Clonidine Reduces Blood Pressure and Heart Rate Oscillations in Hypertensive Patients

Short-term fluctuations in blood pressure (BP) and heart rate (HR) were analyzed in a group of eight men with essential hypertension. Indirect finger BP was measured by a Finapres device. Analog-to-digital conversion of the BP was used to determine systolic (SAP), diastolic (DAP), and mean arterial pressure (MAP) and HR every second. The equidistant sampling allowed a direct spectral analysis using a fast Fourier transform algorithm. The effects of an oral dose of clonidine (150 micrograms) were studied in a double-blind, crossover, placebo-controlled study. Clonidine markedly reduced the variability of BP and HR after 90 min as indicated by a reduction in the standard deviations of BP by 36.7% for SAP, 21.0% for DAP, 22.1% for MAP, and 26.0% for HR. At this time clonidine reduced the average BP by 19.7 mm Hg for SAP, 10.6 mm Hg for DAP, 16.0 mm Hg for MAP, and 1.0 beat/min for HR. Spectral profiles of BP and HR illustrated the alterations in the spontaneous oscillations underlying the standard deviation changes. Clonidine dramatically reduced the amplitude of BP and HR oscillations in the mid-frequency region 66-129 mHz, which depends on the activity of the autonomic nervous system. We suggest that an increased sensitivity of the baroreflex is responsible for the apparent better control of BP and HR with clonidine.