• The effects of applying unbalanced pressures to the head and body were studied in 56 airplane pilots. The head and upper airways of a given subject were exposed to varying pressures within a full-head pressure helmet while the trunk and extremities were exposed to independently adjustable pressures in a separate compartment. Both compartments contained 100% oxygen. When the pressure on trunk and extremities was reduced to that equivalent to an altitude of 40,000 ft. (12,200 m.), marked differences were found among the subjects as to their ability to maintain a normal pulse rate, normal electrocardiogram, and consciousness in the face of increasing difference of pressure between the two compartments. No significant relationship was found, however, between this ability and the results of physical fitness tests or anthropological groupings. The data suggested that unbalanced pressures acting upon the lungs reduce the efficiency of the coronary circulation and cause myocardial hypoxia.
McGuire TF, Talbott GD, Rosenbaum DA, Webber JM, White SC. CARDIOVASCULAR EFFECTS OF BREATHING AGAINST UNBALANCED ATMOSPHERIC PRESSURES. JAMA. 1957;163(14):1209–1213. doi:10.1001/jama.1957.02970490007002
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