evidence, too, that such slowing was accompanied by a fall in ph of the

506 J. Phyaiol. (1960), 152, pp. 506-514 With 3 text-figures Printed sn Great Britain CHANGES IN CARDIAC AND RESPIRATORY FUNCTION, AND IN BLOOD CARBON DIOXIDE PRESSURE AND ph, IN CATS EXPOSED TO OXYGEN UNDER HIGH PRESSURE BY D. W. TAYLOR From the Department of Phy8iology, Univer8ity of Aberdeen (Received 25 January 1960) Earlier experiments on rats (Taylor, 1958) showed that on exposure to pure oxygen under a pressure of 6 atmospheres, slowing of the heart and of respiration took place much earlier when the rats were anaesthetized with chloralose than when barbiturate was the anaesthetic. There was evidence, too, that such slowing was accompanied by a fall in ph of the arterial blood, but there was no evidence of a rise in C02 pressure before the onset of cardiac or respiratory failure. These experiments suffered from the disadvantages that the rats underwent a fairly rapid and considerable fall in body temperature, that the size and frequency of blood samples were strictly limited and that only arterial blood was sampled. It was desirable therefore to repeat and extend the work on larger animals such as cats. METHODS Young cats of both sexes (av. wt. 1-50 kg) were used in all experiments and were anaesthetized by an intraperitoneal injection of either an aqueous solution of 1 % chloralose and 10% urethane (1 ml./100 g body wt.) or with pentobarbitone sodium B.P. (Veterinary Nembutal; Abbott Laboratories Ltd.; 50 mg/100 g body wt.). Any such procedures as vagotomy and section of sinus nerves were then carried out before heparin dissolved in 0-9 % NaCl solution (5000 i.u./ml.) was injected intravenously (0.5 ml./kg body wt.). The femoral arteries were then cannulated with polythene tubing, as was the right atrium via the right jugular vein, the animal was placed in the pressure chamber, and the appropriate connexions made to recording and sampling apparatus. The method of compression was as has already been described (Taylor, 1956). Heart rate, mean arterial blood pressure, respiration, body temperature and blood ph were recorded as described by Taylor (1958). In the first seven experiments the recorded ph was that of the femoral arterial blood; thereafter, it was that of the mixed venous blood drawn from the right atrium. Blood samples were collected and analysed for total COs and 02. The CO2 pressure was calculated as in the earlier experiments, using the form of the Henderson-Hasselbalch equation and the tables given by Severinghaus, Stupfel & Bradley (1956a, b). A series of both arterial and right atrial samples could be collected in any experiment, but in any one experiment the ph could be recorded, and thus the CO2 presure calculated, for one type of sample only. A rapid flow of blood through the chamber containing the glass electrode permitted the ph to be recorded at a blood temperature not significantly different from

CHANGES IN 02 POISONING 507 body temperature, but in some cases the ph was recorded at a fixed time after sampling and the appropriate temperature corrections calculated by Rosenthal's formula (Rosenthal, 1948), the temperatures being checked by a fine thermocouple fitted into the chamber containing the glass electrode. RESULTS As in the rat experiments, abnormalities of the electrocardiogram were frequently observed. These included inverted or absent P waves, increase in P-R interval, changes in the QRS complex, and inverted or biphasic T waves. Unlike the rats, the cats showed no evidence of lung damage, other than mild hyperaemia. And, again in contrast with the rats, convulsions were observed frequently in cats anaesthetized with chloralose and urethane, and rarely when barbiturate anaesthesia was employed. Comparison of changes in cats anaesthetized with chloralose and urethane and with Nembutal. Chloralose and urethane were used to anaesthetize sixteen animals (group A) and nembutal for fifteen (group B), in each case without operative interference other than that required to permit blood sampling. In the first 7 experiments (4 from group A and 3 from group B) only femoral arterial blood samples were taken; in 3 experiments in each group, samples were taken only from the right heart, and in the remainder (9 from each group) samples were taken from both arterial and mixed venous blood. Convulsions. Comparison of the two groups showed that convulsive movements, starting about 30 min after exposure to 02 had commenced, occurred in thirteen out of sixteen animals in group A but only in two out of fifteen animals in group B. Respiration. Obvious hyperpnoea was noted in one cat only in group A but occurred in thirteen out of fifteen cats in group B. Slowing and eventual failure of the respiration occurred in less than 2 hr in ten out of sixteen animals in group A, whereas in group B only two animals showed eventually an appreciable degree of respiratory slowing. Heart rate. In all animals in group A the initial effect, 20-40 min after exposure to 02 commenced, was a slowing of the heart rate, which in eight cats decreased to 50 % or less of the initial rate within 2 hr, and in only three instances did not decrease by at least one third. In group B, however, the initial effect was in every case a slight but distinct rise in heart rate. In no case did the heart rate fall by 50 % within 2 hr and in ten out of the fifteen animals the heart rate did not decrease even by one third in 180-240 min. Thus animals in group B survived for a much longer period, on the average, than did those in group A and, in fact, a number of experiments in group B were terminated simply on account of the passage of time and not because of any apparent deterioration in the condition of the animal.

508 D. W. TA YLOR Hydrogen ion concentration. Since samples of blood were not taken at the same times in all experiments, a strict statistical comparison of ph, total CO2 and 02 was not possible. Nevertheless, in respect of the twelve cats in each group from which samples of mixed venous blood were taken, it was found that blood ph tended to be lower in group A than in group B, falling to less than 7-20 in seven cases in group A. In group B this occurred only in three cases, and it was more usual for the ph to rise early in the experiment to a plateau and later to fall to near its initial level. b, 80 60 / O-L20_ - 76 Cn 77.42 702 Z7*0 0L 68 u c 40 E 30 a.4 20 10 Eu 250 20 '4v 200 40 80 120 Time (min) Fig. 1. Graphs (from above down) of calculated CO2 pressure and ph of mixed venous blood from right atrium, respiratory rate and heart rate plotted against time from beginning of compression, for three consecutive experiments on cats anaesthetized with chloralose and urethane and exposed to 0 under a pressure of 6 atm. *, cat 14; A, cat 16; *, cat 18. Carbon dioxide press8ure of blood. Not unnaturally, the calculated blood CO2 pressure bore a close and inverse relationship to the measured ph. It was on the average a little higher in the earlier stages of experiments in group A than in group B; but even in group A large increases in CO2 were not found except when obvious respiratory failure had supervened. Of the animals, twelve in each group, from which mixed venous blood samples were taken, in 7 experiments in group A the CO2 pressure remained below

CHANGES IN 02 POISONING 509 50 mm Hg in the presence of definite respiratory slowing and, in 4 instances, of gross cardiac slowing. In group B, CO2 pressure tended to show a gradual fall concomitant with the hyperpnoea, followed late in the experiment by a slight rise. No high values were recorded in this group, and as has been noted already respiratory failure did not occur. Three v m"i V 0. >r. 80 60 40 20 0 76 74 72 7-0 68 40 30 20 10 0 300 C E- 250-0 1200 I 1 50 100 40 80 120 160 200 240 Time (min) Fig. 2. Graphs (from above down) of calculated CO2 pressure and ph of mixed venous blood from right atrium, respiratory rate and heart rate plotted against time from beginning of compression, for three consecutive experiments on cats anaesthetized with Nembutal and exposed to 02 under a pressure of 6 atm. *, cat 15; A, cat 17; *, cat 19. animals showed some degree of cardiac slowing without any rise in CO2 pressure. These changes in cardiac and respiratory rates, and ph and CO2 pressure of mixed venous blood are illustrated graphically in Figs. 1 and 2 for 3 consecutive experiments from each group. Oxygen content of blood. When, neglecting time of sampling, all blood samples were taken together and compared, it was found that there was no clear difference between the two groups in 02 content of arterial blood samples, but that the 02 content of the mixed venous blood samples from group B was lower than that from group A by 3 vol. % on average. Effects of cutting both sinus nerves. In 4 experiments, 2 with each

510 D. W. TA YLOR anaesthetic, both sinus nerves were cut and after being tested by carotid compression and observing the blood pressure, the animals were exposed to oxygen in the usual way. This procedure did not apparently modify the results which have been described above. Both animals given chloralose E 160 E 200,A(341) ; @ 80 ) CL 40 0 t 30_ 72) a v10 _sw \ E~~~~~~ 10 3 -- >I 1 7100 40 40 120 Time (min) Fig. 3. Graphs (from above down) of calculated 002 pressure and ph of mixed venous blood from right atrium, respiratory rate and heart rate plotted against time from begbning of compression for four bilaterally vagotomized cats exposed to 0, under a pressure of 6 atm. Cat 32, *, and cat 33, *, were anaesthetized with chioralose and urethane; cat 34, 0, and cat 35, 0, were anaesthetized with Nembutal. and urethane suffered cardiac and respiratory failure in less than 2 hr, one of them exhibiting convulsive movements. Neither Nembutaltreated animal became convulsed, nor showed any evidence of respiratory failure within 3 hr, but in one of them the heart had slowed by about one third at the end of that time. After section of the sinus nerves heparinization of the animal was undesirable, and thus blood samples were not taken

CHANGES IN 02 POISONING 511 in these 4 experiments and in the 6 experiments described immediately below. Effects of cutting both sinus nerves and both vagus nerves. In 6 experiments, 3 with each anaesthetic, both sinus nerves and both vagus nerves were cut. All three cats given chloralose and urethane showed convulsive movements, and heart and respiration failed within 80-100 min. A fall in heart rate was, however, not obvious initially but took place very suddenly after 80-90 min. No Nembutal-treated cat became convulsed, two showed no apparent deterioration after 31 hr and in one there was a slowing of heart and respiratory rates of about one third after 150 min. Effects of cutting both vagus nerves only. In 11 experiments, 4 with chloralose and urethane and 7 with Nembutal, both vagus nerves were cut. In all 4 chloralose experiments, and in 3 with barbiturate, blood samples were taken for analysis. All four chloralose-treated animals had convulsive movements, and three of the four showed evidence of circulatory and respiratory collapse in 1-2 hr, with a rapid and severe fall in blood ph, and a rise in C02 pressure to very high levels, of the order of 100-200 mm Hg, relatively early after exposure to 02 had begun. Of the seven cats given Nembutal five showed evidence of respiratory failure; the heart rates tended to slow slightly, then to increase, and then to slow more abruptly as respiration failed, the pattern thus differing from those in chloralose-treated animals. As respiration failed, blood ph decreased rapidly and C02 pressure rose steeply. These were the only Nembutaltreated cats of the whole series which behaved in this way, and three exhibited convulsive jerking movements. These results are illustrated for two animals from each group in Fig. 3. DISCUSSION The results which have just been described confirm and extend those obtained when using rats and presented in an earlier paper (Taylor, 1958). There was the same difference in the cat as in the rat between animals anaesthetized with chloralose and urethane and those given barbiturate, in the time taken for an appreciable degree of cardiac and respiratory slowing to develop. The role of the vagus nerves in the early onset of bradycardia, emphasized by Whitehorn & Bean (1952) was again difficult to determine on account of early respiratory failure in the majority of vagotomized animals. In the three cats anaesthetized with chloralose and urethane and subjected to section of both sinus nerves and bilateral vagotomy early cardiac slowing was less obvious than usual. Nevertheless, some degree of cardiac slowing in the absence of any large change in respiratory rate certainly took place in three vagotomized animals in 60-80 min from the beginning of exposure to 02- Conversely, it is hard to

512 D. W. TAYLOR believe that the absence of cardiac slowing in animals given Nembutal, but otherwise intact, is due to abolition of vagal function by the anaesthetic, when section of both vagi in such animals resulted in clear and immediate changes in respiration. As in the earlier experiments on rats (Taylor, 1958) there was no apparent connexion between changes in heart rate and changes in arterial blood pressure and section of the sinus nerves with or without vagal section did not alter the differences between chloralose- and Nembutal-treated animals. The results of the blood gas analyses seem to show clearly that considerable cardiac slowing and some degree at least of respiratory slowing can precede any changes in C02 pressure as indicated by the measurements made. Bean (1945) has stated that CO2 pressure of arterial blood is not necessarily an indication of that obtaining in the tissues. The values obtained in the present experiments from analyses of arterial and mixed venous blood agree well with those for arterial and jugular-vein blood of Lambertsen and his colleagues, who have discussed in great detail the relation between arterial and venous blood CO2 pressures and 'central' C02 pressure (Lambersten, Kough, Cooper, Emmel, Loeschke & Schmidt, 1953a, b; Lambertsen, Stroud, Ewing & Mack, 1953). When respiration continues to slow, however, a point seems to be reached when CO2 clearance is no longer adequate and the CO2 pressure then rises, as occurred eventually in the majority of animals given chloralose and urethane. The convulsions so frequently observed in these animals are equally unlikely to be due to a primary rise in CO2 pressure, since they regularly occurred before any significant change in the CO2 pressure was observed, although they presumably contributed to the CO2 load in such cases. In Nembutal-treated animals, on the other hand, the early changes in ph and total C02, and thus in CO2 pressure, pointed to a respiratory alkalosis as a result of the hyperpnoea, and only in those cases where vagotomy resulted in an initial and independent impediment to respiration was there a failure of adequate CO2 excretion. It was not possible to obtain blood from the jugular vein by the methods used in these experiments and, failing such, it is impossible to saywhether or not the observed difference in 02 content of the mixed venous blood between the groups implied a difference in cerebral capillary 02 pressure, the importance of which has been stressed by Lambertsen et al. (1953a). The greater desaturation of the mixed venous blood after Nembutal may have been a result of effort involved in the hyperpnoea, although it is not obvious that this activity entailed a greater 02 consumption than the convulsions of chloralose. It is beyond the purpose of this paper to discuss why animals given Nembutal should exhibit early respiratory failure after bilateral vago-

CHANGES IN 02 POISONING513 tomy but not when this procedure was combined with bilateral section of the sinus nerves, especially as no control observations were made in animals not exposed to oxygen. Von Euler & Soderberg (1952) stated that chloralose strongly depressed the central effects of CO2 and the results, as a whole, seem to show that the key to the differences between the two groups probably lies, as Moruzzi (1955) implied, in a difference in the central control of respiration under the two anaesthetics. Further investigation might therefore be most usefully devoted to studying the influence of a combination of anaesthetic and 02 under high pressure on the functioning of cerebral cortex and respiratory centre. SUMMARY 1. The effects of oxygen under a pressure of 6 atmospheres on cardiac and respiratory rates, and on the CO2 pressure and ph of mixed venous and arterial blood samples, were studied in cats anaesthetized with chloralose and urethane and with Nembutal. 2. There was an obvious difference, confirming earlier experiments on rats, in the average time required for an appreciable degree of slowing ofthe heart and respiration to develop, the changes occurring much sooner in cats given Nembutal. 3. These differences were not affected by bilateral section of the sinus nerves with or without bilateral vagotomy. 4. There was no evidence of a rise in CO2 pressure in the blood before the onset of respiratory failure. 5. When bilateral vagotomy only was performed the differences between the two groups largely disappeared as a result of early respiratory failure in the cats given Nembutal. In both groups of animals CO2 pressure in the mixed venous blood rose to very high levels. REFERENCES BEAN, J. W. (1945). Effects of oxygen at increased pressure. Phy8iol. Rev. 25, 1-147. LAMBERTSEN, C. J., KOUGH, R. H., COOPER, D. Y., EMMEL, G. L., LOESCEKE, H. H. & SCHMIDT, C. F. (1953a). Oxygen toxicity. Effects in man of oxygen inhalation at 1 and 3-5 atmospheres upon blood gas transport, cerebral circulation and cerebral metabolism. J. appl. Phy8iol. 5, 471-486. LAMBERTSEN, C. J., KOUGH, R. H., COOPER, D. Y., EMMEL, G. L., LOESCHKE, H. H. & SCHMIDT, C. F. (1953b). Comparison of relationship of respiratory minute volume to PCO2 and ph of arterial and internal jugular blood in normal man during hyperventilation produced by low concentrations of CO2 at 1 atmosphere and by 02 at 3 0 atmospheres. J. appl. Phy8iol. 5, 803-813. LAMBERTSEN, C. J., STROUD, M. W., EWING, J. H. & MACK, C. (1953). Oxygen toxicity. Effects of oxygen breathing at increased ambient pressure upon pco2 of subcutaneous gas depots in men, dogs, rabbits and cats. J. appl. Physiol. 6, 358-368. MORUZZI, G. (1955). Les mecanismes nerveux des convul8ions causees par l'oxygene. Ministero della difesa-marina. ROSENTHAL, T. B. (1948). The effect of temperature on the ph of blood and plasma in vitro. J. biol. Chem. 173, 25-30.

514 D. W. TA YLOR SEVERINGHAUS, J. W., STUPPEL, M. & BRADLEY, A. F. (1956a). Accuracy of blood ph and Pco, determinations. J. appl. Phy8iol. 9, 189-196. SEVERINGHAUS, J. W., STUPPEL, M. & BRADLEY, A. F. (1956b). Variations of serum carbonic acid pk' with ph and temperature. J. appl. Phy8iol. 9, 197-200. TAYLOR, D. W. (1956). The effects of vitamin E and of methylene blue on the manifestations of oxygen poisoning in the rat. J. Phy8iol. 131, 200-206. TAYLOR, D. W. (1958). Changes in cardiac and respiratory rates, and in carbon dioxide pressure and ph of arterial blood, in anaesthetized rats exposed to oxygen under high pressure. J. Physiol. 143, 149-164. VON EuLER, C. & S6DERBERG, U. (1952). Medullary chemosensitive receptors. J. Phy8iol. 118, 545-554. WMTEHOPRN, W. V. & BEAN, J. W. (1942). Cardiac changes induced by 02 at high pressure, CO2 and low 02, as manifest by the electrocardiogram. Amer. J. Phy8iol. 168, 528-537.