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 determine whether less cardiac vagal withdrawal is associated with the age-related decrease in this response, we assessed high-frequency R-R interval variability (0.15-0.40 Hz), an index of cardiac vagal tone, before (control) and during isometric exercise to exhaustion in healthy young (21-29 yr; n = 12) and older (61-72 yr; n = 11) men. During control, the two groups did not differ in respiratory rate, mean arterial pressure, or heart rate, although the older subjects had a lower high-frequency amplitude (12.2 +/- 1.1 vs. 29.4 +/- 4.6 ms/Hz; P < 0.05). During isometric exercise, the respiratory rate and arterial pressure responses did not differ; however, the older men had a lower absolute heart rate and a smaller increase in heart rate, and only the young men demonstrated a significant decline in high-frequency amplitude. The heart rate responses to exercise were correlated with the declines in high-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, corresponded to smaller increases in heart rate throughout isometric exercise. Therefore, we conclude that the smaller tachycardiac response to isometric exercise in older humans is associated with an inability to decrease cardiac vagal tone below an already reduced baseline level.