Purpose: To evaluate the effects of clonidine on responses to adrenoceptor agonists and baroreflex sensitivity, we examined arterial blood pressure (AP) responses to phenylephrine and heart rate (HR) responses to isoproterenol and baroreflex sensitivity (HR response to AP changes due to phenylephrine or nitroglycerin).
Methods: We studied 60 anaesthetized patients who either did or did not receive 5 micrograms.kg-1 clonidine po before they were anaesthetized. After induction of general anaesthesia, the patients received 3 micrograms.kg-1 phenylephrine, 0.02 microgram.kg-1 isoproterenol, or 2-3 micrograms.kg-1 nitroglycerin, and haemodynamic measurements were taken. Baroreflex sensitivity was expressed as the slope of the linear regression line (msec.mmHg-1; in msec of R-R interval change vs mmHg change in systolic arterial pressure) following the administration of phenylephrine and nitroglycerin.
Results: Patients who received clonidine had greater augmented responses in AP to phenylephrine and in HR to isoproterenol (47.2 +/- 15.6% vs 23.7 +/- 11.9% for increase in systolic AP and 59.8 +/- 22.6% vs 26.2 +/- 11.0% for increase in HR, P < 0.05 respectively). There were no differences between the baroreflex sensitivities in the pressor (phenylephrine) test groups (3.77 +/- 1.08 vs 4.41 +/- 1.66 msec.mmHg-1). In contrast, the slopes of depressor (nitroglycerin) test groups were decreased in patients receiving clonidine (1.98 +/- 0.73 vs 3.68 +/- 1.72 msec.mmHg-1, P < 0.05).
Conclusion: The results suggest that premedication with clonidine might enhance critical hypotension during anaesthesia and surgery, but restoration both of AP and HR decrease can be achieved effectively by phenylephrine and isoproterenol i.v., respectively.