Impedance-derived cardiac indices in supine and upright exercise.

Impedance cardiography was used to determine the classical systolic time intervals (STI's) (i.e., pre-ejection period (PEP), left ventricular ejection time (LVET) and the quotient PEP/LVET), in young, healthy, male subjects during supine and seated exercise. With increasing exercise, there was a tendency toward decreases in PEP, LVET, and PEP/LVET. In the seated position, there was an increase in transthoracic Zo incident to the caudal migration of thoracic blood--a result of the postural change. With seated exercise, there were--in contrast to supine exercise-greater decreases in PEP/LVET and greater increases in the Heather index. Similarly, there was a tendency toward increases in dZ/dtmin and the Rapid Ejection Index. We suggest that these differences are related to increased myocardial contractility resulting from the postural augmentation of cardio-sympathetic activity, added to that of exercise per se. This study, as well as previous ones, indicates that impedance cardiography is reliable, effective, and more practicable than the arteriographic method for monitoring STI's. We also believe that certain impedance-derived indices (i.e., transthoracic Zo, dZ/dtmin and the Heather Index) have considerable potential value for physiologic and clinical investigation.