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 (tau) between systolic blood pressure and interbeat interval variations during stepwise changes in the angle of vertical body axis (alpha). The assumption was that with increasing postural stress, BRS becomes attenuated, accompanied by a shift in tau toward higher values. In 10 healthy young volunteers, alpha included 20 degrees head-down tilt (-20 degrees), supine (0 degree), 30 and 70 degrees head-up tilt (30 degrees, 70 degrees), and free standing (90 degrees). Noninvasive blood pressures were analyzed over 6-min periods before and after each change in alpha. The BRS was determined by frequency-domain analysis and with xBRS, a cross-correlation time-domain method. On average, between 28 (-20 degrees) to 45 (90 degrees) xBRS estimates per minute became available. Following a change in alpha, xBRS reached a different mean level in the first minute in 78% of the cases and in 93% after 6 min. With increasing alpha, BRS decreased: BRS = -10.1.sin(alpha) + 18.7 (r(2) = 0.99) with tight correlation between xBRS and cross-spectral gain (r(2) approximately 0.97). Delay tau shifted toward higher values. In conclusion, in healthy subjects the sensitivity of the cardiac baroreflex obtained from time domain decreases linearly with sin(alpha), and the start of baroreflex adaptation to a physiological perturbation like postural stress occurs rapidly. The decreases of BRS and reduction of short tau may be the result of reduced vagal activity with increasing alpha.

Dynamic Cerebral Autoregulation in Homozygous Sickle Cell Disease

“…14 Afferent, central, and vagal efferent baroreceptor reflex pathways were evaluated by quantifying baroreflex sensitivity using the sequential method. 15 Beat-to-beat values of systolic BP and interbeat interval were interpolated and resampled at 1 second. Cross-correlations were calculated using a 10-second window containing systolic BP for delays in the interbeat interval window of 0 to 5 seconds.…”

Mechanisms of orthostatic intolerance following very prolonged exercise

“…The relationship between spectral components of cardiovascular variabilities and direct measures of muscle sympathetic nerve activity in humans has been shown to correlate closely over a range of arterial pressure changes (37), except when sympathetic activity is reduced (45). In relation to the available techniques, however, it has been reported that spontaneous and pharmacologically determined BRS are complementary (41) and that the BRS methods may contribute to more in-depth examination of the baroreflex circulatory control (59).…”

Validity and variability of xBRS: instantaneous cardiac baroreflex sensitivity

“…In that respect, the method can prove useful when tracking transient changes in BRS during sleep, in exacting conditions (exercise, altitude, daily practice e.g., critical care/anesthesia), or during clinical physiology interventions (standing up and tilt responses (Westerhof et al. 2006)). Clearly, no analysis method can pretend, or needs, to cover all the aspects of the cardiac baroreflex with minute precision.…”

“…Head‐up tilt reduces BRS by a factor of 2–3 (Westerhof et al. 2006). It thus appears that BRS as a level is highly variable depending on the state a subject is in and xBRS ranges correspond to those found in the literature.…”

“…The higher P ‐level of 0.05 almost doubles the number of determinations per minute without an appreciable change in the mean BRS value or its standard deviation (Westerhof et al. 2006). Except for the P ‐level, the xBRS method requires no thresholds, not for pressure‐ or for interval changes (this issue is discussed in more detail in the Appendix).…”