Document 8OzDXwZmgwKQnw9ZO7pRRzbNm

610 HEINZ SPECHT varying degree of pain. The mechanics of the openings to these spaces so well to relief of internal pressure that only with the most serious cases will: than fleeting sensations be experienced.78 On the other hand, there is a more drastic effect to be expected ft, expansion of the gases in the gastrointestinal tract. As in the case of decomps sion from high atmospheric pressures, there is expansion, but the effect :r\ by water vapor is considerably exaggerated. This is due to the fact t'fta- vapor pressure of water is constant with the temperature of the body andfM^ it is an increasingly larger part of the total pressure, as ascent is made.iiUir an increasingly larger distension of the soft-walled gas pockets:*1?,||*k$| decompression from 10 atm. to 1 causes a volume increase of water-satii^, from 1 to 10.59, or a gain of a little more than half a volume over^|h\ expansion; while a decompression from 1 atm. to Vio would result in a. welpvBlj increase of 24.6 times, or a gain of 14.6 volumes over the dry expansion hi) at Vo atm., 38,500 feet, there is a volume increase of 6.66 times rather4^h'^ffi| fivefold dry expansion from one atmosphere. These considerations form|tr of the observation that at moderate altitudes flatus expansion can be^ handled by the normally induced movements of the smooth muscle of thek. intestinal tract, but at an altitude of 30,000 feet or above the tenden||| smooth muscle to give way to gradually increasing internal pressuresjri meteorism with all its attendant symptoms. Besides the excruciating pain caused by these effects there is in aaaS restriction of the movement of the diaphragm and thus a respiratory emn ment. In any case, the prophylaxis of avoiding gas-forming foods andl ment of early symptoms by massage and movement of the abdom|^S put into practice by the individual who has the misfortune to expeiggf distress. According to an extensive series of uniformly conducted induct] ascents fof Army airmen, Swann and Rosenthal79 *reported an incictg|i' pains severe enough to cause descent of 0.59 per cent; of these, per cent occurred during ascent to 30,000 feet; 73 per cent occurred aftei at the working level of 38,000 feet. j^jsT The most critical conditions for mechanical effects from decompVv-i in.the situation in which explosive decompression from pressurized cnhiifsi finder these conditions a volume change theoretically as great?,t^^.'3^i.;.!` original volume was experienced in laboratory tests' in 0.'^08^^f'Td''wi detectable harm. It is probable that the displacement of lung' gasl'^ito^ecd indicated but the expansion of intestinal gases, if any, was sompwhl|| to the tone of the muscles. Cabin pressurization, for the reasons disefissedtf " O. E. Reynolds, H. C. Hutchins, A. Y. Werner, and F. R. Philbrook; VI'S. :.VVH Bull, 46, 845 (1946). .1 " H. G. Swann and T. B. Rosenthal, A Survey o/ the Incidence of Decompressioh^f. with Reference to Some Constitutional and Environmental Variants. AAF Med., Randolph Field, Texas, August 8, 1944. EFFECTS OF ABNORMAL ATMOSPHERIC PRESSURE 611 gd^at great altitudes because of the lack of a margin of safety in the ^^e^ompression.11 Breath holding is an obvious hazard that, although it has ' "|i?in-these tests, might result in pulmonary emphysema, as in the sub;pe conditions cited earlier.04 'ip'-v'JTlE-.EFFECTS OF MAINTAINED LOW ATMOSPHERIC PRESSURE 1. Hypoxia ! ?Jv I^cyijient from the previous treatment of the physical nature of reduction lPfrigip.iressure that the partial pressure of oxygen falls in direct proportion ||h'gjtotal pressure. Thus an ascent to Vs atm. pressure will result in an ||i||al., pressure of oxygen equal to 79.5 mm. Hg. From Figure 4 it is ^^.'theoretical saturation of the blood with oxygen might be as great ^fei;.-,cent if this were the actual partial pressure of oxygen at the lung imV . B.ecause gaseous exchange takes place within the depths of the lungs, ^^fethere is never a complete exchange of the contained gases of the lungs, l&pressure of oxygen is subject to several factors that bring about a c|iom|in,iits magnitude. The first is a matter of dilution with the residual s-ffie'jlungs and their large air passages; the second is an addition of water the air from the moist surfaces of the lungs. The former is a structural T'iat|yaries from one individual to another; the latter varies only with pei&ture of the evaporating surface within the lung. It becomes apparent fly water vapor is_ajort of _constant thatjwe must arbitrarily deduct jMl barometric pressure in order to find the effective pressure and that,' gpibecomes a larger and larger part of the total pressure as we ascend. I,1,1 by analysis, the composition of the gases nearest the circulating Uncalled alveolar gas, has an oxygen partial pressure of only 36 mm. Hg, lawesult in a saturation of the arterial blood of less than 70 per cent. fl"p,efiuction in the saturation of the blood that is the primary factor in liStjtsbifhces attendant upon ascent to high altitudes, since' it interferes- with "transport of oxygen to the tissues, particularly those of the central lyStem that have the highest rate of metabolism and the smallest liffibion. in partial- pressure ofJhe oxygen, in,4he,"ib},Q.od (wilhgxcite the M^ilia "of the. carotid artery bulb, which excitation effects,;an"Increase pot; ventilation of the lungs, resulting in the removal of .carbon dioxide |&ps. some water vapor), with moderate improvementft,in^the..partial llllthe oxygen in the lungs. While this response to. ipjyered.' oxygen gjrifct'llfVaroly hand less effective than a parallel mechanism of the respiratory center |iia' The latter responds to the increase in carbon dioxide that ordij ^^.lp^Sy.oxygen depletion in the body from muscular exertion. This, sm is, however, also responsive to a decrement in the partial pressure 'Ait 'V.Vjf'