THE EFFECT OF INTUBATION ON THE DEADSPACE DURING HALOTHANE ANAESTHESIA

Transcription

1 Brit. J. Anaesth. (1969), 41, 94 THE EFFECT OF INTUBATION ON THE DEADSPACE DURING HALOTHANE ANAESTHESIA BY M. L. KAIN, J. PANDAY AND J. F. NUNN SUMMARY Total functional deadspace (including apparatus deadspace) was measured in anaesthetized patients while breathing from masks and after tracheal intubation. The mean VD/VT ratio was 0.18 larger during mask breathing and the mean value for the total functional deadspace was 82 ml greater than in the intubated patients. The results showed an increase in the VD/VT ratio during the inhalation of higher concentrations of halothane: the difference closely approached but did not attain the 95 per cent confidence limit. Values for VD/VT ratio and VD were uninfluenced by the duration of anaesthesia up to 120 minutes. Many previous workers have reported values for physiological deadspace occurring during anaesthesia (Nunn and Hill, 1960; Askrog et al., 1964; Marshal, 1966; Cooper, 1967a, b). Almost all have chosen to study intubated patients who, in most cases, were ventilated artificially. The volume of the upper respiratory tract and face mask was therefore excluded and little is known about the importance of this factor under the conditions of anaesthesia. Nunn, Campbell and Peckett (1959) suggested that the anatomical volume of the upper respiratory tree (above the carina) was about 70 ml, or more with the head extended. This was challenged by Marshall (1966) who found an increase in physiological deadspace of 27 ml following extubation. However, since he used a nose clip and a mouthpiece of the type used in physiological studies, his results are not stricdy relevant to clinical anaesthesia. Thus there remains some doubt as to the effect of intubation on the deadspace under the conditions of routine anaesthesia. In an attempt to resolve this doubt we have measured the deadspace of anaesthetized patients using alternatively mask or endotracheal tube. We report values for what we have called the total functional deadspace. This equals that part of the tidal volume which does not take part in gas exchange as determined by solution of the Bohr equation (using Pco, of arterial blood) without making the customary subtraction of the estimated apparatus deadspace. It thus equals the sum of the physiological deadspace and the apparatus deadspace (functionally measured). METHOD Patients and anaesthesia. Twelve patients (table I) who were free from clinical evidence of cardiovascular or respiratory disease were selected for this study, which was carried out during routine surgery. They were premedicated with an appropriate dose of papaveretum and hyoscine (papaveretum mg) 1 hour before induction. Anaesthesia was induced with thiopentone (mean dose 250 mg) and mg of suxamethonium chloride was given prior to intubation. Anaesthesia was maintained with a concentration of per cent halothane in oxygen (inspired gas), the patients being allowed to breathe spontaneously throughout the procedure. The project was discussed with the patients prior to premedication and their consent to the study was obtained. M. L. KAIN,* M.D.; J. PANDAY, M.B., B.S., F.F.A.R.C.S.; J. F. NuNN,t PH.D., F.F.A.R.C.S.; Department of Anaesthesia, University of Leeds, England. * Dr. Kain was killed in a motor accident on August 9, 1967, and all correspondence should be addressed to Professor J. F. Nunn. Dr. Kain was supported by U.S.P.H.S. Fellowship No. 1-F3-GM-31, , National Institute of General Medical Sciences. t Present address: Medical Research Council, Division of Anaesthesia, Royal Postgraduate Medical School, London, W.I2.

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3 96 BRITISH JOURNAL OF ANAESTHESIA Sampling Tube\ FIG. 1 Non-rebreathing gas circuit used in the study. The Steen valve had a lower opening pressure than the inspiratory leaf of the Frumin valve. General procedure. Results are presented in two parts. In Series I, the concentration of halothane in the inspired gas was varied between 1 and 2 per cent as required to maintain a steady level of anaesthesia in the face of varying surgical stimulation. In Series II, the concentration of halothane in the inspired gas was deliberately varied in an endeavour to find out whether this influenced the deadspace. An anaesthetic circuit used is illustrated in figure 1. This consists of a non-rebreathing circuit with provision for the collection of expired gas. Surplus fresh gas was prevented from mixing with the expired gas by allowing it to vent through the Steen valve which had a lower opening pressure than the inspiratory leaf of the Frumin valve. The radial artery was cannulated after induction of anaesthesia using a teflon cannula and introducer (Becton, Dickinson "Longdwel" 1 mm i.d.). Arterial blood was collected in a heparinized syringe, the blood being withdrawn slowly over a period of 2 minutes, during the collection of each sample of expired gas. Analytical techniques. Arterial blood was analysed for ph and Pco 2, using the micro-astrup (Radiometer, N.V.), technical details being those described by Kelman, Coleman and Nunn (1966). Expired gas was collected in a 30-litre Douglas bag, collections lasting 4 minutes. Respirations were counted over this period for calculation of tidal volume from minute volume measured with a dry gas meter. The fractional concentration of carbon dioxide in the mixed expired gas was measured with an infra-red gas analyser (URAS, Hartman and Braun), calibrated against two known concentrations of carbon dioxide mixtures immediately prior to the measurement of each sample. The patient's nasopharyngeal temperature was measured while an endotracheal tube was in place. The temperature of the expired gas was also recorded just prior to measurement. Errors. There is no significant systematic error in the determination of Pco 2 by our technique but the random error has a standard deviation of 1.7 mm Hg (Kelman, Coleman and Nunn, 1966). For gas analysis by infra-red absorption the corresponding value was found to be 1.4 mm Hg. Errors in measurement of temperature and gas volumes are relatively unimportant. Data processing. Results were entered on a protocol sheet designed for ease of preparation of a data tape. This was run on the University of Leeds English Electric KDF9 computer with a program (written by M.L.K.) to derive the Pco 2, numerically corrected for time and temperature (Kelman and Nunn, 1966) and to solve the Bohr equation with correction of gas volumes to body temperature and pressure (saturated). The Bohr equation was in the form proposed by Enghoff (1938): VD=VE Pa C02 No allowance was made for estimated apparatus deadspace. RESULTS All individual values are listed in table I. For the whole study (both series) the mean value for VD/VT, while breathing from a mask, was ( 0.084). While breathing from an endotracheal tube, the mean value for VD/VT was ( 0.122). The difference between the two ratios was highly significant (P<0.001). Corresponding values for the total functional deadspace were 196 ( 57) ml during mask breathing (mean VT 312 ml) and 114 ( 49) ml after intubation (mean VT 246 ml), the difference between the mean values being 82 ml and highly significant (P<0.001).

4 EFFECT OF INTUBATION ON DEADSPACE DURING ANAESTHESIA 97 Mask Tube Difference (X,-X 2 ) t P TABLE II Values for VD/VT ratio {Series I). No of observations < Series I. In Series I the difference in VD/VT ratio was between mask and tube breathing (table II), the difference being highly significant (P<0.001) even though the two groups of observations were treated statistically as unpaired. The difference in VD was 70 ml which was also highly significant (P<0.001). VT was 331 ml for mask group and 296 ml for intubated group, but the difference was not significant. Series II. Results in Series II are presented separately for high and low halothane concentrations (table III). In each case the difference was of the same order as that found in Series I and was highly significant. Taking all the patients of Series II together, the mean value of VD was 179 ( 49) ml while breathing from a mask and 90 ( 26) ml after intubation. The difference of 89 ml was highly significant (P<0.001). The mean tidal volume of the mask group was 300 ( 93) ml but only 218 ( 47) ml after intubation. This difference was highly significant (P<0.001). Effect of halothane. Effect of halothane concentration on deadspace was examined by taking paired values of the mean VD/VT ratio for each patient during the inhalation of high and low halothane concentrations. Observations during mask and endotracheal tube breathing were treated separately (table IV). TABLE IV Paired values of mean VD/VT percentage at high and low inspired halothane concentrations (Series II). Patient No. TUBE Patient No. MASK Percentage halothane High t not significant Low t not significant Difference Difference Tube Mask Difference (X.-X,) t P No. of observations TABLE III Values for VD/VT ratio (Series II). Inspired halothane 0-1 per cent per cent < No. of observations <

5 98 BRITISH JOURNAL OF ANAESTHESIA A% Halothane -o: FIG. 2 Changes in VD/VT ratio following changes in concentration of halothane in the inspired gas. Changes are in the same direction in 23 instances out of 28. In the mask group, VD/VT ratio was increased at the higher concentrations of halothane by a mean value of and, in the intubated group, by Neither difference attained the 95 per cent confidence level although the difference in the intubated group closely approached it. The effect of changes in the inspired concentration of halothane on VD/VT are displayed in figure 2. In 23 instances out of 28 the changes in per cent halothane and VD/VT were in the same direction. Sequential measurements of VD/VT in patients 2910 and 2913 are shown in figure 3. Figure 4 shows the effect of a progressive decrease in inspired halothane concentration on the VD/VT ratio. The points on this graph are taken from results obtained during the latter part of our study on patient Here the patient was intubated after the operation had finished and measurements were made in the absence of any surgical stimulation or blood loss. The concentration of inspired halothane was altered in one direction only (progressively decreased) so that the change in alveolar concentration would be in the same direction. With these considerations in mind it was felt that the number of variables would be reduced to a minimum and a clearer picture of the effect of halothane would emerge, although the study as a whole was not designed for this purpose. Effect of time. Figure 5 shows mean values of VD/VT for all the patients in Series II plotted against time. halothane concentrations were calculated at each point (20-minute intervals). All values for VD/VT obtained during mask breathing have been decreased by a factor of 0.18 to render the data compatible with those obtained after intubation. In figure 6 the mean VD values are calculated in a similar way and plotted against time. Ventilatory response. Simultaneous values for arterial Pco 2 and ventilation are shown in figure 7 for all patients in Series I and II, with mean values indicated for intubated and non-intubated patients. There was no significant difference in mean Pco 2 between the two groups but the mean ventilation of the patients breathing from a mask was 0.96 l./min higher than in the intubated patients. This difference was significant (P<0.05). Mask otube «0 200 Time mm Time min FIG. 3 Serial changes of VD/VT ratio in patients 2910 (left) and 2913 (right). The numbers within the graph refer to the concentration of halothane in the inspired gas.

6 EFFECT OF INTUBATION ON DEADSPACE DURING ANAESTHESIA h Volumes (ml) FIG. 4 Progressive changes of VT and VA during the latter part of the study of patient The figures in the graph refer to the concentration of halothane in the inspired gas. The mean value for the total functional deadspace was 70 ml, measured values ranging from 53 to 78 ml. The patient was intubated V l/min 5 : #. o *. * 8 o. o « Paco2 mm Hg FIG. 7 Simultaneous values for arterial Pco 2 and ventilation. The large points indicate mean values. There is a significant difference in the ventilation of the two groups but not in their Pco Time min FIG. 5 values for VD/VT ratio plotted against time (Series II). The numbers in the graph refer to the mean concentration of halothane in the inspired gas o Mask: R =»0-84, P<0001 Tube: R=*0-72, P< ml 50 " > Time min FIG. 6 values for total functional deadspace plotted against time (Series II). FIG. 8 Regression lines of VD on VT. Correlations between paired values are highly significant in both groups. Extrapolated intercept on y axis is not significantly different from zero in either case.

7 100 BRITISH JOURNAL OF ANAESTHESIA Dependence of VD/VT on VT. All simultaneous values of VD and VT are plotted in figure 8 together with the regression lines of VD on VT calculated separately for intubated and non-intubated patients. The line for intubated patients passes close to the intersection of the ordinates, while for non-intubated patients the intercept on the VD axis is 28 ml, but statistically this is not significantly different from zero. DISCUSSION When the geometric values of anatomical and apparatus deadspace are considered, the difference between intubated and non-intubated patients would seem to be as follows: Intubated patient (a) Vol. of endotracheal tube and catheter mount 25 ml (b) Upper airway 0 (c) Lower airway (below carina) 70 ml* Total (* Nunn and Hill, 1960) 95 ml Patient breathing from mask (a) Vol. of mask (on face) and chimney piece 125 mlf (fc) Upper airway (extended head) 90 ml$ (c) Lower airway 70 ml Total 285 ml (f cf. Clarke, 1958) ($ Nunn, Campbell and Peckett, 1959) The difference so calculated is of the order of 200 ml and appears greater than clinical practice suggests, since it implies that with tidal volumes of less than 300 ml patients would inevitably be in ventilatory failure if breathing from a face mask. This is clearly untrue, as tidal volumes of less than 300 ml are commonplace in nonintubated anaesthetized patients. In this study, the mean difference in total functional deadspace between the intubated and non-intubated patients was found to be of the order of 82 ml. The implications are thus less serious than consideration of geometric values would suggest. At first sight it is surprising that the measured values for total functional deadspace are so much less than the total geometric deadspace (anatomical + apparatus). This is probably due to a streamline flow of gas and the mixing effect of the heartbeat (Nunn and Hill, 1960). It is likely that streamline flow is also a factor in external breathing apparatus. Effect of changes in concentration of haloihane in inspired gas. Although this study was not specifically designed to investigate the effects of different concentrations of halothane, the results obtained from patients in Series II suggest that there may be a direct relationship between the inspired concentration of halothane and the VD/VT ratio (figs. 2, 3, 4). Observations on patient 2913 (fig. 3, right) show a fall of VD/VT ratio with decrease of halothane concentration which cannot be explained by any other factor known to influence the deadspace. Further study of the effect of changes in halothane concentration is required and reference should be made to alveolar or arterial levels of halothane rather than the concentration in the inspired gas as in this study. Effect of duration of anaesthesia. Askrog and associates (1966) showed that there was a progressive increase in physiological deadspace with halothane anaesthesia and controlled ventilation, although this has not been confirmed by Cooper (personal communication). In this study the mean values for VD/VT and VD were measured at 20-minute intervals. Corrections were applied to "mask" values in order to present the results of the whole group as "intubated" values (figs. 5 and 6). There was no indication that, over a period of 120 minutes, the VD/VT ratio or the total functional deadspace altered in either direction. With so many factors apparently able to influence the deadspace, the effect of time can only be resolved by a study planned to control all variables. Ventilatory response to additional deadspace. During mask breathing, the increase in total functional deadspace (of the order of 82 ml) was accompanied by a mean increase in tidal volume of the order of 65 ml. The patients thus appeared able to meet the challenge of rebreathing by raising their ventilation almost enough to maintain a

8 EFFECT OF INTUBATION ON DEADSPACE DURING ANAESTHESIA 101 constant alveolar ventilation. The difference in mean Pco 2 in the present study was small (fig. 7) suggesting that patients anaesthetized under the conditions of this study would not have their alveolar ventilation appreciably impaired as a result of failure to intubate. This accords with the observation by Kain and Nunn (1968) that rebreathing from a gas circuit usually results in a markedly raised ventilation with only slight increase in Pco 2. It is, however, important to remember that the ventilatory response may be diminished by deep anaesthesia, excessive narcotic premedication or in certain forms of respiratory disease. Under these circumstances, additional deadspace, from whatever cause, might cause serious increases in Pec. Derivation of alveolar ventilation from minute volume. It is convenient to consider deadspace as a ratio of the tidal volume (VD/VT), but it is not established over what range of tidal volume the ratio remains constant. Figure 8 shows simultaneous values of VD and VT for intubated and nonintubated patients. Correlation coefficients for both groups are highly significant and the calculated regression lines are shown. The line for the intubated patients passes close to zero suggesting that VD/VT may be considered as a constant under these conditions. In contrast the calculated regression line for the mask-breathing patients does not pass through zero but the intercept is based on an extrapolation and in any case is not significant. There is thus no evidence to suggest that it may be inappropriate to consider the VD/VT ratio as constant under these conditions. The fact that no allowance was made for variation in VD or VD/VT due to body size, age, sex or the effect of surgical stimuli during the collection of this data, contributes to the rather wide distribution of points about the regression lines. The correlations are, however, highly significant (P<0.001) and it seems reasonable to use the VD/VT ratio as a rough guide for the calculation of alveolar ventilation from measured minute volumes. For intubated patients the alveolar ventilation averages about half the measured minute volume, the fraction ranging from a third to two-thirds in most patients. These figures agree well with the data of Cooper (1967b). For practical purposes the ratio of alveolar ventilation to minute volume may be considered constant over the range of tidal volumes ml. The frequency varied between 12 and 31 b.p.m. in this study but had no significant effect on VD/VT ratios. For non-intubated patients the alveolar ventilation averages only a third of the minute volume, the fraction generally being within the range quarter to half. This figure is perhaps surprisingly low and the effect of additional deadspace is clearly an important factor in the definition of level of adequacy of ventilation in non-intubated anaesthetized patients. Alveolar ventilation would be further diminished by a gas circuit which permitted rebreathing. ACKNOWLEDGEMENT We wish to thank Professor J. C. Goligher for permission to study patients under his care. REFERENCES Askrog, V. F., Pender, J. W., Smith, T. C, and Eckenhoff, J. E. (1964). Changes in respiratory dead space during halothane, cyclopropane, and nitrous oxide anesthesia. Anesthesiology, 25, 342. Clarke, A. D. (1958). Potential deadspace in an anaesthetic mask and connectors. Brit. J. Anaesth., 30, 176. Cooper, E. A. (1967a). Physiological dead space in passive ventilation. 1: Outline of a study. Anaesthesia, 22, 90. (1967b). Physiological dead space in passive ventilation. 2: Relationships with tidal volume, frequency, age, and minor upsets of respiratory h:alth. Anaesthesia, 22, 199. Enghoff, H. (1938). Volumen inefficax. Bemerkungen zur Frage des shadlichen Raumes. Upsala Ldk.- Foren. Fork. 44, 191. Kain, M. L., and Nunn, J. F. (1968). Fresh gas economics of the Magill circuit. Anesthesiology, 29, 964. Kelman, G. R., Coleman, A. J., and Nunn, J. F. (1966). Evaluation of a microtonometer used with a glass ph electrode. J. appl. Physiol., 21, Nunn, J. F. (1966). Nomograms for correction of blood Po 2, Pco 2, ph, and base excess for time and temperature. J. appl. Physiol., 21, Marshall, B. E. (1966). Physiological shunting and deadspace during spontaneous respiration with halothane-oxygen anaesthesia and the influence of intubation on the physiological deadspace. Brit. J. Anaesth., 38, 912. Nunn, J. F., Campbell, E. J. M., and Peckett, B. W. (1959). Anatomical subdivisions of the volume of respiratory dead space and effect of position of the jaw. J. appl. Physiol, 14, 174. Hill, D. W. (1960). Respiratory dead space and arterial to end-tidal CO 2 tension difference in anesthetised man. J. appl. Physiol, 15, 383.

9 102 BRITISH JOURNAL OF ANAESTHESIA L'EFFET DE L'INTUBATION SUR L'ESPACE MORT AU COURS DE L'ANESTHESIE PAR L'HALOTHANE SOMMAIRE L'espace mort fonctionnel total (y compris l'espace mort de l'appareillage) a ete mesure chez des malades anesthesies pendant la respiration au masque et pendant l'intubation. Le rapport moyen VD/VT etait de 0.18 plus important pendant la respiration au masque et la valeur moyenne pour l'espace mort fonctionnel total etait de 82 ml plus grand que chez les malades intubes. Les resultats montrent une augmentation du rapport VD/VT pendant l'inhalation de concentrations plus importances d'halothane: la difference approche de pres, mais n'atteint pas tout a fait 95% de la limite de confidance. Les valeurs du rapport VD/VT et le VD n'etaient pas influencees par une duree d'anesthesie depassant 120 minutes. DER EFFEKT DER INTUBATION AUF DEN TOTEN RAUM WAHREND DER HALOTHAN- NARKOSE ZUSAMMENFASSUNG Der gesamte funktionelle toten Raum (einschliefilich des toten Raums des Apparates) wurde bei anasthesierten Patienten einmal unter Maskenatmung und dann nach trachealer Intubation gemessen. Wahrend der Maskenatmung war das durchschnittliche VD/VT- Verhaltnis 0,18 grofier und der Mittelwert fur den gesamten funktionellen toten Raum war 82 ml grofier als bei den Patienten nach Intubation. Die Resultate zeigten eine Erhohung des VD/VT-Verhaltnisses wahrend der Inhalation hoherer Halothankonzentrationen: die Differenz kam dem 95-Prozent-Confidence- Limit nahe, erreichte dasselbe jedoch nicht. Die Werte fiir das Verhaltnis von VD/VT sowie von VD blieben bis zu 120 Minuten Narkosedauer unbeeinflufit. BOOK REVIEW Aspects of Resuscitation {Proceedings of the Second International Symposium on Emergency Resuscitation). Edited by Ivar Lund and Bjorn Lind. Acta Anaesthesiologica Scandinavica (1968), Supplementum XXIX. Universitetsforlaget 1 Aarhus. This monograph of approximately 400 pages is an account of the proceedings of a four-day international symposium held in Oslo in the summer of The book is divided into seven sections containing between one and six separate papers each followed by a discussion. The first section entitled "Basic considerations on asphyxia" is excellent and will almost certainly become compulsory reading for all with a direct interest in the problems of resuscitation. The main emphasis of the book is directed towards cardiac resuscitation and this is apparent in the handling of the next three sections on emergency resuscitation, on myocardial infarction, and on drowning. It probably also accounts for the absence of any detailed discussion on resuscitation after burns and scalds, after road accidents and in situations when mass casualties occur. Two further sections deal with the problems associated with the organization of emergency resuscitation and with the education of first-aid workers and public service employees such as policemen and firemen as well as doctors and nurses. The final section on legal and ethical aspects has been greatly influenced by the development of organ-transplant surgery and the differing views expressed reflect the anxiety felt by many doctors about the criteria used to establish death. As a result of their deliberations the participants in this symposium drew up a list of recommendations, most of which are eminently reasonable and sensible. But when such recommendations are made several questions must occur to the reader. Who is to receive them and who is to implement them? More important perhaps, who is to pay for their implementation? No doubt, in an ideal society there will be a medically directed central ambulance-dispatching system capable of selecting hospitals and ambulance vehicles according to individual needs, and all ambulance attendants will be properly trained in the care of the unconscious patient. But with our present inability to provide enough hospital beds and our failure to train enough nurses the reader is bound to ask just how realistic are such recommendations. Most societies cannot find funds to provide enough homes for healthy people let alone provide a special service to resuscitate them when they are nearly dead, and the question of priorities must surely arise. And even if additional finance is available for the health services is it justified to divert funds for the benefit of a relatively small section of the community when other larger sections in need of help are neglected? Or is it simply that in order to be properly treated you need to be nearly dead? These are only some of the questions that a monograph of this nature tends to raise and for this reason alone it deserves to be read. J. P. Payne