RELATIONSHIP OF LEFT-VENTRICULAR STRUCTURE TO MAXIMAL HEART-RATE DURING EXERCISE

Previous investigators using clinical, hemodynamic, or exercise parame ters to predict maximal exercise heart rate (HRmax) have demonstrated age to be the major determinant. Regression coefficients have ranged f rom -0.3 to -0.6, leaving approximately two thirds of the variance in HRmax unexplained. Because cardiac size and function are directly rela ted to stroke volume and should influence HRmax, we studied 114 male s ubjects (aged 19 to 73 years) with tie-dimensional and M-mode echocard iography who underwent maximal treadmill testing with respiratory gas analysis. Seventy-three were normotensive (diastolic BP < 95 mm Hg) an d 41 were hypertensive. As in previous studies, HRmax was inversely re lated to age (HRmax = 199-0.63 [age], r = -0.47, p < 0.001), M-mode le ft ventricular (LV) diastolic dimension (LVD) added significantly to t he explanation of the variance in WRmax (r = -0.57, p < 0.001) (HRmax = 236-0.72 [age]-6.8 [LVD]). Thus, the larger the heart, the lower the HRmax. No other echocardiographic measurement or derived parameter ad ded significantly to the explanation of the variance in HRmax. To eval uate the effects of hypertension on HRmax, we studied hypertensives an d normotensives separately. Only age was significantly related to HRma x in the normotensives (r = -0.50, p < 0.001). In the hypertensive sub jects, however, both age and relative wall thickness (RWT) (which desc ribes LV wall thickness in relation to LV chamber size) were significa ntly related to HRmax, Age explained 45% of the observed variance in H Rmax (r = 0.67, p < 0.001) and RWT added modestly (9%) but significant ly to the relationship (HRmax = 173-0.96 [age] + 94 [RWT], p < 0.001), together explaining 54% of the variance observed in HRmax. Thus, HRma x is inversely related to LVD and patients with larger ventricles achi eve lower HRmax. In hypertensives, the amount of LV muscle mass in rel ation to chamber size is an additional predictor of HRmax. However, de spite controlling for age, sex, and cardiovascular disease, and the in clusion of echocardiographic indices of cardiac size and function, a l arge portion of the variance in HRmax could not be explained. The unex plained variance in HRmax is most likely due to intersubject variabili ty in resting cardiac size, volume, function, and other as yet undefin ed factors.