Hemodynamic Response of Healthy Subjects to Exercise in the Supine Position While Breathing Oxygen

¹ From the Section of Physiology, Mayo Clinic and Mayo Foundation, Rochester, Minnesota

The hemodynamic response to exercise in the supine position was investigated by cardiac catheterization in 12 healthy subjects, breathing 95% O₂ or air. O₂ consumption during exercise ranged from 381 to 560 cc/min/m² and cardiac index from 3.8 to 7.7 l/min/m². No systematic change in stroke index was found, while the heart rate increased by a mean and range of 41 (24 to 54) beats/min. Arteriovenous oxygen difference increased by a mean and range of 4.4 (3.7 to 5.1) volumes per cent, owing chiefly to a fall in the mixed venous blood saturation. The arterial oxygen content increased in every instance as a result of an increase in the oxygen capacity of the blood, which averaged 0.7 (0.1 to 1.1) volumes per 100 cc. A relationship was found to exist, which was different at rest and during exercise, between degree of oxygen saturation of the mixed venous blood and level of the cardiac output. A rise in systemic and pulmonary artery pressure was recorded in every subject, reaching its maximum between the 2nd and the 4th minute of exercise. Following this time a fall was noted with the pressure tending to stabilize after 6–10 minutes at a level significantly less than the maximal level. During recovery the pressure fell to a level less than the pre-exercise value in most cases. Right atrial mean pressures also tended to rise during exercise. Systemic and pulmonary arterial pressures increased in proportion to the rise in O₂ consumption and in cardiac index. The data suggest that this increase becomes significant at levels of O₂ consumption of less than 400 cc/min/m².