Autonomic nervous control of heart rate during blood-flow restricted exercise in man

Summary Power spectra of instantaneous heart rate (f_c) allows the estimation of the contribution of sympathetic and parasympathetic control of f_c during steady-state conditions. The present study was designed to examine autonomic control of f_c as influenced by normal dynamic leg exercise and by ischemic leg exercise. Eight subjects performed supine cycle ergometry at 30% of their control peak work rate, with and without blood-flow restriction. Blood-flow restriction was induced by exposing the exercising legs to a supra-atmospheric pressure of 6.7 kPa (leg positive pressure; LPP). The exercise responses of arterial pressure and f_c increased (P<0.05) by LPP exposure. The exaggerated pressor response may be attributed to a chemoreflex drive originating in the ischemic muscles. Exposure to LPP during exercise also produced a significant decrease in parasympathetically mediated high frequency (HF; 0.15-1.00 Hz) fluctuation of f_c, as indicated by a decrease (P<0.05) in percent HF power compared to the control exercise level. During LPP exercise, the sympathetically mediated very low frequency (VLF; 0–0.05 Hz) fluctuation of f_c increased, as indicated by an increase (P<0.05) in percent VLF power above control exercise levels. Both LPP and control exercise conditions decreased (P<0.05) power in all frequency ranges of interest compared to their respective resting conditions. The results suggest that the increase in f_c associated with normal dynamic exercise was mediated predominantly by parasympathetic withdrawal, whereas the exaggerated f_c response during ischemic exercise resulted from a combination of cardiac sympathetic drive and parasympathetic withdrawal. The increase in sympathetic activity is attributable to a muscle chemoreflex drive, which also may have attenuated parasympathetic activity by reciprocal inhibition. Alternatively, augmented central command mediated parasympathetic withdrawal during ischemic exercise.