Cardiac Output in Exercise

Accurate data on the cardiac output in various types and intensities of exercise are not available, due largely to the very great technical difficulties of measurement. Such data as are available must be accepted with some reservation for the same reason. A great deal is known about the changes in heart rate which accompany exercise, but little is known about the equally important adjustments in the stroke volume.

Even without direct measurements, this could be postulated from the enormous increase in oxygen consumption which occurs. Krogh and Lindhard 3 using an indirect Fick method, found that the cardiac output may exceed 20 liters per minute during heavy exercise. In spite of calculations which indicated a possible total blood flow of 30 to 40 liters per minute during running when the oxygen consumption was 4 liters per minute, it is probable that in the average man the cardiac output during work seldom exceeds 20 liters per minute. An example of the enormous increase in cardiac output which can be achieved by exceptional athletes.

The increase in cardiac output which may be attained in exercise is limited by several factors. First, it is obvious that the heart cannot, for more than a few beats, eject more blood than is returned to it by the systemic veins; in other words, cardiac output cannot exceed the venous return. In numerous conditions, such as shock, hemorrhage, the erect posture with no movement, extreme heat, prolonged bed rest, etc., the cardiac output is reduced below normal by an inadequate venous return. Following strenuous exercise, the sudden withdrawal of the pumping effect of skeletal muscle contractions on venous blood flow results in a sharp fall in cardiac output. On the other hand, the available evidence indicates that the normal healthy adult heart is always able to increase its output sufficiently to handle the greatest amount of blood which can be returned to it in maximal exertion. Additional factors which may limit the increase in cardiac output in exercise are the maximal capacity of the heart for dilating and the reduction in diastolic filling of the heart which results from extremely rapid heart rates.

Finally, such adverse conditions as fatigue, lack of sleep, malnutrition and acute infections may seriously reduce the maximal cardiac output of which the subject would otherwise be capable.