The Respiratory and Circulatory Response of Normal Man to Inhalation of 7.6 and 10.4 per Cent Co2 with a Comparison of the Maximal Ventilation Produced by Severe Muscular Exercise, Inhalation of Co2 and Maxi- Mal Voluntary Hyperventilation1

The effects of inhalation of low concentrations of CO:! have been investigated widely in man (1, 2, 3, 4). The effects produced by higher concentrations (7.5-10 per cent CO,) have been recorded incompletely and only upon small groups. To supplement our knowledge of the quantitative aspects of respiration and circulation it seemed worthwhile to measure the response of a large group of normal subjects to these higher concentrations of carbon dioxide. In addition the maximal respiratory minute volume produced by inhalation of 7.6 per cent and 10.4 per cent CO2 was compared with that resulting from exhausting muscular exercise and maximal voluntary hyperventilation. METHODS. Forty-four male medical students betwen the ages of 21 and 26 served as subjects. Eighteen of these had just completed a six weeks program of athletic training and were in excellent physical condition. A typical experiment was conducted as follows: The subject sat either upon a chair or a stationary bicycle. No attempt was made to secure a basal state. An aviation type half-mask was strapped tightly to the face. The subject breathed through rubber valves (Japanese gas mask type) which offered minimal resistance. Expired air was conducted through tubing l$ inches in diameter to a balanced compensating 300 liter spirometer. Measurement of the volume of expired air (not corrected to S.T.P.) respiratory rate, pulse rate and systolic and diastolic blood pressure were recorded. Some or all of the following procedures were then completed: 1, determination of maximal voluntary breathing capacity (5) over a half-minute period; 2, inhalation of “7.6 per cent COZ” in oxygen (mixtures varied from 7.4 to 7.8 per cent CO,) ; 3, inhalation of 10.4 per cent CO2 in oxygen, and 4, exercise at a maximal rate upon a heavily loaded stationary bicycle. This exercise consisted of a “warm-up” period of several minutes on the bicycle followed by exertion at a maximal rate of pedalling for 60-75 seconds. All subjects except one considered this to be exhausting. Carbon dioxide mixtures were supplied in 6,000 liter high pressure tanks. Each tank was connected through a regulator to a 10 liter rubber bag which was attached to the