Relationship of left ventricular structure to maximal heart rate during exercise

Abstract

Previous investigators using clinical, hemodynamic, or exercise parameters to predict maximal exercise heart rate (HRmax) have demonstrated age to be the major determinant. Regression coefficients have ranged from -0.3 to - 0.6, leaving approximately two thirds of the variance in HRmax unexplained. Because cardiac size and function are directly related to stroke volume and should influence HRmax, we studied 114 male subjects (aged 19 to 73 years) with two-dimensional and M-mode echocardiography who underwent maximal treadmill testing with respiratory gas analysis. Seventy-three were normotensive (diastolic BP<95 mm Hg) and 41 were hypertensive. As in previous studies, HRmax was inversely related to age (HRmax=199-0.63 [age], r=-0.47, p<0.001). M-mode left ventricular (LV) diastolic dimension (LVD) added significantly to the explanation of the variance in HRmax (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 added significantly to the explanation of the variance in HRmax. To evaluate the effects of hypertension on HRmax, we studied hypertensives and normotensives separately. Only age was significantly related to HRmax in the normotensives (r=-0.50, p<0.001). In the hypertensive subjects, however, both age and relative wall thickness (RWT) (which describes LV wall thickness in relation to LV chamber size) were significantly related to HRmax. Age explained 45% of the observed variance in HRmax (r=0.67, p<0.001) and RWT added modestly (9%) but significantly to the relationship (HRmax=173-0.96 [age]+94 [RWT], p<0.001), together explaining 54% of the variance observed in HRmax. Thus, HRmax is inversely related to LVD and patients with larger ventricles achieve lower HRmax. In hypertensives, the amount of LV muscle mass in relation to chamber size is an additional predictor of HRmax. However, despite controlling for age, sex, and cardiovascular disease, and the inclusion of echocardiographic indices of cardiac size and function, a large portion of the variance in HRmax could not be explained. The unexplained variance in HRmax is most likely due to intersubject variability in resting cardiac size, volume, function, and other as yet undefined factors.