Dose-Related Prolongation of Tetracaine Spinal Anesthesia by Oral Clonidine in Humans

Comment: Clonidine and dexmedetomidine are nonspecific and specific a-2 receptor agonists that decrease the release of norepinephrine by stimulation of presynaptic sympathetic neurons. They decrease sympathetic activity in the central nervous system and reduce minimum alveolar concentration (MAC) of inhalational anesthetics. This study has shown that oral clonidine prolongs the sensory block of spinal anesthesia obtained with tetracaine. The authors argue that this probably occurs through undefined supraspinal mechanisms. Clonidine did not alter the maximum sensory level of spinal anesthesia but did significantly prolong the two- and four-dermatome regression time. It is interesting that the only significant hemodynamic effect seen was significantly decreased heart rate in patients receiving 300 μg, the highest dose used in the study. The protocol used in this study for patients undergoing vaginal hysterectomy or transurethral resection of the prostate is different than that used by most practitioners. Isobaric spinal anesthesia was administered with 15 mg of tetracaine to patients averaging 5 foot 3 inches tall. The average sensory level obtained, however, was T-8. Blood pressures were monitored every 2 min for the first 10 min and then only every 10 min. The a-2 agonists may enhance spinal anesthesia through a number of mechanisms. It may be an analgesic effect in the central nervous system similar to the enhancement of MAC. It is unlikely that the doses used in the study would lead to significant concentrations of clonidine in the cerebrospinal fluid. Finally, spinal cord blood flow may be altered, slowing the elimination of tetracaine. Experimentally, it would be interesting to see if small doses of intrathecal clonidine or dexmedetomidine alter analgesic effects of local anesthetics. Clonidine is used clinically as an antihypertensive (Catapres). Anesthesiologists need to be concerned about the effect on MAC and, now, duration of spinal anesthesia. These concerns do not warrant discontinuation of the antihypertensive. Sudden discontinuation may result in a “withdrawal syndrome,” characterized by hypertension and tachycardia.‘