Baroreflex effectiveness index: an additional measure of baroreflex control of heart rate in daily life

In healthy subjects, progressive beat-to-beat increases or decreases in sys tolic blood pressure (SBP) ramps are not always accompanied by baroreflex-d riven lengthening or shortening in pulse interval (PI) ramps, respectively. This phenomenon has been quantified by a new index, the baroreflex effecti veness index (BEI), defined as the ratio between the number of SBP ramps fo llowed by the respective reflex PI ramps and the total number of SBP ramps observed in a given time window. Specificity of BEI was shown in eight cats by a -89% reduction of BEI after sinoaortic denervation. In 14 healthy hum ans, the 24-h average BEI value was 0.21, with a marked day-night modulatio n (approximate to0.25 day, approximate to0.15 night) in counterphase with m odulation of baroreflex sensitivity (BRS). Our analysis indicates that 1) i n normal subjects, arterial baroreflex can induce beat-by-beat PI changes i n response to only 21% of all SBP ramps, possibly because of central inhibi tory influences or of interferences at sinus node level by nonbaroreflex me chanisms and 2) BEI provides information on the baroreflex function that is complementary to BRS.