ABSTRACT
Alpha2-ADRENERGIC receptor agonists bind to peripheral and central nervous system sites modulating autonomio nervous system function. These effects may explain the clinically observed attenuation of sympathetically mediated responses to perioperative stress. They also have an antinociceptive effect in animals in humans premedicated with oral tizanidine an alpha2-adrenoceptor agonist, attenuated the increase in blood pressure associated with laryngoscopy and intubation and the amount of midazolam required for loss of consciousness was significantly reduced. We speculated that the oral administration of tizanidine might reduce the minimum alveolar anesthetic concentration [MAC] of isoflurane. Eighty ASA physical status patients, aged 24-56 yr, were randomly allocated into 4 groups [20 for each]: a Control group and a 2mg Tizanidine group, 4mg Tizanidine group, 10mg Tizanidine group. As premedication, the Control group received a placebo, and the Tizanidine groups received oral tizanidine 70 min. before surgical skin incision. Anesthesia was induced in all patients by propofol 2-2.5 mg/kg intravenously mixed with tidocaine 0.3 mg/kg, given over 30s. until loss of eyelid reflex [determined every 15 s]. After loss of consciousness, appropriate size of laryngeal mask was introduced by Brain method. Then administration of isoflurane was begun; the patients were spontaneously ventilated using a non-rebreathing system. MAC was determined by a technique adapted from the conventional up-down method for quanta! responses. The MAC of isoflurane was 1.3 +/- 0.13in the Control group, 1.24 +/- 0.1 in group It. 0.9 +/- 0.2 in grouplll and 0 7 +/- 0.2 in group IV. In group III and IV the plasma concentrations of epinephrine and norepinephrine were statistically significantly decreased during surgery and remained decreased thereafter. The oral administration of tizanidine successfully reduced the MA C of isoflurane, decreased the release of stress hormones and decreased the postoperative analgesic requirements in human adults