Immediate cardiac response to exercise: physiologic investigation by systolic time intervals at graded work loads.

Immediate cardiac responses to exercise were investigated in five normal male volunteer subjects by measuring heart rate and systolic intervals from rest through the onset of exertion at three different work loads. Recordings were continued for fifty beats and again at one minute with measurements plotted on a beat-to-beat basis and grouped for statistical analysis. During exercise, heart rate and corrected ejection time increased, pre-ejection period and PEP/LVET decreased, and pulse transmission time remained stable. The heart rate acceleration was sudden, occurring in the first few beats of exercise, a phenomenon also demonstrated by previous investigators. A significant new finding was the equally abrupt major change in each of the other parameters. The immediacy of all responses was independent of load and was consistent with experimental studies showing that changes in both neural activity and venous return at the onset of exercise are virtually instantaneous.