RESPONSES IN SIZE, OUTPUT AND EFFICIENCY OF THE HUMAN HEART TO ACUTE ALTERATION IN THE COMPOSITION OF INSPIRED AIR

Abstract

The results are reported from numerous expts. in which acute hypoxia was produced in 27 normal young [male][male], exposed for intervals of 10 to 48 mins. to pO2 corresponding to that at 18,000 to 28,000 feet altitude, with CO2 at 14 to 30 mm. Hg present in some cases, otherwise under constant environmental conditions. Some expts. were made with inspiration of pure oxygen. Roentgenkymograph measurements show that cardiac dilatation does not take place in acute hypoxia with or without added CO2. nor does any significant change in heart size occur during respiration of 4 to 5 times normal pO2. The stroke vol. remains nearly constant in this acute hypoxia and the minute vol. heart output is increased only slightly more than in proportion to the pulse rate change. The presence of CO2 lessened the increase in pulse rate. Blood pressure responses were of no certain predictive value for the 10 cases of complete syncope occurring in the present series. There is always a relative bradycardia in recovery from hypoxic syncope. Indications were seen that acute hypoxia disturbs the stability of the autonomic nervous system. It is concluded that in normal young [male][male] attempts to protect the heart would serve no useful purpose. The heart does not seem to be the limiting factor in tolerance to hypoxia.