Differential effects of propofol, thiamylal and ketamine on the cricothyroid and posterior cricoarytenoid muscles of the canine larynx.
To measure the electromyographic (EMG) responses of the phasic discharge in the cricothyroid (CT; a tensor muscle of the vocal folds) and the posterior cricoarytenoid (PCA; sole abductor muscle of the vocal folds) following intravenous infusion of propofol 1.0 mg.kg-1.min-1, thiamylal 1.0 mg.kg-1.min-1, or ketamine 0.5 mg.kg-1.min-1 for five minutes. Prospective, nonrandomized, controlled animal study. University research laboratory. Fifteen mongrel dogs, including three groups of five animals in each group. Under 0.2-0.3% halothane and oxygen anesthesia with spontaneous ventilation, phasic EMG activities of the CT and PCA muscles were recorded in an identical manner after the administration of each drug. Propofol infusion produced almost equal suppression of EMG activity of the CT and the PCA with time and three minutes after the start of infusion of propofol there was a significant depression of the phasic activities in the both muscles; EMG activity of the CT and the PCA was 33.8 +/- 21.2 and 36.6 +/- 22.9% (% of control, mean +/- SD) respectively P < 0.05). Thiamylal selectively reduced rhythmic discharges in the CT muscle during spontaneous breathing and significant depression of discharge in the CT muscle was observed three minutes after the drug (47.3 +/- 24.9%, P < 0.05). In contrast, both phasic EMG activities of the CT and the PCA were rhythmically active and the differential sensitivity between the CT and the PCA muscles was not observed after ketamine, even after ten minutes of administration. This study confirms a difference in sensitivity between the CT and the PCA muscles, demonstrating that the intrinsic laryngeal muscles do not behave similarly after the administration of conventional intravenous anaesthetic agents.