Cardiac responses to impulse exercise and recovery: systolic time intervals.

Exercise responses depend on work load and its pattern of delivery. Administering a very brief ("impulse") load aims to elicit significant responses through biologic sensitivity to rate - rather than degree - of change. Electrocardiograms, systolic time intervals (STI) and heart rate (HR) were recorded continuously in 10 normal subjects during and after brief (20-sec) bicycle exercise at 50, 100 and 150 W. The purpose of this protocol was to identify a low load impulse-type exercise challenge which would be optimal in terms of (a) reproduction of the time course of exercise changes produced by longer duration (steady-state) exercise, (b) rapid achievement of quantitative responses reaching some or all of the steady-state changes at comparable work load, and (c) absence of ST changes in normal subjects. The onset of exercise produced the greatest rates of change. Directional changes and time course of all measurements paralleled those of steady-state exercise and recovery at the same loads: HR, ejection time index (ETI) and corrected ejection time (ETc) increased sharply; preejection period (PEP) and PEP/LVET fell sharply. Ejection time (LVET), stable through most of exercise, "paradoxically" decreased for up to 15 sec of recovery despite decreasing heart rates. For all measurements, restitution to control levels was complete by one minute of recovery.