CONTROL OF ACIDOSIS IN HYPERBARIC OXYGENATION

Gabriel G. Nahas Departnient of Anesthesiology, College of Physicians and Surgeons

Columbia University, New York, N . Y.

Introduction

A major problem in the clinical application of hyperbaric oxygenation is the occurrence of convulsions during exposure to O2 pressures of three atmospheres absolute or above, levels considered most effective to increase tissue oxygenation. The duration of exposure before the onset of convulsions varies considerably from one subject to another (from 20 to 60 minutes) and onset can be post- poned by central nervous system depressants, such as barbiturates and chlor- promazine or, as reported by N. Haugaard in another paper in this monograph, by y-aminobutyric acid (GABA) . The factors involved in the genesis of convul- sions during hyperbaric oxygenation are not known and there has been much discussion of the role played by an impairment of COz transport.

The experimental work presented here, performed in collaboration with C. Sanger, was intended to study the effect of a titrating agent, tris(hydroxymethy1) aminomethane (THAM) on convulsion time in mice exposed to high 0 2 pres- sures.' THAM will maintain within normal limits acid-base balance of the body over short periods of time in the presence of a respiratory or metabolic a c i d ~ s i s . ~ , ~ At p H 7.40, THAM is 70 per cent ionized and its nonionized fraction will diffuse into the various compartments of the body and can titrate intracellular acid.4 By contrast, the administration of sodium bicarbonate to maintain acid-base equilibrium requires an increase in ventilation in order to eliminate the free C 0 2 formed. Furthermore, bicarbonate ion diffuses very slowly throughout the differ- ent body compartment^.^

Methods and Results

Four groups of mice were exposed to O2 pressures of 3, 3.8, and 4.7 atmos- pheres absolute. One group was given intraperitoneally 1 ml. of a 0.3 M solution of THAM per 30 gm. of body weight (1.2 gm./kg. or 10 mM/kg.), and the three control groups were given a similar volume either of saline, or of 0.3 M NaHC03 or of 0.3 M THAM titrated to pH 7.40 with HCI. FIGURE 1 represents the cumulative frequency distribution curves of the convulsion times observed curing exposure to a pressure of 3 atm. abs. (30 psi). The 50 per cent or median convulsion time in the animals treated with THAM was double the median time in the saline control group (31 and 61.5 minutes). Median convdlsion time was 20.5 minutes in the group injected with bicarbonate and 43.5 in those injected with titrated THAM. Note, of course, the very wide range of values within each group, a variability which, as stated above, is also observed in man. When the mice were exposed to 4.7 atm. abs. ( 5 5 psi), the 50 per cent convulsion time was 8.5 min. in the group treated with THAM as compared to 3 min. in the saline control group, and 6 min. in the two other groups (FIGURE 2) .

In another series of experiments, 24-hour survival was followed in mice ex- posed to 3 atm. abs. for 45 min. or to 4.7 atm. for 16 min. The experimental group was given an injection of 0.3 M THAM ( 1 m1./30 gm. B. W. per injection) 6 hours, 3 hours, and 20 minutes before compression. Survival rate was greatly increased in the treated group, an observation also reported by others (TABLE 1).

Similar oxygen toxicity studies by other investigators were carried out at the 174

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FIGURE 1 . Cumulative frequency distribution curves of convulsion time in mice during exposure to an oxygen pressure of 3.0 atm. abs.' (series I, 30 psi).

same time as the experiments described above. Kylstraa reported that THAM intraperitoneally administered protected Swiss mice from the convulsions of hyperbaric oxygenation. In his experiments, the dose of THAM was 1.8 gm./kg. (14 mM/kg.) and mice were exposed to a pressure of 8 atm. abs. Gottlieb' also reported that 0.3 M THAM (1.2 gm./kg.) injected in the peritoneum protected Swiss mice from the central nervous system symptoms of 0 2 toxicity at 4 atm. abs. Bean8 found that intraperitoneal administration of THAM delayed con- vulsions in rats exposed to 8 atm. of 0 2 . He also reported that this therapy im- proved survival and decreased the severity of pulmonary damage.

Discussion

The mechanism of THAM's protective effect against the acute manifestations of O2 toxicity can be interpreted in various ways. A significant decrease in O2 uptake was observed in mice injected with THAM during exposure to high O2 pressures (TABLE 2) . This decrement in 0 2 uptake, which reflects a decrease in metabolism, could be a factor in delaying the appearance of convulsions. Another interpretation, perhaps, could be based on the fact that THAM forms chelates with rare metals, such as Cu and Zn.O However, such chelations could only occur intracellularly and the protective effect is observed after a short interval, when only small amounts of THAM could have penetrated into the cells.

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Treatment No. of animals 3 a m . (45 min.) 4.7 atm. (16 min.)

18 10 2 18 17 17

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FIGURE 2. Cumulative frequency distribution curves of convulsion time in mice during exposure to an oxygen pressure of 4.7 atm. abs.’ (series I, 55 psi).

THAM THAM-Cl Saline NaHCO, pH 10.2 PH 7.40

2.29 1.86 1.68 2.27

Per cent of control

TABLE 2 OXYGEN UPTAKE (co2) IN MICE (ML./GM./HR.)

- 81% 73 % 99%

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Time

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7.5

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FIGURE 3. pH changes and exchange of CO, in plasma and dialysate during peritoneal dialysis in dogs. The dialysis fluid contained 150 mM/1. THAM, 80 mM/L NaCI, 4 mM/1. KCI and 40 mM/L glucose. The dosage was 30 ml./kg. B.W.

A third factor, namely, a marked fall in arterial pC02, has been observed in dogs receiving a similar intraperitoneal administration of THAM (TABLE 3). After 15 minutes pC02 decreased by 10 mm. Hg. and was maintained at that level for one hour, a time interval corresponding to the observation period used in the experiments on mice exposed to high O2 pressure. Additional studies of peritoneal dialysis in dogs indicated that THAM diffuses slowly across the peritoneum, more slowly than COz diffuses from the plasma into the dialysate (FIGURES 3 and 4) . The passage of THAM across the peritoneum into the blood and the formation of HCOs in the dialysate from the exchange with the endoge- nously produced COz was also observed. In these experiments, 30 ml./kg. of a solution of THAM containing tracer amounts of CI4-tagged THAM was ad- ministered intraperitoneally and the dialysate acted as “a COP trap” for more than one hour.

In another study, performed in association with H. Spalter,lo changes of the retinal vessels of the fundus were recorded in the dog during COz inhalation. The

778 Annals New York Academy of Sciences

60 50

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0 60 120 I80 240 300 360 420 MI NUT ES

Urine excretion of THAM and exchange of THAM and CO, in plasma and dialysate during peritoneal dialysis in dogs. The dialysis fluid contained 150 mM/1. THAM, 80 mM/1. NaCI, 4 mM/1. KCl and 40 rnM/1. glucose. The dosage was 30 ml./kg. B.W.

FIGURE 4.

vasodilation of the retinal vessels produced by the increase in pC02 could be readily corrected by administration of THAM (FIGURE 5 ) .

All these observations indicate that the lowering of K O 2 in blood and tissue might be an important factor in THAM’s protective effect against O2 toxicity. Such an interpretation need not conflict with Lambertsen’s,ll who demonstrated that although there is sufficient C 0 2 accumulation to account for the hyperpnea that occurs at high O2 tensions, convulsions were, in fact, caused by a direct action of O2 on the brain cells. However, the p02 of the brain is a function of cerebral blood flow and of the relative diameter of the brain vessels which are, in turn, influenced by the P C O ~ . ~ ~ If this hypothesis is correct, therapy during hyperbaric oxygenation should include, perhaps, hyperventilation produced voluntarily by the patient or by means of mechanical ventilation.

While THAM protects against the acute toxicity of high O2 pressures, it had no protective effect in rodents breathing pure 0 2 at atmospheric pressures (FIG-

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DAYS FIGURE 6 . Mortality rate of rats during one month of breathing 95 to 98 per cedt 0,.

Most of the deaths occurred in the first four days.

URE 6). Following the early work of Gershman,13 Matteo and Nahasl* observed in rodents marked changes in the lungs and in the gonads within 48 hours after exposure to pure O2 (FIGURE 7) . Neither THAM nor any other agent has afforded protection in these instances. It should also be recalled that exposure to enriched 0 2 mixtures at atmospheric pressure in man is accompanied by early changes in the capillary vessels of the lung followed by severe pulmonary c o n g e s t i ~ n . ~ ~

However, the administration of THAM will delay the onset of convulsions during exposure to O2 pressures of three atmospheres or higher. This experi- mental work would be of little clinical interest if THAM had not proven during the past five years in clinical medicine to be a safe and useful treatment for many forms of acidosis.16 There have been many well-documented reports in Ameri- can and foreign publications describing the use of THAM in open-heart surgery, cardiovascular collapse, pediatrics, severe burns," and barbiturate and salicylate intoxication. Dosage, method of administration, measurements of acid-base parameters, and other physiological variables, as well as precautions and contra- indications, are comprehensively discussed in these reports.

Conclusions

( 1 ) Intraperitoneal injections of THAM delay the onset of convulsions and increase the survival of rodents exposed to hyperbaric 0 2 .

(2) We have interpreted the mechanism of such a protection to be the antacid, basic property of THAM, which can lower pCO2 in tissue as well as in blood and also titrate acid metabolites produced as a result of 0 2 toxicity. In addition, the chelating properties of THAM with Zn and Ca might be operative.

784 Annals New York Academy of Sciences

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(3) However, cellular acidosis is only one aspect of O2 toxicity and the pro- tection afforded by THAM is only temporary. THAM is by no means an antidote and the multiple mechanisms involved that may contribute to O2 toxicity are not yet known.

(4) THAM does not protect rodents against the effects of exposure to high concentrations of O2 at atmospheric pressure.

(5) THAM might be used in association with clinical hyperbaric oxygenation, especially when acidosis is likely to be present, such as in cases of shock or respiratory distress of the newborn. Finally, as further discussed in Benichoux’s paper (see Pages 787-793), THAM might also be useful in surgical procedures associated with temporary apnea or with temporary occlusion of venous return.

References 1. SANGER, C., G. G. NAHAS, A. R. GOLDBERG & G. M. D’ALLEsro. 1961. Effects of 2-amino-

-2-hydroxymethyl-l,3-propanediol on oxygen toxicity in mice. Ann. N. Y. Acad. Sci.

2. NAHAS, G. G. 1963. The clinical pharmacology of THAM [tris (hydroxymethy1)amino- methane]. Clin. Pharmacol. Therap. 4: 784.

3. NAHAS, G. G. 1962. The pharmacology of tris(hydroxymethy1)aminomethane (THAM). Pharmacol. Rev. 14: 447.

4. HOLMDAHL, M. H., & G. G. NAHAS. 1962. Volume of distribution of C“ labeled tris (hydroxymethyl) aminomethane. Am. J. Physiol. 202: 101 1.

5. ROBIN. E. D., R. J. WILSON & P. BROMBERG. 1961. Intracellular acid-base relations and intracellular buffers. Ann. N. Y. Acad. Sci. 92: 539.

6. KYLSTRA, H. 1962. Personal communication. 7. GOTTLIEB, S. F. & R. V. JOGODZINSKI.

92: 710.

1963. Role of THAM in protecting mice against convulsive episodes caused by exposure to oxygen under high pressure. Proc. SOC. Exptl. Biol. Med. 112: 427.

1961. Tris buffer, CO, and sympatho-adrenal system in reaction to 0, at high pressure. Am. J. Physiol. 201: 737.

8. BEAN, J. W.

9. AUYOUNG, N. & G. G. NAHAS. 1963. Unpublished observations. 10. SPALTER, H. & G. G. NAHAS. 1964. Unpublished observations. 11. LAMBERTSEN, C. J., R. H. KOUGH. D. Y. COOPER, G. L. EMMEL, H. H. LOESCHCKE & C. F.

SCHMIDT. Comparison of relationship of respiratory minute volume to pC0, and pH of arterial and internal jugular blood in normal man during hyperventilation oroduced bv low concentrations of Co.. at 1 atmosohere and bv 0, at 3 atmosoheres.

1953.

- - j . Appl. Physiol. 5: 803.

12. KEN. S. S. & C. F. SCHMIDT. 1948. Effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J. Clin. Invest. 27: 484.

13. GERSCHMAN, R., A. E. ARGUELLES & D. 1. IBEAS. Effects of high oxygen tensions of mammalian gonads. (abst. No. 357). Proc. 22nd Intern. Congr. Physiol. Sci. 2.

14. MATTEO, R. S. & G. G. NAHAS. Sodium bicarbonate: Increase in survival rate of rats inhaling oxygen. Science 141: 719

IS. PRATT, J. 1964. Tolerance of the human lung to enriched 0, mixture. Ann. N. Y. Acad. Sci. (in press).

16. HENSCHLER, D. 1963. The therapeutic use of THAM [tris( hydroxymethyl) aminometh- ane; Tris.]. Deut. Med. Wochschr. 8: 423.

17. HINSHAW, J. R., L. M. CRAMEB & H. C. MILLER. 1964. Tris buffer in the treatment of severely burned patients. Surgery (in press).

1962.

1963.