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 rate 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 micrograms/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 rate rise was significantly increased from 47 to 53 beats/min (p < 0.05), although 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.