THE EFFECTS OF CLONIDINE ON CARDIOVASCULAR-RESPONSES TO STANDING IN HEALTHY-VOLUNTEERS

This study assessed the effects of clonidine on blood pressure (BP) and heart rare responses to active standing, recorded continuously using a Finapres monitor. Ten subjects were given a placebo infusion over 1 h, followed by clonidine hydrochloride 1.5 mu g/kg over 2 h. During placebo and at 1, 3 and 19 h following the clonidine infusion, heart rate and blood pressure were recorded during the second half of supine rest for 10 min, active standing and quiet standing for 7 min. Clonidine did not alter the size of immediate drop in BP on standing, although the nadir was lower. BP recovery was impaired, with a loss of the usual BP overshoot in most subjects and with delays in reaching supine levels of diastolic BP (6.1 versus 9.6 s; p < 0.01) and systolic BP (8.1 versus 12.3 s; p < 0.05). The compensatory initial heart rare rise was significantly increased from 47 to 53 beats/min (p < 0.05), athough the peak rate reached was reduced from 114 to 104 beats/min (p < 0.05). These results demonstrate that impairment of central sympathetic vasomotor drive leads to a delay in BP recovery and loss of initial BP overshoot immediately after standing, together with impaired maintenance of early steady-state BP.