Differential evaluation of the baroreceptor cardiac reflex effectiveness as a function of sequence length.

Recently a new index of baroreceptor cardiac function has been proposed, the baroreflex effectiveness index (BEI). BEI was defined as the ratio between the number of systolic blood pressure (SBP) ramps (progressive beat-to-beat increases ("up") or decreases ("down")) followed by reflex changes in heart period and the total number of SBP ramps. In this study we tested BEI as a function of the length of cardiac sequences in which the reflex operates (3, 4, 5, and 6 beats) in human subjects in the laboratory. Results show an overall BEI of .65, with greater values for the "up" (.69) than for the "down" sequences (.62). These values are much higher than previously reported. When analyzed as a function of sequence length, BEI increases as sequence length decreases. This difference is more progressively observed in the "up" than in the "down" sequences. Both slope and overall blood pressure change of the SBP ramps increase as sequence length increases. Overall blood pressure change of the SBP ramps can predict BEI for the 4- (r2 approximately .057) and 3-beat sequences (r2 approximately .20). These results suggest the need to examine the effects of the setting/experimental manipulation on the average length of cardiac sequences, given that an observed change in BEI could be genuine or simply associated with a modification in the pattern of sequence length.