STUDIES ON DYSBARISM I. DEVELOPMENT OF DECOMPRESSION SYNDROME IN GENETICALLY OBESE MICE WILLIAM ANTOPOL, M.D.; JOHN KALBERER, JR., M.S.; SAMUEL KOOPERSTEIN, M.D.; STEPHEN SUGAAR, M.D., AND CHRYSSANTHOS CHRYSSANTHOU, M.D. From the Joseph and Helen Yeamans Levy Laboratories, Beth Israel Hospital, and the Medical Department of the Port of New York Authority, New York, N.Y. Since nitrogen is at least 5 times more soluble in fat than in other tissues,l" the proportion of adipose tissue in the body influences the amount and rate of nitrogen released into the bloodstream after rapid decompression from high atmospheric pressure.5 It was the purpose of this investigation to find a modality in which the decompression syndrome could be produced regularly in small ani- mals, so that a great number of them could be exposed simultaneously to pressure. In view of the influence of adipose tissue in the decompres- sion syndrome, genetically obese mice were employed in these studies. Decompression illness ("bends") could be produced in the obese mice but not in their normal nonobese siblings or other strains of normal mice. These facts are especially significant in the light of recent reports cor- relating air crew obesity with fatal cases of dysbarism.8 MATERIAL AND METHODS Hereditary obese hyperglycemic mice of both sexes, 3 to 6 months of age were used. There were 2 weight ranges, 2I to 38 gm. (average 32 gm.) and 38 to 65 gm. (average 54 gm.), and, in addition, corresponding thin siblings weighing I7 to 27 gm. (average I9 gm.). The mice were obtained from the Roscoe B. Jackson Memorial Laboratory, Bar Harbor, Maine. In addition, thin yellow mice (YBR/W obtained from Dr. J. W. Wilson, Brown University, Providence, Rhode Island), C57 black, C58, and DBA mice were employed. The pressure chamber used for compression was 4 feet, 7 inches long and 2 feet, 4 inches inside diameter. The pressure was automatically controlled and could be maintained at any predetermined level up to go psi (pounds per square inch), ab- solute. Decompression to atmospheric pressure could be accomplished in less than i minute. Air entering the chamber was oil-free. An adjustable air-cooling-circulating system and a humidity control provided constant temperature (72 ± 20 F.) and relative humidity (50 per cent). All the animals were housed in metal cages in animal rooms with controlled tem- perature (71 + 20 F.) and relative humidity (50 per cent) and were fed Purina Laboratory Chow and water ad libitum. This work was sponsored by the Aerospace Medical Division, Air Force Systems Command, U.S.A.F. under Contract #AF 41(609)-1557, and aided by the United States Public Health Service, the Saul Singer Foundation and the Charles H. Silver Fund. Accepted for publication, February io, I964. lI5 I I6 ANTOPOL ET AL. VoI. 45, No. z A total of 438 obese and thin mice of varying weight range, in a series of 32 separate experiments, was placed in the pressure chamber and exposed to 75 psi air pressure (gauge) for 6 hours and then were rapidly decompressed (in less than I minute). Tables I, II and IV show the type, weight range, and number of mice used in each experiment. In I 2 of the experiments, mice of different types or weight ranges were subjected to compression-decompression simultaneously to insure identical experimental conditions. These experiments are designated in the tables by the same experiment number. Following decompression, the mice were observed for at least 2 hours for clinical manifestations. Roentgenologic examination was performed in some mice before and at varying intervals after decompression, in order to observe the progressive accumulation of gas in various tissues and organs. When a mouse died, the survival period from the time of decompression was recorded. In several experiments some animals were sacrificed immediately after decompression. To eliminate the possibility of postmortem formation or fusion of bubbles, necropsy was performed in all cases immediately after death and with minimal manipulation. Sections of various organs were examined histologically. Several obese and thin mice which survived compression-decompression were sacrificed 4 to I2 months after decompression. In some of these, histologic examina- tion of the femur or sternum was performed. In addition, one group of 20 obese mice ranging in weight from 52 to 6o gi. (average 55 gm.) was subjected to 75 psi for 3 hours instead of the usual 6 hours and then rapidly decompressed. Another group of 20 obese mice was subjected to 6o psi for 6 hours and then rapidly de- compressed. RESULTS Obese Mice Shortly after decompression the obese mice exhibited difficulty in breathing, scratching, decreased motor activity, and most of them died within 30 minutes. In those animals which succumbed, severe respiratory distress, with panting, gasping and hiccough-like spells, and wobbling, twitching and erratic running about were observed several seconds pre- ceding death. The mortality of obese mice in the high weight range (39 to 65 gm.), based on a total of I75 mice in I9 separate experiments was 90.8 per cent (Table I). More than 8o per cent of these died less than 30 min- utes after decompression. In the intermediate weight range (2I to 38 gm.) 6o obese mice in separate experiments exposed to identical condi- tions had a mortality of 53.3 per cent (Table II). Mice of the same strain but of different weight ranges, exposed to the same experimental condi- tions had a varied response (Table III). Obese mice of high weight range, exposed to 75 psi for 3 hours (or to 6o psi for 6 hours) and fol- lowed by rapid decompression, had a lower mortality (not exceeding 6o per cent) than animals of same weight range exposed to 75 psi for 6 hours. Gross examination of obese mice which succumbed to treatment re- vealed gas bubbles in the subcutaneous and intra-abdominal fat, the July, 1964 DECOMPRESSION SYNDROME II7 spleen and adrenals. The stomach and intestine were distended with great amounts of gas (Fig. i). The inferior vena cava was usually found filled with air bubbles, as were the right atrium and ventricle; this TABLE I MORTALITY OF OBESE MICE (39 TO 65 GM.) SUBJECTED TO COMPRESSION-DECOMPRESSION Total Number of dead mice Exper. no. of Minutes after decompression no. mice 0-IO 10-30 30-60 over 6o Total I 8 4 3 I 0 8 6 4 0 2 0 0 2 7 4 2 2 0 0 4 8 4 2 2 0 0 4 9 4 2 2 0 0 4 10 5 I 3 0 0 4 II 24 7 5 6 0 18 12 14 3 9 0 0 I2 13 29 20 6 2 0 28 14 4 0 3 I 0 4 15 8 3 5 0 0 8 I6 4 0 4 0 0 4 I7 10 I 8 0 0 9 18 10 7 2 I 0 IO I9 6 o 6 o o 6 20 12 2 4 2 I 9 21 6 I 2 3 o 6 22 9 4 5 0 0 9 23 10 2 7 0 I 10 Total 175 6I 80 I6 2 I59 (9o.8)% TABLE II MORTAITrY OF OBESE MICE (2I TO 38 GM.) SUBJECTED TO COMPRESSION-DECOMPRESSION Total Number of dead mice Exper. no. of Minutes after decompression no. mice 0-10 10-30 3o-60 over 60 Total 2 I5 4 4 I 0 9 3 15 5 3 0 0 8 4 15 4 5 0 I 10 5 15 0 3 I I 5 Total 6o I3 I5 2 2 32 (53.3%) TABLE III RELATIONSHIP OF WEIGHT TO MORTALITY IN DECOMPRESSION SICKNESS Weight Average No. range weight of MIortality Strain (gm.) (gm.) mice (%) Obese hyperglycemic 39-65 54 175 90.8 Obese hyperglycemic 2I-38 32 6o 53.3 Thin siblings 17-27 19 125 0 I I8 ANTOPOL ET AL. Vol. 45, No. z could be demonstrated in roentgenograms (Fig. 2). In only rare in- stances were bubbles found in the left chambers of the heart. Bubbles were not grossly visible in the remaining organs. In the bone marrow, however, the presence of gross bubble formation could not be verified because of the trauma inherent in fracturing the cortex to expose the marrow. For this reason, intact bones were decalcified and examined histologically. Microscopic examination revealed the bubbles to be round, ovoid, or irregularly-shaped clear spaces. On superficial examination the accumu- lations of gas appeared lodged in tissue spaces, probably because of the relative size of the vacuoles in comparison with the small blood channels. However, serial sections usually exhibited a continuity between the bub- ble space and the vessel lumen. Widely separated nuclei of flattened endothelial cells were occasionally noted about the bubble. In the spleen, gas bubbles rendered the organ sponge-like in appearance (Fig. 3); the very small bubbles were usually perifollicular in location in the region of the terminal opening of the sheathed arteriole. Rouleaux formation was evident here; larger bubbles appeared in the pulp and sinuses, often lim- ited and molded by the trabeculae. In the adrenal glands they occurred predominantly in the cortex (Fig. 4), at times forcing apart and disrup- ting cortical cell columns. In the bone marrow, pronounced hyperemia was evident in both diaphysis and epiphysis, and occasionally hemor- rhagic and necrotic foci were observed, predominantly in the diaphysis. In addition an occasional large ovoid space was filled with coagulum. Bubbles were present in both the epiphysis and diaphysis and, when large, were distorted and molded by bony trabeculae (Fig. 5).