Constancy of stroke volume in ventricular responses to exertion

Abstract

Diastolic and systolic dimensions of the left ventricle and the free wall of the right ventricle in intact dogs are affected little by spontaneous exercise. The concept that stroke volume and heart rate in normal man increase by about the same relative amounts was derived from estimations of cardiac output, particularly in athletes, based upon indirect measurements using foreign gases or CO₂. Data for man obtained with the modern cardiac catheterization or indicator dilution techniques confirm the impression derived from intact dogs that increased stroke volume is neither an essential nor a characteristic feature of the normal cardiac response to exercise. Stroke volume undoubtedly increases whenever cardiac output is increased with little change in heart rate (e.g. in athletes or in patients with chronic volume loads on the heart). Tachycardia produced experimentally with an artificial pacemaker in a resting dog causes a marked reduction in diastolic and systolic dimensions and in the stroke change of dimensions. The factors generally postulated to increase stroke volume during normal exercise may prevent the reduction in stroke volume accompanying tachycardia.