Brit. J. Anaesth. (1968), 40, 666. REBREATHING CIRCUITS A Review

Transcription

1 Brit. J. Anaesth. (1968), 40, 666 REBREATHING CIRCUITS A Review BY M, K. SYKES The effects of rebreathing depend on the composition of the gas which is re-inhaled and whether this gas passes to the alveoli (where it interferes with gas exchange) or only to the anatomical deadspace of the patient. It is therefore important to differentiate between the rebreathing of deadspace gas, alveolar gas and mixed expired gas and to know in what order these gases are reinhaled. The deadspace gas may be divided into apparatus and anatomical components. Gas from the apparatus deadspace will vary in composition depending on the arrangement of the circuit. Gas expired from the anatomical (conducting airway) deadspace has a composition similar to inspired gas but is saturated with water vapour and at a temperature of about 32 C. Alveolar gas is also saturated but contains 5-6 per cent carbon dioxide and per cent oxygen, whilst mixed expired gas usually contains 3-4 per cent carbon dioxide and per cent oxygen (when breathing 21 per cent oxygen). Unfortunately, although it is possible to forecast the general behaviour of a circuit on theoretical grounds, it is not possible to make detailed predictions of its behaviour under clinical conditions. There are a number of reasons for this. First, the effective volume of the apparatus deadspace may not equal that determined by volumetric displacement (Clarke, 1958; Harrison, Ozinsky and Jones, 1959). This difference arises from variations in the pattern of gas flow due to channelling, turbulence and the cone-front effect (Briscoe, Forster and Comroe, 1954). However, under certain circumstances this problem can be overcome by utilizing other methods to measure apparatus deadspace (e.g. Voss, 1963; Bethune and Collis, 1967). The second reason why predictions fail is that the size of the anatomical deadspace, and hence the volume of deadspace gas, may vary (Sykes, 1967). The third reason for failure is that the composition of alveolar gas depends not only on the overall relation of alveolar ventilation to oxygen and carbon dioxide exchange but also on the pattern of ventilation and blood flow within the lungs (fig. 1). The effect of rebreathing alveolar gas will therefore depend on which portion of this gas is re-inhaled. Finally, the patient may react to a change in alveolar gas composition by a change in alveolar ventilation (Clappison and Hamilton, 1956; Elam and Brown, 1956). Although these factors lead to difficulties in theoretical analysis there is now general agreement on the underlying principles of most of the common rebreathing circuits. to, EXPIRATION- -t^ INSPIRATION FIG. 1 Expired oxygen tension (Po,) and carbon dioxide tension (PcoJ. Patient with normal longs (solid lines) and patient with ventilation/perfusion inequality (dotted lines).

2 REBREATHING CIRCUITS 667 EFFECTS OF REBREATHING 1. Retention of water and heat. Since the specific heat of air is low (0.24 cal/g= cal/ml at 20 C) whilst the latent heat of vaporization of water is high (580 cal/g), most of the heat loss in the respiratory tract is due to the evaporation of water. Hence there is little difference in heat loss whether hot or cold air is inhaled, providing that it is dry. However, the inhalation of warm, moist, air markedly reduces heat loss. In this respect rebreathing may be beneficial. On the other hand, the inhalation of saturated air at a temperature above 37 C results in the addition of heat to the body. This may lead to hyperthermia, particularly if the normal heat-regulating mechanisms are depressed by the drugs used in anaesthesia. In most anaesthetic circuits heat is rapidly lost to the atmosphere and the gas which is re-inhaled has a lower temperature and water content than expired gas. However, in some circuits, such as the Waters to-and-fro absorption circuit, the tubes are short and heat loss is minimal; in addition, heat is added to the circuit by the chemical reaction taking place in the soda-lime. In this situation the inspired gas may be fully saturated and at a temperature of C (Adriani, 1960; Burton, 1962). Dangerous hyperthermia may result 2. Alteration of inspired and alveolar gas tensions. The general equation governing the alveolar concentration (FA,) of a gas (x) is: where ty, (the volume exchanged in unit time) is positive for uptake and negative for elimination, \^A is the alveolar ventilation in unit time and Fix is the inspired concentration (Nunn and Newman, 1964). From this equation it can be seen that alveolar concentration is dependent on inspired concentration and that the alveolar concentration only equals the inspired concentration when gas exchange is zero or alveolar ventilation is infinite. Normally the partial pressures of the individual components of fresh gas entering the lungs are lowered only by the tension of water vapour. When rebreathing of alveolar gas occurs there will be a further reduction in inspired oxygen tension and an increase in inspired carbon dioxide tension (Swartz, Adriani and Mih, 1953; Ruben, 1953; Firton, 1958, 1963). Similar changes will occur in the alveolar gas tensions unless the alveolar ventilation undergoes a compensatory increase. In addition to its effects on the respiratory gases, rebreathing can also cause an alteration in the inspired and alveolar tensions of an anaesthetic gas or vapour. During induction the alveolar tension is less than the inspired; rebreathing of alveolar gas therefore causes a reduction in inspired and alveolar tension and prolongs induction. During recovery alveolar tension exceeds inspired. In this situation rebreathing maintains a higher alveolar tension and so slows the elimination of an anaesthetic gas or vapour. In a similar fashion rebreathing slows down the change in alveolar gas composition in response to a change in inspired gas composition. This slowing is additional to the delay caused by replacing the gas in the circuit with gas of a new composition. MEASUREMENT OF REBREATHING As the rebreathing of deadspace gas is of little clinical significance most studies have been concerned with the rebreathing of alveolar gas. This has usually been detected by the analysis of carbon dioxide since physical methods for continuous analysis of this gas have been readily available. Furthermore, it is easier to add measured quantities of carbon dioxide to model lung systems than to arrange for a continuous uptake of oxygen. However, in clinical studies similar results may be obtained by continuous analysis of carbon dioxide or oxygen concentrations (Sykes, 1959a; Norman, Adams and Sykes, 1968). There are three main methods for the detection of rebreathing when using carbon dioxide as the indicator gas: (1) measurement of its concentration in the inspired gas; (2) measurement of the increase in alveolar carbon dioxide concentration at constant ventilation; (3) measurement of the increase in alveolar ventilation at constant arterial or alveolar Pco,. The measurement of the quantity of carbon dioxide in the inspired gas is most simply performed by deflecting the whole of one inspiration

3 668 BRITISH JOURNAL OF ANAESTHESIA into a reservoir so that the gas composition and volume can be measured. This method was used in model experiments by Woolmer and Lind (1954). A disadvantage is that the equilibrium of the system is temporarily upset Alternatively, the inspired carbon dioxide concentration can be continuously monitored and replotted against inspired volume (determined by integration of a pneumotachograph signal). The area under the curve is then indicative of the volume of carbon dioxide in the inspired gas (fig. 2). The method is time-consuming and requires impeccable instrumentation. Its main use is in the detailed analysis of the pattern of gas flow in a system. where FAooi=alveolar carbon dioxide concentration; Flcoa = inspired carbon dioxide concentration; Vco,=volume of carbon dioxide entering lungs (constant); VA=alveolar ventilation. Another method of quantifying the degree of rebreathing is to equate the increase in alveolar carbon dioxide or decrease in alveolar ventilation with the effect of an added deadspace. If VD represents the total deadspace and VT and f represent tidal volume and respiratory frequency respectively then V)f or VD=VTnow V*CO,=^AXFA 0O, or VA=CVC VD=VT-[Vco 1 /(FA 0Ol xf)] If VD is determined before and after adding the circuit under test the degree of rebreathing can be expressed as an increase in deadspace over that determined in the absence of rebreathing (Sykes, 1959a; Voss, 1963). ;:.. <} < > Volume Inspired FIG. 2 Mapleson "C circuit, spontaneous ventilation. Fresh gas inflow=15 L/tnin. Inspired carbon dioxide concentration replotted against volume inspired (ml). The initial dip in the carbon dioxide concentration is due to the fresh gas which accumulates under the valve mount during the expiratory period. The second and third methods are based on the assumption that carbon dioxide output is constant and that in a steady respiratory state the volume of carbon dioxide entering the lungs is equal to the volume removed by the alveolar ventilation. Hence any increase in alveolar carbon dioxide concentration or alveolar ventilation must be due to an increase in inspired carbon dioxide concentration. This is illustrated by the application of the general equation (1) to carbon dioxide: CIRCUITS IN WHICH REBREATHING MAY OCCUR These will be classified according to the presence or absence of valves and, if present, on their number and position. Circuits with no valves. The simplest example is the open-drop gauze mask. This becomes an extension of the anatomical deadspace. The degree of rebreathing is determined by the volume of the mask and the thickness of the material used to cover the frame. Rebreathing is increased (and dilution of anaesthetic vapour with fresh air minimized) by the application of a "chimney" (fig. 3). The inhalers of Clover and Ombredanne also act as an extension of the patient's deadspace, but the volume of rebreathing is more clearly defined. Complete rebreathing occurs in the Oxford and Goldman's vinesthene vaporizers since the circuit is completely dosed. Another circuit which has been widely used is the T-piece. This was originally described by Ayre (1937a, b) and further discussed by him in 1956 and The principle has been examined theoretically by Mapleson (1954, 1958),

4 REBREATHING CIRCUITS 669 providing a flow of fresh gas which exceeds the peak inspiratory flow rate. This normally approximates to three times the minute volume (fig. 4). -tlnspiratioi I In 30,- -Expiration- L/rren Schimmelbusch mask FIG. 3 Schimmelbusch mask with towel used to increase concentration of anaesthetic vapour. Onchi, Hayashi and Ueyama (1957), and Lewis and Spoerel (1961). Clinical and experimental investigations have been reported by Woolmer and Lind (1954), Inkster (1956), Voss (1963) and Harrison (1964a). The circuit was designed to eliminate the resistance of the expiratory valve and it was originally suggested that the volume of the expiratory limb should be less than the patient's tidal volume. A number of modifications have since been described (Brooks, Stuart and Gabel, 1958; Voss, 1963: Harrison 1964b) but the manner in which the T-piece functions depends primarily on the volume and shape of the expiratory limb. The expiratory limb may consist of a long tube with a volume greater than the patient's tidal volume, a tube of similar volume with a reservoir bag at the distal end (Rees, 1950), or the expiratory limb may be reduced in length so that the internal volume is less than the patient's tidal volume. The extreme example of this is the modification of Lewis and Spoerel (1961) and of Picken (1950) in which the volume of the expiratory limb is reduced to zero, the expired gases being eliminated through a hole in the wall of the catheter mount With the latter arrangement rebreathing cannot occur, since the patient breathes out directly to atmosphere. However, with this type of T-piece dilution of fresh gases with room air can only be prevented by FIG. 4 Pattern of inspiratory and expiratory flow rate during spontaneous ventilation. Minute volume 10 L/min. As the volume of the expiratory limb is increased so the risk of air dilution diminishes. When the volume of the expiratory limb exceeds the tidal volume dilution is unlikely to occur. The functional analysis of such a T-piece is shown in figure 5. Fresh gas flows continuously Patient I Dead space gas I Alveolar gas FIG. 5 T-piece (Mapleson "E" circuit). Disposition of gases at end of expiration. FGF=fresh gas flow. through the side arm. The peak expiratory flow rate occurs early in expiration and flow then diminishes in an approximately exponential fashion (fig. 4). The proportion of fresh gas which is added to the expired gas as it passes down the expiratory limb therefore increases as expiration progresses and during the expiratory pause fresh gas continues to accumulate at the patient end of the expiratory limb. Rebreathing cannot occur if the fresh gas flow rate exceeds the peak inspiratory flow rate, for all the inspired gas is then supplied from the fresh gas inlet. Under most circumstances a smaller fresh gas flow rate may be used without rebreathing. Favourable factors are a high inspiratory: expiratory time ratio, a slow

5 670 BRITISH JOURNAL OF ANAESTHESIA rise in inspiratory flow rate, a low flow rate during the last part of expiration and a long expiratory pause. All these factors provide time for the fresh gas flow to flush the expiratory limb; this minimizes the quantity of alveolar gas which is re-inhaled. Furthermore, it does not matter if alveolar gas is re-inhaled into the anatomical deadspace since no gas exchange takes place in this portion of the respiratory tract. Under the most favourable conditions during spontaneous ventilation the fresh gas flow rate may be reduced to 2-2^ times the minute volume without rebreathing occurring. Similar considerations apply when ventilation is controlled. It should be noted that the addition of a bag to the expiratory limb (Rees, 1950) will not affect the function of the system providing that the bag is separated from the patient by a tube with an internal volume which exceeds the patient's tidal volume. If the tube volume is too small re-inhalation of mixed expired gas from the bag may occur. The system then approximates to a Mapleson "C" circuit (Mapleson, 1954). One further valveless circuit deserves mention. This is the use of a circle absorption system in which the direction of gas flow is controlled, not by valves, but by a turbine-operated blower (Roffey, Revell and Morris, 1961). The advantage claimed is that resistance to breathing is minimized and that dangers arising from faulty action of the valves are eliminated. This system behaves as a T-piece at the mouth; circulating volume flow must therefore exceed three times the patient's minute volume to ensure that expired gas is not rebreathed until it has passed through the absorber. One of the dangers of this type of apparatus is that any increased resistance in the absorber or other part of the circuit may so reduce the flow of gas round th: circuit that rebreathing results. The principle of the T-piece is also utilized in many oxygen masks and tracheostomy humidifier boxes: since high oxygen flows are often undesirable, rebreathing is usually minimized by dilution with room air. T-piece circuits have the advantage that they are small, simple and have small apparatus deadspace. However, they require relatively large gas flows. They are, therefore, most suitable for paediatric use. Circuits with Expiratory Valves only. Spontaneous ventilation. The classification used by Mapleson (1954) is shown in figure 6. - Constant gas flow from anaesthetic machine Reservoir bag Corrugated tubing -i~i- Expiratory valve / \ Face mask FIG. 6 Classification of circuits (Mapleson, 1954). The Magill circuit (Mapleson "A") has been studied in most detail. The efficiency of this circuit during spontaneous ventilation depends on the separation of expired gas into deadspace and alveolar portions (Wynne, 1941; Molyneux and Pask, 1951; Domaingue, 1951). On expiration the deadspace portion travels up the tube towards the bag, followed by the alveolar gas portion (fig. 7). The deadspace gas meets the fresh gas entering the circuit and, as the bag fills and overflows during the expiratory pause, the deadspace gas is driven back towards the patient. This causes the expiratory valve to open and to discharge the alveolar gas lying at the patient's end of the corrugated tube. If the fresh gas Sow

6 REBREATHING CIRCUITS ?-^ [ used without the occurrence of rebreathing. Using this technique on patients who were breathing spontaneously under light halothane anaesthesia they found that the fresh gas flow could be reduced to two-thirds, and in some cases to half the minute volume, before rebreathing occurred. Norman, Adams and Sykes (1968) repeated this work in conscious volunteers and showed that rebreathing was not seen until the fresh gas flow was approximately 70 per cent of the minute volume. These authors also demonstrated that there was a fall in inspired and alveolar Po a when [~*1 Fresh gas rebreathing occurred. I Dead space gas ( The Mapleson "B" circuit was originally introduced in an attempt to ensure that changes in Alveolar gas fresh gas composition would be rapidly reflected in the alveoli. However, when the circuit diagram is examined it becomes apparent that there is a closed limb terminating in the reservoir bag which is never flushed with fresh gas. The analysis of this system is complicated, but it is obvious that rebreathing will occur if the flow of fresh gas is less than the peak inspiratory flow rate. Under these conditions it is a mixture of alveolar gas and fresh gas which is re-inhaled. A flow of fresh gas more than double the minute volume will, in most clinical circumstances, reduce the degree FIG. 7 Circuits with expiratory valves: spontaneous ventila- of rebreathing to acceptable levels. tion. Disposition of gases at end-expiration. From The Mapleson "C" system (Waters to-and-fro above down, Mapleson "A", "B", "C", "D" circuits. P=patient FGF=fresh gas flow. absorption circuit without a carbon dioxide absorber) may be regarded as similar to system "B" rate is great enough, the deadspace component except that there is no corrugated tubing to will also be washed out through the valve; how- maintain the separation of expired gas into deadever, as mentioned earlier, rebreathing of this space and alveolar components. When rebreathportion of the expired gas will not affect the ing occurs it is therefore a combination of mixed patient. Mapleson (1954) predicted that rebreath- expired gas and fresh gas which is inhaled (fig. 2). ing would not occur if the fresh gas flow rate For safety in clinical practice the fresh gas flow equalled or exceeded the alveolar ventilation. should be greater than twice the minute volume. Since this prediction was based on a number of The Mapleson ad" circuit is similar to the assumptions (the main one being that there would Mapleson "B" except that the expiratory valve be no mixing of alveolar, deadspace and fresh is situated close to the reservoir bag. If the tidal gas), he suggested that, in clinical practice, the volume is less than the volume of the corrugated fresh gas flow should always exceed the minute tube (500 ml) the system behaves as a T-piece. volume. Woolmer and Lind (1954) and Bracken If, however, the tidal volume exceeds this figure and Sanderson (1955) confirmed these predictions mixed expired gas from the bag is re-inhaled. experimentally and De d i v e Lowe (1956) and To prevent rebreathing at normal tidal volumes Davies, Verner and Bracken (1956) confirmed the fresh gas flow should exceed twice the them under clinical conditions. Kain and Nunn patient's minute volume. At fresh gas flow rates (1967) approached the problem by determining below the peak inspiratory flow and with a northe lowest fresh gas flow rate which could be mal pattern of breathing and tidal volume the

7 mt BRITISH JOURNAL OF ANAESTHESIA system is more efficient than the "B" or " C circuit. This is due to the fact that all the fresh gas passes into the patient whereas in the " B " and " C " circuits it is chiefly fresh gas which passes out through the valve during the expiratory pause. When the tidal volume is increased the difference between the circuits becomes less marked. Controlled t>entilation. The Magill attachment behaves very differently when ventilation is controlled (fig. 8). To force consists mainly of deadspace and fresh gas. Rebreathing under these conditions is therefore marked (Sykes, 1959a). The effects can be diminished by increasing the tidal volume or by increasing the fresh gas flow rate. Elimination of carbon dioxide can be further improved by applying a high pressure early in inspiration so that more alveolar gas is discharged to the atmosphere. PrtJsurt FGF-i Flow through valvt -HNSPIRATION* Fresh gas Dead space gas Alveolar gas Pro. 8 Circuits with expiratory valves: controlled ventilation. Disposition of gases at end-expiration. FGF=fresh gas flow. gas into the lungs the expiratory valve spring is tightened and pressure applied to the bag. As the pressure in the circuit is increased gas flows into the lungs, the alveolar gas being the first portion to return. Eventually the pressure under the valve causes it to open and the excess gas is discharged to the atmosphere (fig. 9). This gas -HNSPllWtON- FIG. 9 Flow of gas through expiratory valve with two differently-shaped inspiratory pressure curves. The Mapleson "B" circuit behaves in a manner similar to that described for spontaneous ventilation. However, rebreathing is slightly less since the expiratory valve is closed during the expiratory pause and the fresh gas therefore accumulates at the patient end of the corrugated tubing. When the bag is compressed most of the fresh gas passes into the lungs whilst the mixed expired gas tends to pass out through the valve. Similar remarks apply to the Mapleson " C " system although in this case there is a smaller volume available for the storage of fresh gas during the expiratory pause, most of the fresh gas mixing with the expired gas in the bag. In both these circuits a fresh gas flow of at least twice the minute volume is required to reduce rebreathing to clinically acceptable levels during controlled ventilation. The Mapleson " D " system tends to cause less rebreathing than systems "A", "B" or "C" during controlled ventilation because the valve is situated away from the patient. In the Mapleson "B" circuit some of the fresh gas passes out through the valve during inspiration whereas in

8 REBREATHING CIRCUITS 673 the "D" circuit practically all the fresh gas input is driven into the patient, whilst the gas which is eliminated from die system consists mostly of alveolar gas and deadspace. The order of merit of the four systems considered is therefore "D", "B", "C", "A" (Waters and Mapleson 1961). For this reason Waters (1961) has suggested a combined system using a Mapleson "A" circuit for spontaneous ventilation and a Mapleson "D" system for controlled ventilation. It must be remembered, however, that the tidal volume during controlled ventilation is usually greater than during spontaneous ventilation. The fresh gas flow required to ensure the complete absence of rebreathing may therefore approach 20 l./min. In clinical practice the rebreathing which occurs under conditions of controlled ventilation is usually compensated by the increased alveolar ventilation which is achieved. Smaller fresh gas flow rates than those mentioned may therefore prove acceptable (Nightingale, Richards and Glass, 1965; Marshall and Henderson, 1968). Circuits with Valves controlling the Direction of Gas Flow to and from the Patient. These may be divided into circuits with the valves close to the patient and those with the valves placed remotely from the patient. Valves placed close to the patient are usually termed non-rebreathing valves. Although there are many different designs (Forreger, 1959; Sykes, 1959b) these valves basically consist of an inlet port, which prevents reflux of fresh gas, and an outlet port, which prevents rebreathing of expired gas or dilution with room air. If the valve functions perfectly rebreathing is therefore related to the size of the added apparatus deadspace. Normally this is of the order of ml Unfortunately, in a number of these valves gas leaks past the valve flaps, either because the movement of the flaps is sluggish or because the valve fails to seat correctly (Kerr and Evers, 1958; Loehning, Davis and Safar, 1964). The reflux of patient's expired gas into the inspiratory limb may amount to per cent of the tidal volume if the inlet valve is sluggish and the pressure in the bag is lowered slowly. Fortunately it is usually only the deadspace portion of the expired gas which is re-inhaled; harmful rebreathing therefore only occurs when the valve sticks in the inspiratory position. During controlled respiration some inspired gas often slips past the expiratory valve during the initial part of inspiration. This reduces the tidal volume delivered. During spontaneous ventilation the expiratory valve may stick in the open position so that air may be inhaled. If the expiratory port is connected by a tube (e.g. to an absorber) the gas which is re-inhaled is alveolar gas; harmful rebreathing might therefore occur under these conditions. When the valves are situated at a distance from the patient two additional factors may cause rebreathing. During spontaneous ventilation some mixing of inspired and expired air streams occurs at the T-piece due to turbulence, delay in valve action and the pressure fluctuations in the system. During controlled ventilation this mixing may be accentuated by variations in the elasticity of the breathing tubes and by differences in the compressible volume on the two sides of the circuit Finally, the elimination of carbon dioxide from circle systems without an absorber must be considered. This depends largely on the fresh gas inflow and on the position of the expiratory valve. If the valve is situated close to the patient's mouth and the fresh gas flow equals the patient's minute volume almost complete elimination of alveolar carbon dioxide may occur. However, if the valve is placed at some distance from the patient considerable rebreathing may occur unless the fresh gas flow is increased (Brown, Seniff and Ham, 1964). CONCLUSION The elimination of carbon dioxide from semidosed circuits without absorbers depends on the fresh gas flow, tidal volume and to a lesser extent on the pattern of breathing. Some circuits arc surprisingly efficient; this is because the expired carbon dioxide is concentrated in the alveolar portion of the expired gas. If the design of the circuit is such that this portion is eliminated through the expiratory valve rebreathing will be minimal Circuits which do not maintain the separation between fresh gas, deadspace gas and alveolar gas tend to be inefficient. REFERENCES Adriani, J. (1960). Disposal of carbon dioxide from devices used for inhalations] anesthesia. Anetthesiology, 21, 742.

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Anaesthesia, 23, 75. Nunn, J. F., and Newman, H. C. (1964). Inspired gas, rebreathing and apparatus deadspace. Brit. J. Anaesth., 36, 5. Onchi, Y., Hayashi, T, and Ueyama, M. (1957). Studies on the Ayre T-piece technique. Far East J. Anaesth., 1, 8, 30. Picken, D. K. W. (1950). An endotracheal connector for infants. Brit. med. J., 1, 954. Rees, G. J. (1950). Anaesthesia in the newborn. Brit, med. J., 2, Roffey, P. J., Revell, D. G., and Morris, L. E. (1961). An assessment of the Revell circulator. Anesthesiology, 22, 583. Ruben, H. M. (1953). Concerning the concentration of inhaled gases in semiclosed anesthesia systems. Anesthesiology, 14, 459. Swam, C H., Adriani, J., and Mih, A. (1953). Semiclosed inhalers: studies of oxygen and carbon dioxide tensions during varying conditions of use. Anesthesiology, 14, 437. Sykes, M. K. (1959a). Rebreathing during controlled respiration with the Magill attachment Brit. J. Anaesth., 31, 247. (1959b). Non-rebreathing valves. Brit. J. Anaesth., 31, 450. (1967). Aspects of dead space in anaesthetic practice and artificial pulmonary ventilation. Chapter in Modem Trends in Anaesthesia (eds. Evans and Gray), vol. 3, p. 80. London: Butterworths. Voss, T. J. V. (1963). Deadspace in paediatric anaesthetic apparatus. Brit. J. Anaesth., 35, 454. Waters, D. J. (1961). A composite semidosed anaesthetic system suitable for controlled or spontaneous respiration. Brit. J. Anaesth., 33, 417. Mapleson, W. W. (1961). Rebreathing during controlled respiration with various semiclosed anaesthetic systems. Brit. J. Anaesth., 33, 374. Woolmer, R., and Land, B. (1954). Rebreathing with a semiclosed system. Brit. J. Anaesth., 26, 316. Wynne, R. L. (1941). Mechanism of partial rebreathing in anaesthesia. Brit. med. J., 1, 155.