Circulatory and Respiratory Adaptation during Prolonged Exercise in the Supine Position

Abstract Circulatory and respiratory changes during prolonged, non‐steady state exercise in the supine position were studied by the heart catheterization technique in six healthy young men of average physical fitness. The work load was chosen so that they could perform supine exercise for one hour. This load gave a mean heart rate of about 128 beats/min after 10 min and 142 beats/min after 50 min of work. The circulatory changes during these experimental conditions were characterized by a constant cardiac output and a continuous increase in heart rate, indicating a continuously falling stroke volume. The systemic arterial blood pressure fell continuously, but the mean pulmonary arterial pressure fell markedly between 10 and 20 minutes' work and then remained unchanged. The acid base status was fairly stable during exercise. The respiratory response was characterized by a slight increase in total ventilation, an unchanged alveolar ventilation and a slight increase in dead‐space ventilation. The small increase in total ventilation was due to a small increase in respiratory rate and tidal volume. The overall ventilation‐perfusion relationship was unchanged. Both circulatory and respiratory changes may be explained by a decreased tonus in the capacity vessels with a shift in the distribution of the blood.