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Keywords:

  • Equipment;
  • airway, laryngeal mask;
  • Anaesthesia;
  • depth;
  • Anaesthetics, intravenous;
  • propofol

Summary

We have studied the efficacy of the loss of response to jaw thrust as a clinical test to assess adequate depth of anaesthesia for insertion of the laryngeal mask in 60 patients. After induction of anaesthesia with propofol (infused using a syringe driver), the patients were randomly allocated to one of two groups. In one group, insertion of the laryngeal mask was attempted immediately after the loss of verbal contact and in the other group, after the loss of motor response to a jaw thrust. Conditions for insertion of the laryngeal mask were assessed. The mean dose of propofol required to obtain loss of verbal contact was 1.94 mg. kg−1 (SD 0.39, 95% confidence intervals (CI) 1.79–2.08 mg. kg−1) and that for the loss of response to jaw thrust was 2.55mg. kg−1 (SD 0.46, 95% CI 2.38–2.72 mg. kg−1). When depth of anaesthesia was assessed using jaw thrusting, it was always possible to insert the mask and the conditions were optimal in 87% (95% CI 72–95%) of patients. Neither coughing nor gagging occurred. In contrast, conditions were almost always less than optimal when insertion was attempted after the loss of verbal contact. Conditions were significantly better when jaw thrust was used as a clinical test compared with loss of verbal contact (p = 0.001). No marked haemodynamic depression occurred in any patient. Thus, jaw thrust is a reliable clinical test to assess the adequate depth of anaesthesia for uncomplicated insertion of the laryngeal mask after induction of anaesthesia with propofol.