CHANGES IN RESISTANCE TO MOUTH OPENING INDUCED BY DEPOLARIZING AND NON-DEPOLARIZING NEUROMUSCULAR RELAXANTS

Mouth opening was measured in 43 children anaesthetized withisoflurane and paralysed with vecuronium or suxamethonium. Measurementsof mouth opening were made for up to 10 min after loss of theadductor pollicis twitch and cessation of muscle fasciculations.In 22 patients receiving suxamethonium, a significant (P <0.001) reduction in mean mouth opening occurred in the 60 safter loss of twitch and cessation of fasciculations. Mouthopening reductions could last for up to 10 min after the lossof twitch, beyond the return of the twitch. One patient experienced"masseter spasm"; he did not develop malignant hyperpyrexiaduring 2.5 h of isoflurane anaesthesia. Patients receiving vecuroniumshowed a significant (P < 0.0006) increase in mouth opening.In 20 subjects, mouth opening was generated with a small (1.67N) and a larger (4.32 N) force. Proportionally equal reductionsin mouth opening were obtained with either force after suxamethoniumadministration. Relatively equal increases with either forcefollowed vecuronium administration. Isolated masseter spasmis not pathognomonic for malignant hyperpyrexia. If the diagnosisof malignant hyperpyrexia is contemplated, signs of hypermetabolism,such as increases in end-tidal carbon dioxide concentrationduring constant minute ventilation, should be sought.