system is inefficient because the FGF needs to be more than three times MV to prevent CO2 buildup from rebreathing.

5. The Miller modification of the Magill system keeps the APL closed during inspiration thus decreasing FGF requirements.

2. Lack system (Coaxial Mapleson A) 1. The Lack coaxial system has two tubes.

1. The outer tube is similar to that of the Mapleson A with FGI and RBB close to the machine end.

2. The expired gases from the patient are, however, conducted by the inner tube to the machine end so that the APL is closer to the machine end.

2. Functionally similar to the Mapleson A, the arrangement allows for easier scavenging and manipulation of the APL.

3. Mapleson B and C systems 1. In these systems, the APL and FGI are close to the patient

connector.2. The RBB and corrugated tubing when present (Mapleson B) form a

blind end where mixed expired and fresh gases collect.3. A FGF >2×MV is needed to prevent rebreathing in both

spontaneous and controlled modes.4. The C system is also called Waters to & fro system without

absorber.5. These systems are rarely used today.

4. Mapleson D system 1. This system is basically a T-tube system with the FGI close to the

patient forming one limb of the T-tube.2. Expired gases flow through the other limb of the T, which is a long

corrugated tube with a reservoir bag and a pop-off valve.3. Spontaneous Ventilation

1. In expiration, the dead space gas, alveolar gas and FGF mix in the corrugated tubing and the RBB.

2. When the bag fills, the pressure in the system rises and the APL opens to vent the mixture.

3. In the expiratory pause, the FGF continues to push the expired Alveolar gas toward the RBB and APL.

4. During spontaneous breathing, 2×MV of FGF is needed to prevent rebreathing.

5. It is less efficient than Mapleson A but more efficient than Mapleson B and C.

4. Controlled ventilation 1. On expiration, the corrugated tubing and RBB fill up with a

mixture of FGF, Dead Space and Alveolar gas.2. In the expiratory pause, the FGF pushes the expired gas

toward the RBB and APL.3. Compressing the bag manually allows this FGF in the

corrugated tubing to enter the lungs, whereas the higher pressure opens the APL to vent the FGF expired gas mixture from the bag.

4. During controlled ventilation, 1 to 2×MV of FGF is needed to prevent rebreathing.

5. The circuit is often used for manual ventilation during transport of patients (Fig. 18-3).

Figure 18-3 Mapleson D Used in…

5. Bain System (Co-axial Mapleson D) 1. Fresh gas enters the inner small-bore tube and is delivered to the

patient connection.2. Expired gases go into the outer corrugated tubing to the RBB

and APL close to the machine end (Fig. 18-4). 1. Spontaneous ventilation

1. During expiration, dead space and alveolar gas enter the outer corrugated tubing and RBB.

2. During the expiratory pause, FGF from the inner tube fills the outer tube, pushing the expired gas to the RBB and APL.

3. Keeping the FGF adequate (2.5 to 3 × MV) prevents rebreathing.

2. Controlled ventilation 1. During controlled ventilation, 70 mL/kg/min or 1

to 2 × MV prevents rebreathing.2. For controlled ventilation, the reservoir bag can be

removed for ventilator attachment.

Figure 18-4 Bain Circuit Adapted from Dorsch…

3. Pethick test 1. If the inner tube that delivers FGF gets disconnected or

kinked at the proximal end, the entire corrugated tubing becomes additional dead space, which is difficult to compensate for, leading to respiratory acidosis.

1. Prevention by checking for the patency of the inner tube connection uses the Pethick Test.

1. Occlude the patient end and close the APL valve.

2. Fill the circuit with the Oxygen Flush button, distending the RBB.

3. Release the patient end.4. The resulting Venturi effect flattens the RBB

confirming inner tube connection patency.4. CPAP system

1. Occasionally, during one-lung-ventilation, a modified Mapleson D system can be attached to the nonventilated lung and a secondary O2 source.

2. This provides CPAP to the nonventilated lung and decreases the shunt fraction, thus improving oxygenation.

6. Mapleson E and F systems 1. These are valveless T-piece systems, which have low resistance.2. Ayre’s original T-piece system with additional expiratory limb of

open corrugated tube (which acts as a reservoir) forms the E-system.

3. Loss of moisture because of the high FGF can be significant.4. Partial occlusion of the expiratory limb opening (Mapleson E) or

the RBB (Mapleson F) during manual ventilation can lead to high pressures.

1. A pressure relief mechanism is necessary to prevent barotrauma.

5. Spontaneous ventilation 1. The Mapleson E is similar to the D system.2. During inspiration, no rebreathing occurs if there is no

expiratory limb. 1. With an expiratory limb, however, rebreathing is

avoided if the FGF is adequate. 1. If the volume of the expiratory limb is less

than the TV of the patient, air dilution can occur.

2. A FGF of 2 to 3× MV prevents rebreathing (Fig. 18-5). Figure 18-5 Mapleson F: The Jackson…

6. The Mapleson F 1. The Jackson-Rees’ modification has a RBB with an open

end attached to the expiratory limb of the Mapleson E. 2. The addition of a bag acts as a reservoir to prevent dilution

with air and allows manual ventilation when necessary.3. FGF requirements are similar to that of the Mapleson E.

4. Nonrebreathing systems 1. Circle system Table 18-2 The Advantages and Disadvantages of…

The circle system is the most popular system in the United States. It allows low FGF, conserves heat and humidity, and allows efficient scavenging.

1. This is currently the most popular system in the United States.2. The system was first described by Brian Sword in 1926.

1. It allows low FGF.2. It conserves heat and humidity.3. It decreases pollution by allowing efficient scavenging.

3. The earlier problem of predicting inspired gas composition is no longer an issue because gas concentrations are sampled for measurement at the patient yoke (Y-connector).

4. There is no further increase in patient dead space compared to other systems since gas sampling begins at the patient yoke.

5. The resistance in the circle is <3 cm of water.6. Rebreathing of CO2 is prevented by absorption of CO2 from the exhaled

gas using the carbon dioxide absorber. 1. The position of components of the circle is also important in this

respect.7. Classical circuit configuration

Figure 18-6 Classic Circle System Reused with…

1. Vaporizer is outside the circle.2. Unidirectional valves lie between the patient yoke and the RBB in

both inspiratory and expiratory limbs.3. Two additional requirements are that the FGI, APL, and RBB

should all be in between the two valves and not on the patient side of the circle.

4. Spontaneous ventilation 1. Inspiration

1. The unidirectional expiratory valve closes.2. Gas flows to the patient from the RBB (via the

absorber) and the FGI through the inspiratory limb and the open unidirectional inspiratory valve.

2. Expiration 1. The unidirectional inspiratory valve closes. 2. Expired gas flows through the open unidirectional

expiratory valve in the expiratory limb to the RBB and to the CO2 absorber.

3. The APL is positioned just before the CO2 absorber in the expiratory limb.

4. When flows are high CO2 rich expired gas is the first to spill out of the circle thus conserving the CO2 absorber.

5. Manual ventilation 1. Inspiration

1. Excess gases are vented through the partially open APL valve during inspiration.

2. The gas flowing through the absorber to the patient will be a mixture of FGF and expired gases.

3. The composition of the mixture depends on the FGF.

2. Expiration 1. Exhaled gases flow into the RBB.2. There may be some retrograde flow of fresh gas

through the absorber depending on FGF rate.3. Mechanical ventilation

1. Inspiration 1. During inspiration, gas flows from the

ventilator through the absorber and unidirectional inspiratory valve to the patient.

2. The gas in the bellows of the ventilator would be a mix of FGF and exhaled gas that has been scrubbed of CO2.

2. Expiration 1. During expiration, exhaled gases flow to

into the bellows where they will mix with FGF flowing retrograde through the absorber.

2. Excess gases spill through the APL in the latter part of expiration.

8. Circle system function 1. During the initial 5 to 10 minutes of use, large amounts of

inhalation agent and nitrous oxide are taken up by the patient.2. The circuit also contains nitrogen from the lungs of the patient.

1. Dilutions of the anesthetic agents are decreased by using high flows during this period.

2. When high flows are used, the concentration of inspired agent is very close to vaporizer settings.

3. The use of high flows for prolonged periods is uneconomical, increases environmental pollution, and does not conserve heat and humidity.

9. Use of low flows in the circle system 1. When the Circle system has stabilized, the FGF can be decreased

to 1 to 1.5 liters.

The use of high gas flows for prolonged periods, is uneconomical, and does not conserve airway heat and humidity.

2. The inspired gases are then made up of the FGF and the expired gas cleared of CO2 by the absorber, but also with a lower O2 concentration.

1. Because of dilution, higher FGF O2 concentrations are needed to keep inspired O2 concentrations above 30%.

2. Changes of anesthetic concentration when desired will need higher flows to achieve a more rapid change.

1. Advantages of low flows include economy, decreased environmental pollution, and conservation of heat and humidity.

2. Disadvantages include greater use of the absorber and possible accumulation of undesired breakdown products from the absorber.

10. Closed circle system 1. In this system, the APL is completely closed, and the minimum

FGF flow should be equivalent to the oxygen consumption (250mls/min in the average 70kg patient).

11. CO2 absorber 1. Soda Lyme and BaraLyme are the most commonly used

absorbers. 1. Small amounts of Silica or Kieselguhr are added for

hardening to reduce dust formation.2. The absorptive efficiency is inversely related to hardness.3. Absorptive surface area depends on granule size.

1. Smaller granules increase the absorptive surface but also increase resistance to flow of gas, while larger granules decrease the absorptive surface and decrease resistance leading to channeling of gas flow.

2. The usual mesh size is therefore 4–8 mesh (1/4” to 1/8”).

4. Up to 26L of CO2 can be absorbed by 100g of absorbent.5. Composition of Soda Lyme: NaOH 4%; KOH 1%; H2O

15%; 80% Ca(OH)2.6. Composition of Bara Lyme: Ba(OH)2 20%; Ca(OH)2 80%;

plus KOH indicator7. The moisture or water content of the absorbent forms

carbonic acid with CO2, which reacts with alkali forming carbonate and generating heat.

8. H2O + CO2 ⇔ H2CO3 ⇔ H+ + HCO3−

NaOH + H2CO3 ⇔ NaHCO3 + H2O

2NaHCO3 + Ca(OH)2 ⇔ 2 NaOH + CaCO3 + H2O

2. Amsorb is a new CO2 absorbant. It contains Ca(OH)2 and CaCl2. The hardening agents are CaSO4 and Polyvinylpyrrolidine.

1. Since it does not contain NaOH or KOH, there is no CO or Compound-A production.

2. It is more expensive, and the CO2 absorption capacity is less.

3. Indicators 1. These are pH sensitive and added to the granules to show

when they are exhausted. 1. Phenlophthalein: White when fresh and pink when

exhausted2. Ethyl Violet: White when fresh and purple when

exhausted3. Clayton yellow: Red when fresh and yellow when

exhausted4. Interactions with anesthetic agents

1. Carbon monoxide 1. When the absorbents become dry and desiccated,

they can react with anesthetic agents containing the CHF2-O functional group resulting in carbon monoxide production.

1. CO production can be significant with Sodalyme and Baralyme leading to carboxyhemoglobin concentrations of up to 30%.

2. CO production increased by dehydrated absorbent, higher temperatures, lower FGF, and higher anesthetic agent concentration.

3. CO production is also dependent on anesthetic agent

1. Desflurane > Enflurane > Isoflurane > halothane = sevoflurane

4. CO production is dependent on absorbent 1. BaraLyme > SodaLyme

2. The production of CO can be decreased by decreasing the drying of absorbent by shutting off gas flow after cases, changing the absorbent if gas flow had not been shut off, rehydrating the absorbent, and using absorbents such as Amsorb that do not contain NaOH and KOH.

2. Other Breakdown compounds 1. Compound-A

1. Sevoflurane reacts with dry absorbent to produce an Olefin compound called Compound A (Fluoromethyl-2–2-difluoro-1-trifluoromethyl vinyl ether).

1. The production is worse in dehydrated Baralyme with higher concentrations of Sevoflurane, low FGF, and higher absorbent temperatures.

2. The concentration is low and is not known to cause toxic effects in humans.

2. Other compounds produced in small quantities include Acetone, methane, hydrogen, and ethanol.

3. Fire 1. There is a small risk of fire within the absorber and

breathing circuit with high temperatures and dry absorbents when Sevoflurane is used.

Chapter Summary for Anesthesia Breathing Circuits

References

1. Mapleson WW. The elimination of rebreathing in various semiclosed anaesthetic systems. Br J Anaesth. 1954;26:323–332.

2. Dorsch JA, Dorsch SE. Understanding anesthesia equipment. 5th ed. Philadelphia, PA: Williams and Wilkins, 2008.