Time course analysis of baroreflex sensitivity during postural stress

Abstract

Postural stress requires immediate autonomic nervous action to maintain blood pressure. We determined time-domain cardiac baroreflex sensitivity (BRS) and time delay (τ) between systolic blood pressure and interbeat interval variations during stepwise changes in the angle of vertical body axis (α). The assumption was that with increasing postural stress, BRS becomes attenuated, accompanied by a shift in τ toward higher values. In 10 healthy young volunteers, α included 20 degrees head-down tilt (−20°), supine (0°), 30 and 70 degrees head-up tilt (30°, 70°), and free standing (90°). Noninvasive blood pressures were analyzed over 6-min periods before and after each change in α. The BRS was determined by frequency-domain analysis and with xBRS, a cross-correlation time-domain method. On average, between 28 (−20°) to 45 (90°) xBRS estimates per minute became available. Following a change in α, xBRS reached a different mean level in the first minute in 78% of the cases and in 93% after 6 min. With increasing α, BRS decreased: BRS = −10.1·sin(α) + 18.7 ( r2 = 0.99) with tight correlation between xBRS and cross-spectral gain ( r2 ∼ 0.97). Delay τ shifted toward higher values. In conclusion, in healthy subjects the sensitivity of the cardiac baroreflex obtained from time domain decreases linearly with sin(α), and the start of baroreflex adaptation to a physiological perturbation like postural stress occurs rapidly. The decreases of BRS and reduction of short τ may be the result of reduced vagal activity with increasing α.