Factors Influencing Blood Pressure and Heart Rate Variability in Hypertensive Humans

We examined the influence of baroreceptor reflex sensitivity (the increase in pulse interval in response to a phenylephrine-induced increase in blood pressure), age, blood pressure, and β-adrenergic receptor blockade on the variability of blood pressure and heart rate in essential hypertension. Fifty-six subjects were studied before treatment; intra-arterial blood pressure was recorded outside the hospital for 24 hours. Variability was defined (from all beats occurring while subjects were awake) as the standard deviation about the average waking value for mean arterial pressure (MAP) or pulse interval. The correlation (r) between baroreceptor reflex sensitivity and blood pressure variability was −0.47 (p < 0.0002). Baroreceptor reflex sensitivity was the only independent determinant of blood pressure variability on multiple regression analysis. Thirty subjects were restudied after 5 months of β-adrenergic receptor blockade. Ambulatory blood pressure was lower during treatment, whereas pulse interval, its variability, and baroreceptor reflex sensitivity were higher. Blood pressure variability was unchanged. The variability of MAP was inversely correlated with baroreceptor reflex sensitivity before (r=−0.42, p < 0.02) and during (r= −0.45, p < 0.02) treatment, but it was unrelated to the average ambulatory MAP or to the variability of pulse interval either before or during β-blockade. Sixteen subjects whose average waking ambulatory blood pressure was 140/90 mm Hg or less were not treated. This group of borderline hypertensive subjects had less variable MAP than did the remaining 40 subjects (12.4 ± 2.3 [SD] vs 14.5 ± 2.5 mm Hg; p < 0.01). We conclude that 1) the decline in baroreceptor reflex sensitivity is the principal determinant of increased blood pressure variability in hypertension, 2) MAP and its variability are regulated independently, 3) heart rate variability is not influenced by baroreceptor reflex sensitivity and is unrelated to blood pressure variability, and 4) the blood pressure of subjects with borderline hypertension is not excessively labile.