Baroreflex sensitivity assessed by complex demodulation of cardiovascular variability.

We used complex demodulation of cardiac interval and systolic arterial blood pressure oscillations in the low-frequency band (0.04 to 0.14 Hz) to investigate baroreceptor control of heart rate. Baroreflex sensitivity was defined as the instantaneous amplitude of complex-demodulated oscillations in the RR interval divided by the instantaneous amplitude of complex-demodulated oscillations in systolic blood pressure. We evaluated the method using both simulated and actual data obtained from 33 healthy nonsmokers during supine and standing postures. To test the validity and reliability of the method, we compared the mean values of baroreflex sensitivity calculated using complex demodulation with the values obtained using power spectral analysis and sequential analysis of spontaneous variations in blood pressure and RR interval. All three methods applied to the simulated data yielded the same values of baroreceptor sensitivity. Mean values of baroreflex sensitivity assessed by complex demodulation of the actual data were similar to those calculated by both power spectral analysis and sequential analysis (13.9 +/- 5.2 versus 13.7 +/- 6.7 or 14.3 +/- 6.5 ms/mm Hg for supine and 7.3 +/- 2.8 versus 7.0 +/- 3.0 or 7.2 +/- 2.8 ms/mm Hg for standing, respectively). In addition, a significant correlation existed between the values obtained by complex demodulation and power spectral analysis (r = .97, P = .0001) and sequential analysis (r = .98, P = .0001). Furthermore, complex demodulation-derived baroreflex sensitivity fluctuated across time during both the supine and standing postures, and this could not be discerned by power spectral analysis. The results indicate that complex demodulation provides a dynamic assessment of baroreflex sensitivity and may be a useful tool in exploring reflex autonomic control of the cardiovascular system.