Cardiac output and contractility indices: establishing a standard in response to low-to-moderate level exercise in healthy men.

Measurement by impedance cardiography (ZCG) of cardiac output and contractility indices during exercise is a recent addition to exercise testing of the cardiac patient. To provide a needed standard of normal responses to exercise at integrals of resting oxygen consumption (METS), 10 healthy men were tested. Basal oxygen consumption (1MET) under standardized conditions was measured by mass spectrometry. Over several days the subjects were exercised on a treadmill at 2METS through 6METS with monitoring of heart rate, electrocardiogram, oxygen consumption per minute (VO2) and ZCG. The exercise routine was then repeated to look for test-retest effect. Values closely approximating presently accepted basal oxygen consumptions were achieved under these standardized but practical conditions. Cardiac output during sitting, fully supported rest was slightly higher than that at 2METS on treadmill owing to higher stroke volume. From 2METS through 6METS the curvilinear mean heart rate and stroke volume were mirror images, producing a linear increase in mean cardiac output which at 6METS was 145% of basal and 160% of the 2MET value, while the contractility index increased to 217% of basal. Comparison of the 2 trials revealed no significant test-retest effect.