Effect of oral clonidine and intrathecal fentanyl on tetracaine spinal block.

We studied the effect of oral clonidine and intrathecal (IT) fentanyl on the onset and duration of a hyperbaric tetracaine-induced spinal block. Forty adult males undergoing elective surgery were studied according to a randomized, double-blind, placebo-controlled protocol involving four treatment regimens: Group I, placebo per os (PO) + tetracaine 12 mg IT; Group II, placebo PO+tetracaine 12 mg IT+fentanyl 10 micrograms IT; Group III, clonidine 200 micrograms PO+tetracaine 12 mg IT; Group IV, clonidine 200 micrograms PO+tetracaine 12 mg IT+fentanyl 10 micrograms IT. Onset time to highest sensory level was 8.5 +/- 3.1, 8.2 +/- 2.3, 6.1 +/- 1.6, and 6.8 +/- 1.4 min in Groups I, II, III, and IV, respectively. The time required for sensory regression to the L1 dermatome level was 153 +/- 34, 151 +/- 51, 183 +/- 63, and 209 +/- 39 min in Groups I, II, III, and IV, respectively. Similarly, the duration of Grade III motor block was 132 +/- 37, 140 +/- 55, 160 +/- 51, and 189 +/- 52 min in Groups I, II, III, and IV, respectively. IT fentanyl 10 micrograms did not change the onset or duration of tetracaine-induced spinal block. Furthermore, there was no significant interaction between clonidine and fentanyl in Group IV. Episodes of bradycardia and hypotension were more frequent in the clonidine-treated patients (40%-50% vs 10%). We conclude that oral clonidine (200 micrograms) shortened the onset time of tetracaine's sensory block and prolonged the duration of sensory and motor block. However, clonidine premedication increased the risk of hypotension and bradycardia.