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

Aim: This study evaluated the influence of muscle mechanical afferent stimulation on the integrated arterial baroreflex control of the sinus node during dynamic exercise. Methods: 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. Results: 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). Conclusion: 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.