Change in phase relationship between SBP and R-R interval during lower body negative pressure.

We have investigated the hypothesis that beat-by-beat interaction of systolic blood pressure (SBP) to R-R interval (the spontaneous baroreflex) is dependent on the length of the R-R interval. Data were collected from eight healthy men while heart rate was slow (R-R interval 1,043 +/- 34 ms) and accelerated (R-R interval 804 +/- 18 ms) by application of lower body negative pressure (LBNP greater than or equal to -40 mmHg). Time series data of SBP and R-R interval were searched for spontaneous baroreflex sequences in which R-R interval changed in the same (lag 0), next (lag 1), or next following (lag 2) beat as SBP. This phase relationship was also quantified by cross-spectral analysis. At rest, 85% of all spontaneous baroreflex sequences occurred with no lag (lag 0). With LBNP, there was a significant reduction in the number of lag 0 sequences (26%), whereas lag 1 and lag 2 sequences increased (10-26% and 5-29%, respectively). Cross-spectral phase also changed significantly from -2.3 +/- 6.3 degrees at rest to 70.5 +/- 7.4 degrees during LBNP. These data supported the hypothesis that the lag of a baroreflex event was dependent on the prevailing R-R interval.