Oxygen convulsions are believed by many workers to be caused by an accumulation

272 J. Physiol. (I949) I09, 272-280 6I2.223.II:6I2.26I THE ROLE OF CARBON DIOXIDE IN OXYGEN POISONING BY H. J. TAYLOR From the Royal Naval Physiological Laboratory, Alverstoke, Hants (Received 26 March 1948) Oxygen convulsions are believed by many workers to be caused by an accumulation of carbon dioxide in the body, one suggestion being that the oxyhaemoglobin is not reduced in the capillaries in contact with the tissues, and this interferes with the mechanism for the transport of carbon dioxide. In addition, there is some evidence that the toxic properties of oxygen are increased as the tension of carbon dioxide breathed at the same time is increased. Campbell (1929) showed that animals breathing oxygen showed a rise in carbon dioxide tissue tensions but made no systematic study, particularly with regard to rate of rise and value reached in convulsions. Taylor (1949), studying the effect on cats of breathing oxygen at atmospheric pressure, obtained evidence to support the view that carbon dioxide plays some part in the toxic action of oxygen. The object of the following experiments was to seek further information on the effect of oxygen under pressure on the behaviour of carbon dioxide in the tissues. METHODS Oxygen and carbon dioxide tensions in the tissues were measured by the method of injection, developed by Campbell (1924), of 40-50 ml. of nitrogen subcutaneously. The gas gradually reached equilibrium with the tissue gases. Depots of gas were thus established, samples withdrawn when required for analysis by the method described by Campbell & Taylor (1935), and the oxygen and carbon dioxide tensions calculated. If the animal was under increased pressure it was decompressed to atmospheric pressure before a sample was taken. Cats were used and the nitrogen was injected subcutaneously in the back. In this site the gas does not spread as it does if it is injected subcutaneously in the region of the abdomen. 20-30 ml. of nitrogen were injected and the gas left for at least 5 hr. in order that equilibrium with the tissue gases should be established. The animal was then placed in a small cylindrical pressure chamber of capacity 1381. and the oxygen pressure raised to the required value. Control experiments were carried out by subjecting animals to equivalent pressures in air. In order that the nitrogen in the injected bubble should not be rapidly removed the nitrogen in the chamber was not washed out of the chamber by oxygen so that the partial pressure of nitrogen remained constant at about 610 mm. Hg. As soon as a convulsion occurred the gas in the chamber was rapidly decompressed to atmospheric pressure, the animal removed, and a sample of the subcutaneous gas taken for analysis. Values were thus obtained for the tissue carbon dioxide and oxygen tensions when a convulsion occurred.

CARBON DIOXIDE IN OXYGEN POISONING 273 Next the rate of rise of oxygen and carbon dioxide tensions in the tissues under these conditions was determined. Two methods were employed. (a) A series of determinations of oxygen and carbon dioxide tissue tensions after varying times of exposure to high oxygen pressure were made. A bubble of nitrogen was injected into the animal and when equilibrium with the tissue gases was established the animal was placed in oxygen at the required pressure for 0 5 hr. It was then decompressed and a sample of gas removed for analysis. The animal was left for 24 hr. for tissue gas tensions to return to normal and the experiment repeated. The time of exposure to oxygen under pressure was, however, lengthened to 1 hr. before decompressing and taking the gas sample. This was repeated for gradually increasing times of exposure until a convulsion intervened. (b) The results obtained by (a) may be open to objection on the grounds that day to day variations in the state of the animal and of the ambient conditions may produce variations in the rate at which equilibrium is achieved. The experiment also took considerable time and the following procedure was therefore adopted. An animal, with a bubble of gas in equilibrium with the tissues was placed in the pressure chamber in oxygen at the required pressure for 0.5 hr. It was then rapidly decompressed, a sample of gas withdrawn for analysis and the oxygen pressure raised to its original value as soon as the animal was back in the chamber (the whole operation occupied less than 1*5 min.). At the end of another 05 hr. the animal was again decompressed and another gas sample taken for analysis. This procedure was carried out until the animal convulsed. The value found at the end of the second 0-5 hr. exposure was taken as being due to a total compression time of 1 hr. and so on. The whole experiment was repeated using different oxygen pressures. In order to test the effect of various factors, such as intermittent rapid decompression and recompression, added carbon monoxide, and added carb6n dioxide to the inspired oxygen, on the oxygen convulsion time of an animal, it was necessary to 'calibrate' the animal for convulsion time in oxygen alone. This was done by exposing the animal on alternate days for about 2-3 weeks to a certain oxygen pressure and noting the time of exposure required to produce an oxygen convulsion. The effect of the various factors on convulsion time can then be estimated. 40 be - I30 E 20 - E 20 E10 0 50 100 150 200 Time (min.) 50-40,,-* I 30- E 2Q E 10l,,. I Fig. 1. 0 10 20 30 40 Time (min.) Changes in oxygen and carbon dioxide tensions in the tissues after the injection of nitrogen. Above, oxygen; below, carbon dioxide. RESULTS Equilibrium rate of subcutaneous gas bubbles. Fig. 1 shows the rate at which equilibrium between the nitrogen bubble and the tissue gases is reached. This equilibrium is established in about 18-20 min. for carbon dioxide and about

274 H. J. TAYLOR 2-5 hr. for oxygen. The normal values for cats, breathing air, in the subcutaneous region used, were found to be 28-32 mm. Hg for oxygen and 40-43 mm. Hg for carbon dioxide. Oxygen and carbon dioxide tissue tensions after an oxygen convulsion. The experiment was carried out as described previously and the results are given in Table 1. The results agree with the observations made by Campbell (1929) that TABLE 1. Values of carbon dioxide and oxygen tissue tensions after convulsion Oxygen Carbon dioxide Pressure Convulsion Calculated Calculated Cat of oxygen time tension tension no. (lb./sq.in.) hr. min. Percentage* (mm. Hg) Percentage (mm. Hg) 1 55t 3 14 12-3 410*0 7*2 246*0 2 55 3 05 11*8 393 0 6-4 213-0 3 55 2 45 9-03 301-0 5*5 183-0 4 55 2 55 10*3 343*0 6*0 200-0 5 55 3 45 12-3 410-0 6-8 226-0 6 55 2 45 12*9 430.0 6-1 203-3 * The normal value for an animal breathing air is just under 5% for oxygen and just over 5% for carbon dioxide. t The value of 55 lb. abs. was chosen because at this pressure the convulsion time is comparatively long. This is necessary as the t,ime taken for the oxygen tension in the bubble to reach equilibrium with the tissues is a slow process, and with short convulsion times the value obtained will not even approximate to the true value. It is probable that the value given for oxygen tension in Table 1 will, for this reason, be always on the low side. exposure of rabbits to high oxygen tensions results in increased tissue carbon dioxide tensions. The values given are calculated on the assumption that during decompression little or no change occurs in the oxygen or carbon dioxide concentration in the bubble. The time of decompression is so short (30 sec.) that little change could be expected in that time (see Fig. 1). It should be pointed out that the figures for oxygen and carbon dioxide tissues shown in Table 1 are lower than the real tensions in the tissues since under these conditions equilibrium is not established. The results, however, give a good indication that the rise in these tissue tensions is a significant one. The results of control experiments on cats taken to equivalent pressures in air are shown in Table 2. They show that although a small rise in carbon dioxide and oxygen tensions was found when air is used instead of oxygen at high pressure, these rises are in no way comparable with the figures obtained in oxygen. TABLE 2. Effect of breathing air under pressure on oxygen and carbon dioxide tissue tensions Calculated Calculated Time of tissue tissue Cat Pressure exposure tensions tensions no. (lb./sq.in.) hr. mi. Percentage (mm. Hg) Percentage (mm. Hg) 1 67 3 14 1-30 44*0 1-40 47-3 2 67 3 05 1-34 45.5 1-60 54-3 4 67 55 1-27 43-1 1X60 54X3* 6 67 2 45 1-21 41-1 1-3 44

CARBON DIOXIDE IN OXYGEN POISONING 275 The rate of rise of carbon dioxide and oxygen gas tensions in the tissues. Experiments were carried out on three cats by methods (a) and (b) described previously. The results for one animal are shown in Fig. 2. They show the rate of change of oxygen and carbon dioxide tensions in the bubble and the equivalent calculated tissue tensions. 0.2~~~\ O nit n 1doo g, I o 100 0 100 200 300 400 250 > = O200 -* d 50 Time (min.) 0 10 200 300 400 Time (min.) Fig. 2. Rate of rise of tensions of oxygen and carbon dioxide in the tissues during the breathing of oxygen at high pressure. Above, oxygen; below, carbon dioxide; broken line, oxygen 55 lb./ sq.in., nitrogen 12 lb./sq.in.; continuoue line, oxygen 43 lb./sq.in., nitrogen 12 lb./sq.in. 'Calibrated' The effect of intermittent rapid decompression on convulsion time. cats with a reasonably constant convulsion time were selected for this experiment. These were exposed to oxygen at high pressure, were decompressed and then rapidly recompressed at approximately 30 min. intervals. The time taken from beginning of decompression to end of recompression was approximately 1-5 min. The results are recorded in Table 3. No further test of significance is TABLE 3. Effect of intermittent decompression and compression in convulsion time Catno.......... 1 2 7 6 Pressure of oxygen 55 lb./sq.in. Convulsion time (min.) 225 185 135 165 195 225 150 170 180 149 120 165 210 210 127 120 225 235 210 135 195 - Mean 207 216-5 153*7 150-9 Standard deviation 19-6 28*4 34*8 19.6 Coefficient of variation (%) 9.5 11.3 22-6 13-0 Convulsion time with 230 220 150.160 decompresion (min.) Difference from mean +25 +3-5 -3*7 +941 Difference (%) +11.1 +1.6-2-4 +6.0 PH. CIX. 18

276 H. J. TAYLOR required for these figures, since in no case does the observed difference from the mean exceed two standard deviations. In fact, in only one case does it exceed one standard deviation. The correctness of the assumption made when using method (b) (above) for estimating the rate of rise of tissue oxygen and carbon dioxide tensions is thus supported. Effect of carbon monoxide on oxygen convulsion time. Whole blood exposed to a partial pressure of oxygen of three atmospheres has 6 ml. % of oxygen in simple solution. This is the normal amount lost per 100 ml. of circulating blood in man and some other animals at rest. In these circumstances the needs of the body for oxygen can therefore be satisfied without any reduction of oxyhaemoglobin. Thus the potential alkalinity of haemoglobin is completely lost from the point of view of carbon dioxide transport. If part of the oxyhaemoglobin is replaced by carboxyhaemoglobin and sufficient oxygen is supplied in solution in the plasma to satisfy the normal oxidation processes in the body it may be that the toxic effect of oxygen in the system would be unaffected. Experiments were carried out with this in mind. Animals whose convulsion times were well known (selected animals) were exposed to an atmosphere of oxygen (55 lb./sq.in.) in which the relative partial pressures of carbon monoxide: oxygen are 1: 506. Since the relative affinity for haemoglobin of carbon monoxide and oxygen is 310: 1, then one would expect that under these conditions the relative amounts of carboxyhaemoglobin and oxyhaemoglobin would be approximately 3: 5. Table 4 gives the results. It will be seen that the time to convulsions is scarcely altered, i.e. the toxicity of this particular pressure of oxygen is not appreciably modified. TABLE 4. Effect of added carbon monoxide on oxygen convulsion time Convulsion time when Mean convulsion breathing CO in time at 55 lb./sq.in. concentration of 02% of oxygen CO at same 02 pressure Cat no. hr. min. hr. min. 1 3 30 3 30 5 3 3 15 2 3 20 3 29 6 2 27 2 50 The effect of added carbon dioxide on the oxygen convulsion time of animals. This effect has been studied by many workers and the hastening of convulsions at a given high oxygen pressure has been noted. In particular, Haldane & Spurway (1943) emphasize this fact. These authors carried out experiments on man and they concluded that 'the effect of carbon dioxide in enhancing the toxicity of high pressure oxygen begins at a partial pressure corresponding to about 3% of one atmosphere'. Table 5 shows the effect of added carbon dioxide upon the time to convulsion of a cat breathing oxygen at pressure. It will be noted that the convulsion time of this cat shortens to 10-13 min. and does not decrease any more, however much carbon dioxide is added. All animals used

CARBON DIOXIDE IN OXYGEN POISONING 277 in this experiment (4) show exactly the same effect, whatever the original convulsion time in pure oxygen. In other words, the animals all had a minimum convulsion time and further addition of carbondioxide had no effect. Pratt (1944) found an exactly similar effect by increasing the oxygen pressure. He exposed cats to increasing oxygen pressures and noted the convulsion times. These fall to a minimum at about 80-100 lb./sq.in. when further increase of oxygen pressure fails to decrease the time (approx. 10-12 min.). TABLE 5. Effect of added carbon dioxide on convulsion time of a cat exposed to high oxygen pressure (55 lb./sq.in.) Partial pressure of added carbon dioxide Convulsion time (% of 1 atmosphere) (mm.) 0 31, 41, 38, 30, 26, 32, 42, 25 1-40 42, 25 2-35 15, 23, 18 3-30 16, 15, 22 4-70 12 7-00 10 1030 13 12-20 13 Two other animals exposed to 63 lb./sq.in. of oxygen together with carbon dioxide of approximately 11 % of an atmosphere show a minimum convulsion time of 3.5-5 min. These cats were exposed to oxygen pressures of 21 lb./sq.in., 29 lb./sq.in. and 36 lb./sq.in. together with 11 % of an atmosphere of carbon dioxide and again the animals convulsed in 3 5-5 mi. These oxygen pressures alone failed to produce oxygen convulsions, however long (up to 2 hr.) the animal was exposed. This is of interest in that Taylor (1949) has shown that convulsions may be produced in cats breathing oxygen at atmospheric pressure and that carbon dioxide tissue tensions in such animals have a high value. Recent work shows that the subcutaneous injection of carbon dio'xide (about 300 c.c.) into cats also has some effect in shortening their convulsion time in oxygen. DISCUSSION A full discussion on the etiology of reactions to oxygen at high pressure is to be found in reviews of the subject by Stadie, Riggs & Haugaard (1944) and Bean (1945); Bert (1878) and subsequent workers have shown that the outstanding sign of the effects of exposure to high oxygen pressures is a convulsion associated with respiratory difficulty. Thompson (1889) suggested that an accumulation of carbon dioxide in the body as a result of an interference with normal diffusion processes was responsible for these convulsions. Hill & Macleod (1903) also produced evidence to show that compressed air impeded the diffusion of carbon dioxide in the lungs. Gesell (1923) pointed out that a failure of reduction of oxyhaemoglobin should interfere with the normal function of haemoglobin in the transport of carbon dioxide from the tissues by 18-2

278 H. J. TAYLOR the blood. This would lead to an accumulation of carbon dioxide in the tissues. As pointed out earlier on in this paper, the amount of oxygen dissolved in the plasma when blood is exposed to oxygen at a pressure of three atmospheres (45 lb./sq.in.) is sufficient for the normal needs of the body; there will then be no reduction of oxyhaemoglobin. The base provided by the reduction of oxyhaemoglobin, and which normally serves for the transport of most of the carbon dioxide from the tissues would, under these conditions, not be available. The first direct evidence of accumulation of carbon dioxide in the tissue was provided by Campbell (1929). He measured oxygen and carbon dioxide tissue gas tensions by the method described earlier in this paper. Campbell & Hill (1931) suggested that it is the accumulation of carbon dioxide which causes the convulsion, but that this increased carbon dioxide tension may be due to vasoconstrictor effect of oxygen. Hill (1933) showed that a preliminary exposure of an anmal to a mixture of 5% carbon dioxide and 95% oxygen lowered the critical pressure at which convulsions occurred. On the other hand, Shaw, Behnke & Messer (1934) reported that anaesthetized animals when exposed to high oxygen pressure and caused to over-ventilate artificially still gave signs of oxygen poisoning. This over-breathing should lead to a fall in alveolar carbon dioxide and thus to a fall in tissue catbon dioxide tension. Behuke & Shaw (1937) therefore concluded that carbon dioxide had little effect on the production of oxygen poisonin'g. The assumption that tissue carbon dioxide and alveolar carbon dioxide are always interdependent would obviously not be valid if a breakdown in carbon dioxide transport from the tissues took place. In fact, under these latter conditions one would expect a lowered alveolar carbon dioxide content. Bohr & Bean (1942) subjected tracheotomized, urethanized or decerebrate cats to controlled artificial respiration in oxygen at pressures of six atmospheres. They found that over-breathing postponed the onset of reactions to high pressure oxygen. This may be explained on the basis of an increased carbon dioxide diffusion gradient from the plasma to the alveolar gas. On the other hand, this over-ventilation would hasten the uptake of oxygen. If the rate of attainment of a certain oxygen partial pressure in the tissues controlled the convulsion time one would expect this over-ventilation to hasten the onset of a convulsion. Further evidence of the part carbon dioxide plays in oxygen poisoning is provided by the work of Bean (1929, 1931) who found a definite decrease in pulse rate in dogs subjected to 4-5 atmospheres of oxygen which he compared to a similar result from the administration of carbon dioxide. The present work produces confirmatory evidence of the rise in tissue carbon dioxide tensions produced by exposure to oxygen at high pressure. This carbon dioxide tension reaches its maximum some time before a convulsion occurs and this would suggest that if a rise of carbon dioxide is a causative factor in producing a convulsion the critical factor is not a particular value of the carbon

CARBON DIOXIDE IN OXYGEN POISONING 279 dioxide tension but rather of this value and the time the tissue tension is raised. However, it may be that in other tissues, notably the brain, the rate of rise of carbon dioxide tissue tension is different. Experiments on the tissue gas tensions in the brain under similar conditions of high oxygen pressure are being undertaken. It will also be noted, in the experiments described, that exposure to air at equivalent pressures, raises the oxygen and carbon dioxide tissue tensions to a small extent. These values, however, are not comparable with those obtained by breathing oxygen at the same pressure. The small rise in carbon dioxide tissue tension may be due to interference with the diffusion of carbon dioxide in the alveoli owing to the increased density of the inspired gases. This effect would also come into play when oxygen at high pressure is breathed but, as suggested by Gesell (1923), is probably insignificant compared with the interference with carbon dioxide transport. No effect on convulsion time was found when cats breathed a mixture of carbon monoxide and oxygen under pressure so that the calculated carboxyhaemoglobin: oxyhaemoglobin ratio was 3: 5. These convulsions had the appearance of normal oxygen convulsions. It thus appears that oxygen transported by oxyhaemoglobin has little effect on the production of oxygen convulsions, but on the other hand from the point of view of carbon dioxide transport it would be of little moment whether the haemoglobin were present as oxy- or carboxyhaemoglobin. Since carrying out this work it has come to my notice that Lorrain Smith (1899) has already considered this point. He tried to show that the toxic effect of oxygen was related to its tension in the inspired air, and not to its quantity in the blood. By the latter expression he presumably meant the relative quantity of oxyhaemoglobin, and he does not take into account the oxygen dissolved in the plasma. He found that larks developed convulsions as usual when exposed in a chamber containing 04 % carbon monoxide as well as three atmospheres of oxygen, although, as he says, 'the arterial blood at the end of the exposure was only 38% saturated with oxygen'. SUMMARY 1. The work of Campbell (1929), who showed that when animals are exposed to high oxygen partial pressure then both oxygen and carbon dioxide tissue tensions reach high values, is confirmed. 2. Intermittent rapid decompression and compression have little effect on the rate of rise of these gaseous tensions in the tissues, and have no effect on the resultant time of exposure required to produce convulsions (in cats) compared with that of continuous exposure. 3. Breathing a concentration of 02 % carbon monoxide such that, by calculation, the relative amounts of carboxyhaemoglobin and oxyhaemoglobin were 3: 5 (apprpx.) did not modify the time to convulsions of animals exposed to high oxygen partial pressures.

280 H. J. TAYLOR 4. The well-known effect of carbon dioxide on the time required to produce oxygen convulsions is further discussed. Carbon dioxide given by injection was also found to have some effect. Much lower carbon dioxide tensions than those usually considered to be effective are shown to produce a shortening of the time required for the onset of oxygen convulsions. This effect is most marked in cats whose convulsion time in oxygen, without added carbon dioxide, is comparatively long. The convulsion time was found to reach a limiting value however much carbon dioxiide was added. I wish to express my gratitude to Dr C. L. G. Pratt for all the help and advice he has given during the course of this work. REFERENCES Bean, J. W. (1929). Proc. Soc. exp. Biol., N.Y., 28, 832. Bean, J. W. (1931). J. Phyiol. 72, 27. Bean, J. W. (1945). Phy8iol. Rev. 25, 1. Behnke, A. R. & Shaw, L. A. (1937). Nav. med. Bull., Wa8h., 35, 61. Bert, P. (1878). La Presion Barometrique. English translation by M A. Hitchcock and F. A. Hitchcock (1943). Columbus: Ohio College Book Co., U.S.A. Bohr, D. F. & Bean, J. W. (1942). Fed. Proc. 1, 8. Campbell, J. A. (1924). J. Phy8iol. 59, 1. Campbell, J. A. (1929). J. Phy8iol. 68, vii. Campbell, J. A. & Hill, L. (1931). J. Physiol. 71, 309. Campbell, J. A. & Taylor, H. J. (1935). J. Physiol. 83, 219. Gesell, R. (1923). Amer. J. Phy8iol. 66, 5. Haldane, J. B. S. & Spurway, H. (1943). Personal communication. Hill, L. (1933). Quart. J. exp. Phy8iol. 23, 49. Hill, L. & Macleod, J. J. R. (1903). J. Hyg.,(Camb., 3, 401. Pratt, C. L. G. (1944). Personal communication. Shaw, L. A., Behnke, A. R. & Messer, A. C. (1934). Amer. J. Physiol. 108, 652. Smith, L. (1899). J. Phy8iol. 24, 19. Stadie, W. C., Riggs, B. C. & Haugaard, N. (1944): Amer. J. med. Sci.. 207, 84. Taylor, H. J. (1949). J. Phy8iol. 108, 264. Thompson, W. G. (1889). Med. Bet. 36, 1.