LESSER VAGAL WITHDRAWAL DURING ISOMETRIC-EXERCISE WITH AGE

The tachycardia that accompanies isometric exercise decreases with age in humans; however, the mechanism of this decline is unknown. To dete rmine whether less cardiac vagal withdrawal is associated with the age -related decrease in this response, we assessed high-frequency R-R int erval variability (0.15-0.40 Hz), an index of cardiac vagal tone, befo re (control) and during isometric exercise to exhaustion in healthy yo ung (21-29 yr; n = 12) and older (61-72 yr; n = 11) men. During contro l, the two groups did not differ in respiratory rate, mean arterial pr essure, or heart rate, although the older subjects had a lower high-fr equency amplitude (12.2 +/- 1.1 vs. 29.4 +/- 4.6 ms/Hz; P < 0.05). Dur ing isometric exercise, the respiratory rate and arterial pressure res ponses did not differ; however, the older men had a lower absolute hea rt rate and a smaller increase in heart rate, and only the young men d emonstrated a significant decline in high-frequency amplitude. The hea rt rate responses to exercise were correlated with the declines in hig h-frequency amplitude (initial, r = -0.808; midpoint, r = -0.714; peak , r = -0.632; all P < 0.005), which were, in turn, correlated with the control high-frequency amplitude (initial, r = -0.727; midpoint, r = -0.643; peak, r = -0.610; all P < 0.01). Thus smaller declines in high -frequency amplitude, related to lower baseline amplitude, corresponde d to smaller increases in heart rate throughout isometric exercise. Th erefore, we conclude that the smaller tachycardiac response to isometr ic exercise in older humans is associated with an inability to decreas e cardiac vagal tone below an already reduced baseline level.