Electromyographic monitoring of profound surgical muscle relaxation during cardiac anesthesia

Quantitative assessment of neuromuscular block produced by large doses of nondepolarizing neuromuscular blocking agents during cardiac surgery is not possible with conventional methods of monitoring. Various posttetanic responses can, however, be elicited, even when no twitch response is present. Posttetanic responses measured by electromyography were used in this study. Twenty-four male patients undergoing coronary bypass surgery were anesthetized with sufentanil plus diazepam. Neuromuscular block was provided either with pancuronium 0.1 mg/kg or with vecuronium 0.07 mg/kg initially and supplemented with small increments when indicated. Neuromuscular block was monitored from the hypothenar muscle. The ulnar nerve was stimulated by train-of-four, with supermposed periodic tetanic stimuli to evoke posttetanic responses, once every 7 to 15 minutes. The tetanically potentiated responses were detectable during 96% ± 3.6 (vecuronium) and during 97% ± 3.7 (pancuronium) of the entire intraoperative period, while the nonpotentiated electromyographic responses were present for less than 50% of the time. The sum (of the amplitudes) of 6 posttetanic responses is significantly (p<0.05) greater than the sum of 6 nonpotentiated responses and than the size of a single-peak posttetanic response when compared with the normal, nonpotentiated responses. Higher-frequency tetanic stimuli (100 or 200 Hz) produced greater posttetanic responses (p<0.05) than did the 50-Hz tetanic stimulus. There were only slight or no significant differences in the degree of posttetanic potentiation between pancuronium and vecuronium either before, during, or after cardiopulmonary bypass. With posttetanic responses, we could detect changes in the level of neuromuscular block that occur during cardiac surgery and that are related to cardiopulmonary bypass, cooling, rewarming, and large doses of corticosteroids and antibiotics. Furthermore, it was not necessary to extend the arm or to use an arm board (on which the hand is immobilized when using mechanical monitoring methods) during cardiac surgery.