Altered frequency-dependent characteristics of the cardiac baroreflex in essential hypertension

Blood pressure (BP) oscillations contain rhythmic components of low-, mid-, and high-frequency bands (0.02-0.06, 0.07-0.14, 0.15-0.40 Hz, respectively). Baroreceptors may sense each BP oscillation and induce a buffer reaction. To clarify the frequency-dependent baroreflex response and its possible alteration in patients with high BP, both BP (Finapres) and the RR interval (electrocardiogram) were continuously monitored in untreated patients with essential hypertension (EH) (n = 52) and normotensive subjects (NT) (n = 43). The magnitude and phase response of cardiac beats to BP oscillations were examined by transfer function analysis. Spontaneous baroreflex sensitivity was assessed by linear regression analysis of the BP and RR oscillations. The heart rate responded linearly to BP oscillations in more subjects at mid- and high-frequency bands (83% or more) than at the low-frequency band (60% or less). The phase was approximately zero at the high-frequency band and was consistently negative at the mid- and low-frequency bands. In general, all frequency gains were significantly and positively correlated with spontaneous baroreflex sensitivity. Each frequency gain was smaller in EH patients than in NT subjects, the high-frequency gain more so than the gains at the lower frequencies. In seven young EH patients, treatment with a beta 1-adrenoceptor selective blocker normalized the high-frequency gain and tended to increase the gains at the lower frequencies. These results suggest that the spontaneous baroreflex modulates RR oscillations over a broad frequency range from 0.02 to 0.40 Hz; the effect was most marked at frequencies higher than 0.07 Hz. Furthermore the frequency-dependent characteristics of the cardiac baroreflex were altered in essential hypertension partly because of an increased beta-adrenergic activity.