Systolic time intervals during submaximal and maximal exercise in man

Abstract

This study describes the serial responses of the systolic time intervals (STI) to submaximal and maximal supine exercise in normal subjects. Regression equations relating STI and heart rate during supine submaximal and maximal exercise are derived. It is shown that within the heart rate range of 120 to 170 beats per minute, total electromechanical systole (QS 2) and left ventricular ejection time (LVET) are related inversely and linearly to heart rate for both levels of exercise. At comparable heart rates, the QS 2 and LVET are greater during maximal than submaximal exercise, with differences widening significantly with increasing heart rate. This finding points out the importance of taking into account the intensity of exercise when evaluating STI responses to exercise stress in different groups. The pre-ejection period (PEP) significantly decreases from rest to the initial phase of both maximal and submaximal exercise with a subsequent tendency to plateau, indicating that a minimum PEP is reached early during exercise and that cardiovascular adaptation no longer includes additional shortening. This study also shows that STI obtained during exercise cannot be corrected for heart rate according to regression equations which were developed in resting supine subjects. Thus, there appears to be no justification for extrapolating the heart rate range of resting supine subjects to the exercise state for correcting intervals in this manner. In addition, STI measurements made during the immediate recovery period following supine exercise are significantly different from those obtained just prior to cessation of exercise. Therefore, recovery STI do not reflect the true hemodynamic changes occurring during the course of exercise. The application of the levels of exercise described in this study to the assessment of changes in STI in patients with documented or suspected cardiac dysfunction appears warranted. Alterations in left ventricular performance not evident at rest or following mild exercise might be detected employing these more strenuous exercise protocols. Using the regression equations established in this study, exercise-induced deviation in STI corrected for heart rate may now be determined with the realization that these relationships were derived from relatively young men. However, in the adult population the duration of LVET corrected for heart rate varies little with advancing age, and PEP shows only a slight increase. 34,35