Anaesthesia Breathing Circuits John W.R

Anaesthesia breathing circuits John W.R. Mclntyre FFARCS(Eng) FRCPC

During clinical anaesthesia gases pass through a No further reference is made to open circuits. complex conduit interposed between the anaesthe- They are of historical interest, unless an anaesthetist sia machine and the patient. This conduit brings is faced with the need to construct his own gases and vapours to the patient and is an egress for equipment under primitive circumstances. Insuf- expired substances. New circuits continue to be flation techniques may be employed under special developed 1-4 because many factors, including circumstances that do not fall within the scope of users' priorities, influence design. Attempts at this review. Consideration of the issues of humidity classification of anaesthetic breathing circuits have in breathing circuits, and of transmission of been numerous and nomenclature is confusing. The infection is not possible due to space limitations. three main functions of anaesthesia circuits are (i) delivery of anaesthetic gases and vapours, (ii) Carbon dioxide washout circuits oxygenation of the patient, (iii) carbon dioxide These comprise those systems not intended to elimination from the patient. include a carbon dioxide absorption device. As will In this review a functional classification based on be examined for each circuit, their performance is carbon dioxide elimination5 is used (Table I). influenced by the clinical circumstances under Carbon dioxide is eliminated either by "washout," which the circuit is used, particularly whether or not produced by the fresh gas inflow, or by absorption the patient is breathing spontaneously. (soda lime or equivalent). A functional classification is useful because it emphasizes differences in Magill circuit (Figure (A)) circuit performance based on the manner in which it is used. SPONTANEOUSRESPIRATION Finally, additional important information of The dynamics of this system are complex. While secondary consideration is summarized under the the fresh gas flow required to avoid rebreathing of headings: carbon dioxide is not influenced by the respiratory - Breathing circuit resistance, work of breathing waveform, critical factors include the fresh gas and wasted ventilation. flow, setting of the excess gas venting valve, - Breathing circuit malfunction. reservoir bag, and volume of the corrugated tubing. Theoretical consideration shows that rebreathing TABLE I Classification of anaesthesia breathing circuits has probably been eliminated if the fresh gas flow is Carbon dioxide washout circuits at least equal to the minute volume6'7 which is Open (no reservoir bag) sufficient to maintain normocarbia. In another study Open mask (drop) a mean fresh gas flow of 82ml-kg-l'min-t was lnsufflation required to prevent respiratory stimulation due to T-piece rebreathing, s It is difficult to know exactly what gas Semi-open (with reservoir bag) Magill circuit composition the patient is receiving. Bain system Jaekson-Rees system ARTIFICIAL VENTILATION Lack system The variabiity and unpredictability of the rebreath- Mera F system ing of alveolar gas and the venting of fresh gas is Non-breathing valve systems

Carbon dioxide absorption circuits From the Department of Anaesthesia, Faculty of Closed (fresh gas flow = uptake by patient) Medicine, 3B2-36 Walter MacKenzie Centre, University Semi-closed (fresh gas flow > uptake) of Alberta, Edmonton, Alberta, T6G 2B7.

CAN ANAESTH SOC J 1986 / 33: 1 / pp98-t05 Mclntyre: ANAESTHESIA BREATHING CIRCUITS 99

(A) (E) ./VVV~VVVVVVV~

(B) LT~J

(c)

(D)

Valve ~ Relief Valve ~ Oxygen Analyzer

--'-'1 Flow Volve

FIGURE (A) Magill circuit; (B) Bain circuit; (C) Jackson-Rees system; (D) Lack system; (E) Mera F system; (F) Non-breathing valve system; ((3) Circle carbon dioxide absorption circuit; (H) To and fro carbon dioxide absorption circuit. such that Sykes believed artificial ventilation should ing patient has long been a subject of controversy. never be attempted. 9 Opinions differ, however, Evidence exists that the system can safely be used and probably normocarbia can be maintained in the with a fresh gas flow of 100ml'kg-~'min -~ in adult if the fresh gas flow exceeds 5 L-rain -1 with suitable patients anaesthetised to retain an adequate less emphasis on the tidal volume has been carbon dioxide response. It-13 The concern about recommended, 9 but in the light of available the adequacy of such a gas flow is largely based on information Sykes' conclusion is a wise one in any unpredictable variations in carbon dioxide produc- clinical situation where the anaesthetist must tion ('(/CO2), minute ventilation, ratio of dead space predict accurately the constitution of inspired gases. ventilation to tidal volume (~/l)]~rT), and respiratory waveform that occur in the clinical situation. These Bain Circuit (Figure (B)) variations can all influence fresh gas flow requirements. ~4"15 Various formulae to predict fresh gas SPONTANEOUSRESPIRATION flow requirements during spontaneous respiration Use of the Bain Circuit for the spontaneously breath- with a Bain circuit have been presented. 6't3J6-2~ 100 CANADIAN ANAESTHETISTS 1 SOCIETY JOURNAl.

TABLE I! Formulae proposed for the calculation of fresh gas substantially exceeds the fresh gas flow, the fresh flows necessary when a patient is breathing spontaneously gas flow becomes the controlling factor for carbon through a Bain circuit. Superscripts indicate reference sources. dioxide elimination, i.e., rebreathing occurs. Thus

F.G.F. = 2-3 • WE6 different combinations of fresh gas flow and minute = 100ml.kg-t.min-1 13 volume can be selected to produce a desired PaCt2, = 3 x VE TM the choice being based on the clinical circum- = 2 X VE 17 stances. Many formulae have been devised relating = 8L unless VCO2 is more than 250ml TM fresh gas flow to a normal carbon dioxide tension or = Estimated normal ventilation '9 = 2.5 x 6 • kg body weight x f(15 • kg • f)2o to the prevention of rebreathing (Table III). A commonly used figure is 100 ml.kg-t'min -' with appropriate attention to the patient's ventilation. (Table II). The anaesthetist who customarily uses a Jackson-Rees system (Figure (C)) calculation giving a relatively low fresh gas flow This is a T-piece arrangement with a reservoir bag relative to alternative calculations should take into that incorporates a relief mechanism for gases to be account the prevailing clinical circumstances. The vented. When the patient exhales, exhaled gases critical issue is whether rebreathing of a character pass down the expiratory limb, mixing with fresh that will have an important effect on that patient's gases entering via the fresh gas inlet. During the PaCO2 is occurring. If specific blood gas values are expiratory pause fresh gases continue to flush essential for that patient's welfare these should be exhaled gases away from the T-piece and down the measured, because clinical signs of hypercarbia are expiratory limb. During inspiration the gases not always obvious. inhaled come from the fresh gas flow and the expiratory limb which includes the reservoir bag. If ARTIFICIAL VENTILATION the expired gas cannot readily escape from the When the Bain circuit is used in this fashion two reservoir bag reinhalation of mixed gas may relationships are important. When the fresh gas occur. 2~ flow is very high the PaCt2 is ventilation limited, i.e., by virtue of flow the patient does not rebreathe SPONTANEOUS RESPIRATION exhaled gases. However, when the minute volume The fresh gas flows necessary to prevent rebreathing are similar to those used for the Bain system. TABLE III Formulae proposed for the calculation of fresh gas However, if a positive end-expiratory pressure flows when a patient is being artificially ventilated with a Bain exists an increase in end-tidal carbon dioxide is circuit. Superscripts indicate reference sources. likely to develop unless the fresh gas flow is at least three times the minute volume. 2t Adults F.G.F. = 70 ml.kg -l.min-' at ventilation of 150 ml.kg-l-min -~ 6,~6 ARTIFICIAL VENTILATION F.G.F. The fresh gas flows required are similar to those = 100 ml.kg -l.min -t at ventilation of 120 ml-kg-~-min -t ,7 described for the Bain system. However, the manner in which the ventilation and venting is CMIdren t~ (Body weight 10-30 kg) managed during ventilation will have varying Toproduee a PaCt2 FGF = 1(300 + 100 ml.kg-l.min -' effects on rebreathing and in some clinical of4.9 kPa (37 ton') circumstances other circuits can be employed more To produce a PaCt2 FGF = 1600 + 100 ml,kg- ~. min- J easily. of 4.0 kPa (30 ton') Lack system (Figure (D)) (Body weight 30-70 kg) This is a coaxial system, but in contrast to the Bain To produce a PaCt2 FGF = 2000 + 50 ml.kg -~ .min -t circuit the fresh gas flow passes up the outer tube of 4.9 kPa (37 tort) and expiration passes through the central conduit. To produce a PaCt2 FGF = 3200 + 50 ml'kg -1 .rain -1 This circuit has not been subjected to the same of 4.0 kPa (30 tort) intensive study that alternative systems have McIntyrc: ANAESTHESIA BREATHING CIRCUITS I01 enjoyed. Available information is summarized as ventilation the fresh gas flow is adjusted to keep the follows: reservoir bag about three quarters full at the beginning of inspiration. Distension of the bag SPONTANEOUSRESPIRATION should be avoided and too low a gas inflow is A fresh gas flow of the order of expired minute equally hazardous. In the opinion of many volume is likely to avoid rebreathing, 22 but it may anaesthetists, the difficulties and potential hazards be wiser to use a greater flow. 23 A flow of associated with non-rebreathing valve systems 58 ml'kg-l'min -l has been proposed. 24 outweigh their usefulness for clinical anaesthesia except for emergency resuscitation or during the ARTIFICIAL VENTILATION transport of patients. The same flow considerations apply as for the Magill circuit. More recently, in an endeavour to Carbon dioxide absorption circuits reduce the fresh gas flows necessary to prcvcnt rebreathing, a Lack circuit with an injector device Closed systems (Figures (G) and (H)) has been used successfully.2s This was achievcd by In a completely closed system, the fresh gases and introducing thc oxygen through a Venturi dcvice vapours provide the metabolic and anaesthetic needs (Wolf Injectomat - Richard Wolf (U.K.) Ltd) of the patient and venting of excess gases should be positioned between the Lack circuit and the patient. unnecessary. Gases may follow a repetitive circular pathway (Figure (G)) or pass to and fro through an MERA F system (Figure (E)) absorber (Figure (H)). The relative placement of the This multipurpose breathing system 3' ~6 is designed items comprising the circle system do influence its to be used either as a coaxial circle attached to the function and the Figure illustrates a commonly used inspiratory and expiratory valves of conventional arrangement, circle anaesthesia delivery systems (Figure (G)) or as a modified type of T-piece circuit. Semi-closed carbon dioxide absorption circuits The breathing circuits just referred to can also be SPONTANEOUSRESPIRATION used in a semi-closed manner by opening the Rebreathing is avoided with a fresh gas flow of venting port. The to and fro system (Figure (H)) is 100 ml.kg-t-min -~ and it has been used at flows as little used though it is useful for anaesthetists who low as 50 ml.kg- train -I in some patients, without do not have a circle carbon dioxide absorption demonstrable rebreathing. 6 system available. However, circle systems are frequently used with a fresh gas flow greater than ARTIFICIALVENTILATION the patient's requirements, thus requiring venting of The function and fresh gas flow requirements are excess gases. This is commonly referred to as "a likely to be similar to those described later for a closed circuit with a leak." Employed in this fashion closed circuit or a "closed circuit with a leak." the fresh gas flow can exercise partial control of arterial carbon dioxide tension and can be used to Non-rebreathing valve systems (Figure (F)) regulate it. A variety of non-rebreathing valves have been If the carbon dioxide absorber is switched out of incorporated with a conduit and reservoir bag to the circuit rebreathing of expired gases will not form a breathing circuit (Figure (F)). There is no occur until the fresh gas flow is less than alveolar mixing of inhaled and exhaled gases proximal to the ventilation. If the configuration is as in (Figure (G)) mask or endotracheal tube connector. The appara- the fresh gas flow should equal the minute tus deadspace is confined to the deadspace of the ventilation predicted necessary to produce the valve. Fresh gas comes from the reservoir bag and desired arterial carbon dioxide value. 26 Such a the valve directs exhalations into the atmosphere or technique is unusual and customarily the carbon a scavenging device. Assuming the system is not dioxide absorber is switched into the circuit and the defective and is used correctly during artificial fresh gas flow is deliberately less than the minute ventilation the fresh gas flow should equal the ventilation. patient's minute volume. During spontaneous It is particularly important to realise that the 102 CANADIAN ANAESTHETISTS' SOCIETY JOURNAL smaller the fresh gas flow of nitrous oxide and In general the higher the fresh gas flow the greater oxygen the greater the difference between the fresh the work imposed on the patient. Mechanical gas flow oxygen concentration and the inspired ventilation, manual ventilation, or alternatively the oxygen concentration. Inspired oxygen concentra- contractility of a distended reservoir bag, could tion should always be measured when an anaesthe- reduce the work of inspiration but under some tic is being administered, but it is useful to know circumstances this work could be transferred to the the circumstances under which it may be fairly expiratory muscles. However, regardless of the successfully predicted. If the fresh gas flow is breathing system used, the major factor governing 1.0 L.min-i or less, the inspired oxygen concentra- resistance to a patient's respiration is the endotra- tion may be at least 20 per cent less than that in the cheal tube. fresh gases. 27 It has been shown that inspired oxygen concentration can be predicted after twenty Breathing system malfunction minutes steady state with a fresh gas flow of Many and varied problems have been publicized. 1-5L.min-l. 2s In respect of inspired oxygen Some are due to errors in design or manufacturer, concentration, perhaps a useful compromise be- others occur in transit or the hospital. It is because tween a totally closed system and a "closed carbon of these problems and their serious consequences dioxide absorption system with a leak" is the finding for all concerned that a pre-use check of an that in normal patients a fresh gas flow of nitrous anaesthesia delivery system including the breathing oxide 1200 ml. min- i oxygen 800 ml.min- l ensures circuit, should always be done prior to starting a an inspired oxygen concentration of 30 per cent or case. A detailed account of the manoeuvres that more. 27 should be performed is inappropriate here but specific directions 33 are summarized as follows: Mechanical factors Circle systems Breathing circuit resistance, work of breathing 1 Pressure gauge: make certain it reads zero. and wasted ventilation 2 Carbon dioxide absorber: ensure it is active and Reported differences between breathing circuits are switched into or out of the circuit as desired. small and for adults the circle absorber system with 3 Unidirectionalvalvesandcircuitpatency:ensure a standard 22 mm diameter is likely to have the least valves are present and working correctly. Testing resistance during spontaneous breathing. 29 The the patency of inspiratory and expiratory limbsas magnitude of this resistance is important because of well as the absorber itself. A method for detect- the potentially wasted ventilation and the increased ing incompetent unidirectional drive valves in a work of breathing. Any increase in volume of a circle carbon dioxide absorption system is as breathing circuit produced during the inspiratory follows34: phase of positive pressure ventilation is likely to (a) Preparation: remove breathing tubes from result in clinically important variations in delivered the circle, turn off all gas flows, and close ventilation, so the pressure within the system and relief valve. the material it is made of are very important. Highly (b) Checking inhalation valve: connect a reser- compliant adult circuits can result in a compression voir bag at the usual bag connector site and volume loss that exceeds the tidal volume setting of one limb of the corrugated tube on the inhala- the ventilator. 3~ The situation is even more tion outlet of the absorber; while the exhalation complicated if small infants are ventilated at high inlet is covered with the palm of a hand, apply rates. Gas flow in the infant airway lags that positive pressure (approximately 5-10cm generated by the ventilator and at the beginning of a H20) to the corrugated tube with "gently slow breath ventilation may be wasted. 31 blow" (see (d) for interpretation). The total work of breathing through a system is (c) Checking exhalation valve: connect a reser- important as is the rate at which work is imposed voir bag to the exhalation inlet and a and the fraction done during expiration. Laboratory corrugated tube on the bag connector site; studies 32 show that the Lack and Bain circuits are while the inhalation outlet is covered with the more acceptable than the Magill and circle systems. palm of hand, apply gentle positive pressure Mclntyre: ANAESTHESIA BREATHING CIRCUITS 103

to the corrugated tube as described above should bring to light existing problems but will not (see (d) for interpretation). indicate problems that develop during the conduct (d) Interpretation of test: if the valve being of a case. These are almost invariably manifested tested is competent, the reservoir bag will not early by clinical signs 37 so vigilant monitoring is the fill; filling of the reservoir bag indicates only protection for patient and anaesthetist. Once a incompetency of the valve tested; if the problem has been detected the anaesthetist may incompetency is not corrected after replace- immediately identify the cause in the breathing ment of the suspected valve, look for an system and correct it or postpone elucidation of the irregularity on the annular seat. cause until later and take active steps to protect the 4 System intregity: ensure hoses and reservoir bag patient. are attached in their correct positions. Check for Certain axioms for the user are obvious but bear leaks within the system by generating a positive repetition. A breathing system should be used only pressure of 30 cm H20 within it and making sure for the purpose for which it was designed. The the pressure does not fall more than 5 cm H20 configuration of the system should not be altered during the ensuing 30 seconds. unless the user is fully conversant with the 5 Relief valves: ensure the low pressure relief valve principles under which the old and new version - located between the wall outlet or gas cylinder function. The breathing system should only be used and the flow meters and the high pressure relief in the recommended manner, bearing in mind that a valve - located between the flow meters and the device may be in use for some time before problems breathing circuit, are functioning adequately. become evident, particularly when a combination These respectively protect the patient against a of devices is employed. For example, it has been dangerous decrease or increase in breathing shown that positioning and temperature are critical circuit pressure. Once system integrity has been factors when certain filters and humidifiers are in checked the pressure should be released by use. 38 slowly opening the relief valve. There should be

a gradual loss of pressure from the system. The Conclusion low pressure relief valve is checked by first Traditional breathing systems used for anaesthesia closing the high pressure relief valve and purposes perform well if they are cared for properly occluding the patient port. The reservoir bag is and the user understands the principles that underlie partially filled via the oxygen flush. A 4 L.min -1 their function. The circuit selected will depend on flow is set on the oxygen flow meter. After one the task to be performed and the professional minute the bag should be filled but not distended environment. Much valuable information omitted and the pressure in the breathing system should in this review is available in specialized books not increase. The bag is then given a quick dealing with anaesthesia equipment and should be squeeze. The relief valve should close and the easily accessible in any department of anaesthesia. pressure in the system increase. Releasing the Continuing changes include the adoption of materi- bag should cause the valve to open and the als that absorb inhalational agents less than does pressure to fall. conductive rubber and design changes to combine as many desirable qualities as possible in one Carbon dioxide washout circuits breathing system. Whether greater safety lies in The aspects of function to be tested are similar to having available two or three separate near perfect those listed for circle systems. Among potential systems for different clinical situations or in problems with the Baln circuit is disconnection of developing a single circuit that will function in the inner tube or a leak from it. Several safety different modes according to a selector setting checks have been reviewed, a5 The most widely remains to be seen. However, a sound knowledge of used consists of flushing the circuit with oxygen the potential problems in breathing circuits is flush valve and watching the reservoir bag. essential for the safe practice of anaesthesia and can Collapse of the bag secondary to the Venturi effect effectively be derived from the Canadian Standards in the outer tube shows the circuit is intact. 36 Association publication Standard Z 188.9, Breath- The conscientious use of appropriate checks ing Systems for Use in Anaesthesia. 104 CANADIAN ANAESTHETISTS' SOCIETY JOURNAL

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