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SUMMARYThe baroreceptor control of the sinus node was evaluated in 10 normotensive and 10 age-matched essential hypertensive subjects in whom ambulatory blood pressure was recorded intraarterially for 24 hours and scanned by a computer to identify the sequences of three or more consecutive beats hi which systolic blood pressure (SBP) and pulse interval (PI) progressively rose (+ PI/ + SBP) or fell ( -PI/ -SBP) in a linear fashion, according to a method validated in cats. In normotensive subjects, several hundred +PI/+SBP and -P I / -S B P sequences of 3 beats were found whereas the number of sequences of 4,5, and more than 5 beats showed a progressive drastic reduction. The mean slopes of + PI/ + SBP (7.6 ± 2.0 msec/mm Hg) and -P I / -SBP (6.4 ± 1.5 msec/mm Hg) sequences were similar, but in both instances there was a large scattering of the values around the mean (variation coefficients: 64.2 ± 4.7 and 62.6 ± 2.4%). The slopes decreased as a function of the sequence length and baseline heart rate and increased to a marked extent during the night as compared with daytime values. All sequences were more rare (-33.2% for +PI/ + SBP and -31.7% for -P I / -S B P ) and less steep in hypertensive subjects ( -4 0 . 3 and -3 6 . 2 % , respectively), who failed to show the marked nighttime increase in slope observed in normotensive subjects. To our knowledge, these observations provide the first description in humans of the baroreceptor-heart rate reflex in daily life. This reflex is characterized by marked within-subject variations hi sensitivity due in part to hemodynamic, temporal, and behavioral factors. All features of the baroreceptor-heart rate reflex are unpaired hi essential hypertension. (Hypertension 12: 214-222, 1988) KEY WORDS • baroreceptor reflexes • ambulatory blood pressure monitoring • hypertension sleep • humans • heart rate W E have previously reported 1 -2 that blood pressure in unanesthetized cats exhibits spontaneous rises or falls that are accompanied by linearly related increases or reductions in pulse interval (PI). We have also reported 12 that sinoaortic denervation abolishes these events, which therefore reflect baroreceptor modulation of the sinus node. We have concluded that evaluation of these events by computer analysis of intra-arterial blood pressure tracings represents a powerful tool for studying the baroreceptor-heart rate reflex in daily life.

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