520 RESPIRATION 



altitudes. Most generally, -however, an elevation of about 4000 m. 

 suffices to produce definite discomforts and especially if the consump- 

 tion of oxygen has been markedly increased on account of the muscular 

 exertions incurred during climbing. At a height of 5000 m., at which 

 the pressure of the air is reduced to about one-half and the oxygen 

 tension to about 11 per cent, of an atmosphere, scarcely anybody 

 escapes the sensations of fatigue and respiratory oppression. Neither 

 is it possible to obviate these difficulties by ascending to these heights 

 in a balloon, because even in the absence of all unnecessary muscular 

 activity, the body is in need of more oxygen owing to an increased 

 action of the heart and a compensatory augmentation of the cellular 

 oxidations. More favorable conditions, however, may be established 

 during balloon ascensions, and hence, somewhat higher altitudes may 

 be attained in this way. Altitudes of 7000-8000 m. and over may be 

 reached by resorting to inhalations of pure oxygen, but even this arti- 

 ficial means does not afford an absolute protection against the develop- 

 ment of dangerous conditions. This is shown by the experiences 

 which Tissandier 1 had while ascending in a balloon to a height of 

 8600 m. At an altitude of 7500 m. he and his two companions became 

 so weak that they could not make effective use of the oxygen bags. 

 All three persons finally lost consciousness but without having pre- 

 viously experienced a decided dyspnea. Tissandier was the only 

 survivor. 



Henderson 2 and his collaborators have produced acute effects of 

 oxygen deficiency at sea-level by breathing into an apparatus con- 

 sisting of a spirometer and a canister containing alkali. The exhaled 

 carbon dioxid is absorbed by the alkali, while the oxygen is gradu- 

 ally diminished by the continual rebreathing. The increase in the 

 frequency of the heart is slight at first, only about one to three beats, 

 but a marked acceleration sets in when the oxygen has fallen to be- 

 tween 13 and 9 per cent. (14,500 to 22,000 feet of altitude). In men 

 who do not tolerate low percentages of oxygen an increase of from 

 40 to 70 beats was not uncommon. The systolic blood pressure re- 

 mains about the same until the oxygen has been lowered to between 

 14 and 9 per cent., when it may rise 15 to 20 mm. Hg above normal. 

 The diastolic pressure remains fairly normal, but falls somewhat 

 after the oxygen has been reduced to 9.5 per cent, or less. The best 

 type of men may tolerate as low an oxygen content as 6 per cent., 

 which corresponds to an altitude of close to 30,000 feet. The hemo- 

 globin showed a well defined increase in at least 25 per cent, of all 

 the men. No cardio- vascular lesions could be noted in men in 

 "optimum" condition; others, on the other hand, developed mur- 

 murs and hypertrophic conditions. 



In accordance with Bert, it is generally held that the disturbances 

 just described, are due to a failure of the diffusion pressure which 



1 La nature, 1875, 337. 



2 Medical Studies in Aviation, Jour. Am. Med. Assoc., Ixxi, 1918. 



