THE EFFECT OF CHANGES IN POSTURE AND OF GRADED EXERCISE ON STROKE VOLUME IN MAN *

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Abstract
Cardiac output, heart rate, and stroke volume were measured in 4 healthy, untrained men at rest in the supine position, at rest standing and during exercise in the upright position. The exercise varied in severity from gentle movements of the calf muscles and marking time to walking at 4.5 mph up a treadmill inclined at 12 degrees from the horizontal. This severe excercise was associated with cardiac ouputs of 15 to 25 1/min. and O2 consump-tions of 2.0 to 2.8 1/min. The cardiac index of subjects at rest in the supine position averaged 3.5 1, and the stroke index averaged 54 ml. When the subject stood, there was a fall in cardiac output and an increase in heart rate. The stroke index averaged 32 ml, a fall of 41%. Mild exercise was sufficient to restore the stroke index almost to the value obtained with the subject at rest in the supine position. With moderate exercise in the upright position the stroke index was similar to that obtained when the subject was at rest in the supine position and with the severest excercise it increased to 59 ml. It is probable that the apparent discrepancies in previous reports were due to variations in the circumstances under which resting values for stroke volume were obtained rather than to inadequate techniques for measuring cardiac output. Thus, if the stroke volume obtained during severe exercise in the upright position is compared with that obtained when the subject is at rest in the supine position, the increase is slight (-9 to +23%) compared with the increase in heart rate (+156 to +190%). If the comparison is made with the value obtained when the subject is standing at rest, the increase in stroke volume (+61% to +110%) is comparable with that in the heart rate (+77 to +122%).