Baroreflex control of sinus node during dynamic exercise in humans: effect of muscle mechanoreflex

This study evaluated the influence of muscle mechanical afferent stimulation on the integrated arterial baroreflex control of the sinus node during dynamic exercise. Systolic blood pressure (SBP) and pulse interval (PI) were measured continuously and non-invasively in 15 subjects at rest and during passive cycling. The arterial baroreflex was evaluated with the cross-correlation method (xBRS) for the computation of time-domain baroreflex sensitivity on spontaneous blood pressure and PI variability. xBRS computes the greatest positive correlation between beat-to-beat SBP and PI, and when significant at P = 0.01, slope and delay are recorded as one xBRS value. Heart rate variability (HRV) was evaluated in the frequency domain. Compared with rest, passive exercise resulted in a parallel increase in heart rate (67 ± 3.2 vs. 70 ± 3.6 beats min⁻¹; P < 0.05) and mean arterial pressure (87 ± 2 vs. 95 ± 2 mmHg; P < 0.05), and a significant decrease in xBRS (13.1 ± 1.8 vs. 10.5 ± 1.7 ms mmHg⁻¹; P < 0.01) with an apparent rightward shift in the regression line relating SBP to PI. Also low frequency power of HRV increased while high frequency power decreased (56.7 ± 3.5 vs. 62.7 ± 4.8 and 43.2 ± 3.4 vs. 36.9 ± 4.9 normalized units respectively; P < 0.05). These data suggest that the stimulation of mechanosensitive stretch receptors is capable of modifying the integrated baroreflex control of sinus node function by decreasing the cardiac vagal outflow during exercise.