Document LJvDDQQp7QQMXVYpajZLnwEgX

612 HEINZ SPECHT of carbon dioxide, and to the exclusion of excitations of the carotid bulk ,, nism due to reduced oxygen pressures in the circulating blood. At altitffa||kie fore, a dilemma soon arises. Ventilation increases in response to the'lwe oxygen pressure. This -washes out carbon dioxide from the lungs and blKtfclj?' the result that ventilation falls off again. Thus a vicious cycle of respiration sets in, with a progressive drop in oxygen pressure andf^safral of the blood until central nervous system metabolism is so depressed-it^ffi function and failure occur. Cyclic respiration has been described frequcnt'yR't? early symptom of mountain sickness and is symptomatically equi^'AiV n, Cheyne-Stokes breathing. At intermediate altitudes, breathing air:f equivalent altitudes, breathing oxygen, it is observed that equilibria'a'i|spi tion may be attained by normal individuals in such a manner that-iefftf^ ventilation for oxygenation may be maintained by the adjustment ofin the blood and accommodation of the respiratory centers to this altcrid?nf!?5" A concise exposition of alveolar gases and ventilation phenomena . formulated by Fenn, Rahn, and Otis80 with numerous charts illustratirigifliSeuC of different levels of altitude. In practice this information may be `appVifdSi dustrially at high terrestrial altitudes, for example, in mining, but -thtSuSH pressurization and supplementary oxygen in commercial aviation"'make necessary for consideration only in emergencies. While the acute response to oxygen lack may be unconsciousness itncl ,dl?im death, the more chronic exposure to low-grade hypoxia may produce1 a^M response ranging from inefficiency in both muscular control and rngntnIt;Ms to disturbances of central nervous system structures, inducing a-neurocinHihftg' collapse, or at higher altitudes a peripheral circulatory collapsi 1 lip in a state of shock with its attendant progressive deterioration.6'11* It- i.#!pr that nearly all these derangements have a common mechanism rootek|ii]|ffiE tissue hypoxia,81 but the variable responses that can be elicited--ffonnjwf complex structure take on such a confusing aspect that analysis t,6;a'*srRIcm is not practicable.82 8. Hypocapnia As suggested above, the partial pressure of carbon dioxide unde: e'ocsii-lmri .at_altitude-as-a-secondary-response-to-the-hypoxiar-Its-ei quite marked and unfortunately unadapted to the circuiflkln<l|' carbon dioxide from the blood can be seen from Figure 4 to1 caU's^r&drewxw^1 be combined with hemoglobin at the same pressure of oxygen !than befolj'vn? a depletion. While this may at first seem to be satisfactory, iy-nf K W. 0. Fenn, H. Rahn, and A. B. Otis, Am. J. Physiol., 146, 637 (1946) , ` "W. P. Yant, J. Chornyak, H. H. Sohrenk, F. A. Patty, and R. R,,'.',- Health Bull. No. 211 (1934). ' "G. A. Brown, C. L. Cronick, H. L. Motley, E. J. Kocur, and W. 0:K1ingnian 7, 167 (1945). ' tfimkP - .vI'W' a 1 ... . -'AMS EFPECTS OF ABNORMAL ATMOSPHERIC PRESSURE 613 of oxygen, it will be effective in the tissues only if the carbon dioxide 'maintained or raised through increased metabolism. The hemoglobin -^correspondingly increased affinity for oxygen .in the capillaries if the -Islt'carbon dioxide .is decreased there as in the lungs. Unfortunately, the onSphat cause the hypoxia are not metabolic and thus carbon dioxide ^'effure falls throughout the body. This falling of the pressure brings on lion in ventilation discussed above, and also a train of other effects JpES of thiB condition.11 Thus, hypocapnia brings on peripheral sensory ..i-ifl,described as a tingling, especially of the hands, apprehension and jSSiKeilSychologioal changes which are precursors of a more drastic central [y'CVie'm response, often inducing strong autonomic stimulations resulting latter is particularly hazardous in aviation because the presence ill result in aspiration of the vomitus and complete asphyxia unless latifaid is rendered.6 liclhypoxia caused by breathing ambient air at altitude is avoided by ngjory'gen, then normal carbon dioxide pressures will be maintained; but l-faltitudes are reached, the internal maintenance of carbon dioxide plough metabolic activity imposes a growing restriction that has been ^ij.prfeviously; that is, the partial pressure of carbon dioxide tends to Ecifistant and thus becomes a larger and larger part of the total pressure _ fSiprevents the entrance of sufficient oxygen into the lungs at critical w M' : -~3; -W-ater-Vapor'of~the Hungs - tions above of the physical effects of carbon dioxide in dis- iix^gen at altitude are shared equally by water vapor that is constantly g&rated from the lung surface. Traditionally, the assumption has been Je gases in the lung are saturated with water vapor at body tempera- (i^^eflection it will be seen that the physics of this-system cannot justify ofroliision without modification, particularly at high'-rates of ventilation. fgSftexpended in evaporating water from the alveolar surface must ^gclic variation in temperature of the blood leaving the lung capillaries. fkg.pjyn that the saturation of the gases depends on the diffusion of Ijltfaroughout the space, and thus the ratio of surface to- volume will gree of saturation attained at any time. The facts of the case are ||af;as saturation at the temperature of the alveolar, bronchial, and HgSses, with respect to the time of exposure to the evaporating surface. eSIjyiyi.t....h..a..s....b.e..e..n....s..h..o..w..n....t.h...a..t...t.h..e.....d..e..g.r..e.e.....o..f...s..a..t.u..r.a...tion of the whole -SBnSte.M^'bl^'M^tMn;-cMeulated-on-the--assum:ption--5f-::8a;iufa-tion-atriS||ture,88 but there is little reason to doubt that at the alveolar surface ^|||nMs practically that imposed by the local temperature. JjttjBBfch. Science, 102, 619 (1946). L. H. Marshall and H. Specht, Am. J. Physiol