CLINICAL ANAESTHESIA AND THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER

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Br. J. Anaesth. (1974), 46, 773 CLINICAL ANAESTHESIA AND THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER C. A. SHANKS SUMMARY Airway temperatures and humidities were examined during adult anaesthesia before and after use of a heat-and-moisture exchanger (HME).

1 Br. J. Anaesth. (1974), 46, 773 CLINICAL ANAESTHESIA AND THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER C. A. SHANKS SUMMARY Airway temperatures and humidities were examined during adult anaesthesia before and after use of a heat-and-moisture exchanger (HME). The unmodified anaesthetic circuits delivered gases at room temperature, with three levels of relative humidity. End-phase temperatures and absolute humidities were higher with the HME in circuit, most noticeably when the inspired gases were saturated. Use of the HME with a dry non-rebreathing system gave minimal improvement in the end-inspired humidity of gases entering the trachea, and the respiratory heat loss remained high. "Few anaesthetists consider it necessary to humidify anaesthetic gases, even during prolonged anaesthesia, whereas most accept the need for humidification during long-term mechanical ventilation of the lungs " (Editorial, 1970). Most reviews of humidification are largely concerned with long-term effects (Chamney, 1969; Graff and Benson, 1969). Boys and Howells (1972), however, analysed the anaesthetic considerations, particularly the changes which have been thought to result from the use of dry anaesthetic gases, and the dangers of systems used to add moisture to the inspiratory gases. They concluded that humidification is worthwhile for all patients undergoing anaesthesia, particularly when endotracheal and non-rebreathing circuits are used. They recommended that the dry fresh gases be diverted through water before being delivered to a condenser-humidifier (heat and moisture exchanger, HME). This recommendation was based on the study by Mapleson, Morgan and Hillard (1963). Using a different form of model lung, we found some discrepancies (Shanks and Sara, 1973b). If anaesthetic conditions vary from those examined by Mapleson's group, clinical application of their theoretical analysis might be altered. This study was performed to examine the respiratory temperatures and humidities obtained when patients were ventilated via an endotracheal tube from an unmodified non-rebreathing or circle absorber system. The heat and moisture patterns in the artificial airway were calculated, and the changes resulting from insertion of HME were studied. C. A. SHANKS, F.F.A.R.A.CS., Royal Prince Alfred Hospital, Camperdown, N.S.W. 2050, Australia. METHOD Eighteen patients aged years, having a weight range kg, were investigated. They were premedicated with a belladonna alkaloid and a narcotic, and anaesthesia was induced with thiopentone. Relaxation was obtained with pancuronium, and the patients were ventilated mechanically with a nitrous oxide, oxygen and halothane mixture. The expired tidal volume was set at 500 ml, and the rate was set at 19 or 20 b.p.m. The patients were divided into three groups of six, according to the anaesthetic circuit used (fig. 1). Group 1 patients were ventilated from a dry nonrebreathing system which incorporated an Ambu E valve. In Group 2 the circle system included a Boyle Mark III absorber unit, and the 6 litre/min fresh gas inflow entered the circle alongside the inspiratory unidirectional valve, after the absorber. For Group 3 the circle system was a Drager type IIIA, and the same amount of gas flowed in at a point on the expiratory side of the circle, before the absorber. Temperatures were measured with nichrome-constantan thermocouples (Elektrolaboratoriet, Copenhagen). Body core temperatures were measured in the nasopharynx and at the tympanic membrane. Gas temperatures were measured in room air, in the gas to be inspired, and in the airway. Pairs of airway probes (TRA-1) were placed axially in the lumen of the catheter mount, and at the bevelled end of the endotracheal tube (fig. 2). This was a modification of the system described by Ingelstedt (1956), a technique which provides end-phase temperatures and humidities. The end-inspired and end-expired temperatures were obtained from an

2 774 BRITISH JOURNAL OF ANAESTHESIA DRY GAS - NROW 1 I»~vww VENTILATOR unmodified thermocouple. A second probe faced the first at a distance of about 2 mm. This was adapted to provide "wet bulb" temperatures from which the humidities were derived by psychrometry (Shanks and Sara, 1973a). The bare wires of the thermocouple had a fine thread spiralled around them, and before each reading this was moistened from a remotely placed tuberculin syringe. Calibration was performed in gases of predetermined relative humidity (Loew, Klein and Chalon, 1972) obtained with known proportions of dry and saturated gases. For a tidal volume of 500 ml, the derived humidities were slightly high in the middle range (fig. 3) and this tendency increased with relative humidities below 30%. Larger tidal volumes improved the agreement between the predetermined value and the humidity observed in the patient limb of a T-piece. Measurements were made at 10-min intervals, and those shown in the tables represent readings which had been stable for min. The HME was then inserted between the circuit and the catheter mount (fig. 2), and further measurements were made. Control measurements were made again after removal of the HME. Using the calculations shown in the Appendix, DRY GAS INFLOW VENTILATOR FIG. 1. The three groups were differentiated by the type of anaesthetic circuit used. The patients of Group 1 were ventilated with dry gases from a non-rebreathing system. The circle absorber system for Group 2 had the dry fresh gas inflow join the circuit on its inspiratory side. In Group 3 the fresh gas entered before the absorber resulting in saturated gases in the inspiratory tubing. 100 r 2 60 > CATHETER MOUNT 40 UJ s 20 ENDOTRACHE TUBE * PROBE SITES FIG. 2. The assembly with the HME inserted. The probes for measurement of end-phasic temperatures and humidities were sited in the tidal airway of the apparatus, as near as possible to the trachea and to the HME PREDETERMINED RELATIVE HUMIDITY % 100 FIG. 3. Calibration of relative humidity obtained from end-inspired values at a T-junction. High volumes of fresh gas flowed at room temperature, and contained different proportions of saturated and dry gases to provide a known relative humidity. The curve shown is for a tidal volume of 500 ml, with 20 b.p.m., and higher volumes produced better agreement.

THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER 775 the respiratory heat loss was calculated from the mean end-phasic temperatures and absolute humidities.

3 THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER 775 the respiratory heat loss was calculated from the mean end-phasic temperatures and absolute humidities. RESULTS The mean values and standard deviations for gas temperatures are shown in table I. The inspired gases left the corrugated tubing of the anaesthetic system at close to room temperature. At the catheter mount the end-inspired temperatures were not much higher until the HME was inserted (table IB). The highest end-inspired temperatures at this site were seen in the saturated gases of Group 3. During use of the unmodified circuits, gases entered the trachea with end-inspired temperatures near 29 C. With the HME in the circuit these were higher, and in Group 3, gases in the trachea had a mean minimum temperature of 32 C. The relative humidities were not corrected, and those in the lower range are thus too high (fig. 3). In Group 1 the mean end-inspired relative humidity at the catheter mount was 30%, and at the trachea 32%. Insertion of the HME increased the values to 34% and 39% respectively. In Group 2 the unmodified circuit gave a mean end-inspired relative humidity of 58% in the catheter mount and 66% in the endotracheal tube. In Group 3 all end-inspired measurements were saturated, and this did not change during use of the HME. In Group 2 A. The unmodified circuit TABLE I. Airway temperatures ( C) during anaesthesia with Catheter mount three circuit variants. Mean end-phasic and core temperatures Endotracheal tube are shown, with SD. For the three group categories seefigure1; Core they represent gases supplied by the anaesthetic systems at three humidity levels. A. The unmodified circuit Inspiratory tubing 22.4 (0.8) Catheter mount 23.9 (1.1) Endotracheal tube 28.1(0.6) Core Auditory canal or nasopharynx Endotracheal tube 33.8 (0.6) Catheter mount 30.6 (0.8) B. With a multiple gauze HME in Inspiratory tubing 21.9 (1.2) Catheter mount 26.5 (0.7) Endotracheal tube 29.6 (0.8) Core Auditory canal or nasopharynx Endotracheal tube 34.4 (0.4) Catheter mount 31.7(0.7) 23.4(0.7) 22.9(1.3) 24.2 (0.6) 29.2(1.5) 24.0 (1.0) 29.2 (0.9) 35.9 (0.6) 36.1 (0.7) 36.1 (0.6) 34.7 (0.4) 30.8 (0.6) the circuit 22.7 (0.4) 27.8 (0.9) 30.2 (1.0) 34.5 (0.4) 30.9 (0.9) 22.7 (1.4) 28.9 (1.2) 32.0(1.0) 35.8 (0.5) 36.0 (0.4) 36.0 (0.6) 35.1 (0.6) 32.1 (0.7) 35.1 (0.5) 32.9 (0.7) addition of the HME produced mean end-inspired relative humidities of 58% and 69% at the catheter mount and trachea. A further group of patients were anaesthetized under the same conditions as Group 2, except that the fresh gas inflow was reduced to 3 litre/min. This gave end-inspired relative humidities which were close to 80%. Endexpired gases were saturated in all groups. The end-phasic absolute humidities shown in table II are those associated with the temperatures shown in table I. As the relative humidities were much the same after inclusion of the HME, the increase in the end-inspired humidities in table II was directly affected by the associated increase in temperature. From the temperatures and humidities shown in tables I and II, the respiratory heat loss was estimated, by the method shown in the Appendix. Table III indicates that the patients in Group 1 lost about 10 kcal per hour from the airway. During use of the HME, the patients in Group 3 lost about one-third this amount. TABLE II. The absolute humidities associated with the temperatures in table I. These are expressed as mg water vapour j litre of gas. Means with SD. The alveolar values were assumed to be the saturated value for the deep body temperature. Endotracheal tube Catheter mount 6.4 (0.7) 8.9 (1.5) 41.6 (1.2) 27.3(1.2) 31.3(1.3) 12.9 (1.4) 18.9 (2.7) 42.0(1.3) 39.1 (0.8) 31.7(1.1) B. With a multiple gauze HME in circuit Catheter mount 8.6(1.1) 15.6(2.0) Endotracheal tube 11.5(2.0) 22.1(3.4) Core 41.3 (1.0) 41.7 (0.7) Endotracheal tube Catheter mount 38.3 (0.9) 33.2(1.3) 39.8 (1.3) 33.9(1.3) 21.8 (1.6) 28.1 (2.0) 42.0 (1.3) 38.6 (0.9) 31.9(1.6) 28.3 (3.0) 33.2 (2.3) 41.7(1.2) 39.8 (1.0) 35.4(1.4) TABLE III. Respiratory heat loss was calculated from the end-phasic temperatures and humidities shown in the tables by the method outlined in the Appendix. The heat loss estimates are in kcal per hour, based on a respiratory minute volume of 10 litres (measured at 22 C, saturated). At the catheter mount A. Unmodified B. With HME In the endotracheal tube A. Unmodified B. With HME II

776 BRITISH JOURNAL OF ANAESTHESIA DISCUSSION The multiple-gauze heat and moisture exchanger (HME) is designed to trap expired heat and water, so that it may be returned in the inspired gases.

4 776 BRITISH JOURNAL OF ANAESTHESIA DISCUSSION The multiple-gauze heat and moisture exchanger (HME) is designed to trap expired heat and water, so that it may be returned in the inspired gases. During use with normal room air, it has been reported that the inspired gas temperature may be "C and contain mg/litre of water vapour (Toremalm, 1960; Sara, 1965). After measuring the water loss during ventilation with dry gases, Mapleson, Morgan and Hilland (1963) described a theoretical basis for the function of the HME. They indicated that the HME was least efficient when the fresh gases were dry, and table II confirms this. However, their model system was arranged so that the saturated air was expired at 37 C, a temperature unlikely to be achieved in practice (Webb, 1951; Ingelstedt, 1956). Table I shows that gases reached the HME with an endexpired temperature about 32 C, and this would place the clinical results on the less favourable side of Mapleson's graph of percentage return. They suggested that a cooler environment would improve the efficiency of the HME, but this may be offset by the greater cooling of the expired gases en route to the HME. Han and Lowe (1968) examined respiratory water losses during mouth breathing. They showed that all types of HME were able to reduce the losses, and the largest deficits occurred with inspiration of arid gases. They found that 23.4 mg of water vapour was lost in each litre exhaled from a non-rebreathing system, but they did not use an HME during these anaesthetics. During studies with dry gases and a non-rebreathing valve, Caldwell, Gomez and Fritts (1969) found that their subjects lost 22 mg of water vapour per litre, whereas without the valve the exhaled water was 29 mg/litre. The latter is comparable to the 30 mg/litre which Chase, Kilmore and Trotta (1961) found was lost via an anaesthetic mask. There is reasonable agreement between the first half of table I and other reports of end-phasic temperatures in the airway (Dery et al., 1967; Whitby and Dunkin, 1969; Dery, 1973). However, the inspired relative humidities are dissimilar, particularly in the dry gases of the non-rebreathing system (Dery, 1973). This is partly the result of differences in method, as his inspiratory gas samples were not end-phasic. Sato (1961) used a dewpoint technique, obtaining mean temperatures and humidities for the whole of each phase of respiration. From these mean values he calculated the respiratory heat losses, and those he derived for the non-rebreathing system are in good agreement with the data in table III. The respiratory heat losses during use of the circle system agree less well and his lower figures could follow his use of mean phasic values, and might be related to his measurements of inspiratory and expiratory gases being made at different sites. Dery and his co-workers (1967) demonstrated that the "inert" wall of the artificial airway participated in heat and moisture exchange. With the non-rebreathing system ventilating dry gases at 10 litre/min the calculated respiratory heat losses exceeded 10 kcal per hour, and this has been confirmed when changes in total body heat were examined (Shanks, 1974). It can be seen (table III) that delivery of saturated gas reduces this respiratory loss. Tracheal conditions are improved by full humidification at room temperature. During use of the HME, the gases in Group 3 reached the trachea saturated, with an average end-inspired temperature of 32 C. Ingelstedt (1956) showed that air entering the subglottic space during nasal breathing had an average end-inspired temperature of 32.3 C with a relative humidity of 98%. However, the work of Noguchi, Takumi and Aochi (1973) would suggest that the temperature at which saturated gas is delivered is not critical between 20 and 30 C, and studies on the tracheobronchial ciliated epithelium would support this (Tayyab, Ambiavagar and Chalon, 1973). HEAT APPENDIX BALANCE CALCULATIONS (1) Gas exchange. Specific heat of a gas mixture = (wici + W2C2)/(w, + w 2 ) Weight of 1 litre of nitrous oxide at STP (wi)= g Weight of 1 litre of oxygen at STP (w,)= g Specific heat of nitrous oxide (ci) = cal/g Specific heat of oxygen (C2)=0.219 cal/g Therefore specific heat of nitrous oxide and oxygen mixture with a 3:1 ratio=0.222 cal/g. This mixture at STP would have a specific heat (c m ) of cal/litre. Heat lost by respiratory gas exchange (corrected to STP) would be: c m (inspired temperature expired temperature) cals per litre. (2) Water exchange. From the absolute humidity of the inspired gases (h ; ) is deducted the weight of the water expired (h e ). This requires 580 cal/g for its evaporation at 30 C. Timed heat loss from this cause is 0.58 (hi h e ) cal/ljtre when the absolute humidities are expressed as mg/litre. REFERENCES Boys, J. E., and Howells, T. H. (1972). Humidification in anaesthesia. Br. J. Anaesth., 44, 879.

5 THE MULTIPLE-GAUZE CONDENSER-HUMIDIFIER 777 Caldwell, P. R. B., Gomez, D. M., and Fritts, H. W. (1969). Respiratory heat exchange in normal subjects and in patients with pulmonary disease. J. Appl. Physiol, 26, 82. Chamney, A. R. (1969). Humidification requirements and techniques. Anaesthesia, 24, 602. Chase, M. R, Kilmore, M. A., and Trotta, R. (1961). Respiratory water lost via anesthesia systems. Anesthesiology, 22, 205. Dery, R. (1973). The evolution of heat and moisture in the respiratory tract during anaesthesia with a non-rebreathing system. Can. Anaesth. Soc. J., 20, 296. Pelletier, J., Jacques, A., Clavet, M., and Houde, J. J. (1967). Humidity in anaesthesiology. Ill: Heat and moisture patterns in the respiratory tract during anaesthesia with the semi-closed system. Can. Anaesth. Soc. J., 14, 287. Editorial (1970). Humidification. Br. J. Anaesth., 42, 271. Graff, T. D., and Benson, D. W. (1969). Systemic and pulmonary changes with inhaled humid atmospheres. Anesthesiology, 30, 199. Han, Y. H., and Lowe, H. J. (1968). Humidification of inspired air. J.A.M.A., 205, 907. Ingelstedt, S. (1956). Studies on the conditioning of air in the respiratory tract. Acta Otolaryngol. (Stockh.) (Suppl.), 131. Loew, D. A. Y., Klein, S. R., and Chalon, J. (1972). Volume-controlled relative humidity using a constant temperature water vaporizer. Anesthesiology, 36, 181. Mapleson, W. W., Morgan, J. G., and Hillard, E. K. (1963). Assessment of condenser-humidifiers with special reference to a multiple-gauze model. Br. Med. J., I, 300. Noguchi, H., Takumi, Y., and Aochi, O. (1973). A study of humidification in tracheostomized dogs. Br. J. Anaesth., 45, 844. Sara, C. (1965). The management of patients with a tracheostomy. Med. J. Aust., 1, 99. Sato, T. (1961). Studies in respiratory humidity. II: Humidity in anesthetic circuits and water loss via anesthesia systems. Acta. Med. Okayama, 15, 335. Shanks, C. A. (1974). Humidification and loss of body heat during anaesthesia. I: Quantification and correlation in the dog. Br. J. Anaesth., 46, (in press). Sara, C. (1973a). Airway heat and humidity during endotracheal intubation. I: Inspiration of arid gases via a non-rebreathing system. Anaesth. Intensive Care, 1, 211. (1973b). A reappraisal of the multiple gauze heat and moisture exchanger. Anaesth. Intensive Care, 1, 428. Tayyab, M. Z., Ambiavagar, M., and Chalon, J. (1973). Water nebulization in a non-rebreathing system during anaesthesia. Can. Anaesth. Soc. J., 20, 728. Toremalm, N. G. (1960). A heat-and-moisture exchanger for post-tracheotomy care. Acta Otolaryngol., 52, 461. Webb, P. (1951). Air temperatures in respiratory tracts of resting subjects in cold. J. Appl. Physiol., 4, 378. Whitby, J. D., and Dunkin, L. J. (1969). Temperature differences in the oesophagus. Br. J. Anaesth., 41, 615.

CARBON DIOXIDE ELIMINATION FROM SEMICLOSED SYSTEMS

Brit. J. Anaesth. (1956), 28, 196 CARBON DIOXIDE ELIMINATION FROM SEMICLOSED SYSTEMS BY RUSSELL M. DAVIES, I. R. VERNER Queen Victoria Hospital, East Grinstead AND A. BRACKEN Research and Development Centre,