Duration of anesthesia before muscle relaxant injection influences level of paralysis

Dosage guidelines for muscle relaxants are based on dose-response studies, normally performed after several minutes of stable nitrous oxide (N O)-opioid anesthesia. However, relaxants are used immediately after induction of anesthesia. The study was designed to determine the influence of the duration of anesthesia and N O on the onset time at the adductor pollicis (AP) and the corrugator supercilii (CS) muscles of maximum neuromuscular blockade after mivacurium.

After institutional approval and informed consent, patients were randomly allocated into three groups. Anesthesia was induced with alfentanil and propofol. Group A (n = 10) received mivacurium (0.1 mg/kg) immediately after loss of consciousness. Groups B (n = 10) and C (n = 10) received mivacurium after 15 min of anesthesia with propofol alone (B) or propofol with N O (C). The evoked response to train-of-four stimulation was measured by acceleromyography at the AP and the CS.

Maximum neuromuscular blockade (%T1, median [range]) was significantly less in group A than in groups B and C ( < 0.001) at both the AP (81 [47-90]; 90 [35-100]; 100 [93-100], respectively) and the CS (19 [5-63]; 68 [61-100]; 89 [72-100], respectively). Maximum neuromuscular blockade was less in group B than in group C ( < 0.001) at the AP. Onset time of maximum neuromuscular blockade was not different between groups but was shorter at the CS than at the AP.

Duration of anesthesia and N O before mivacurium injection affect intensity of neuromuscular blockade but not onset time. Neuromuscular blockade obtained at the AP after several minutes of stable anesthesia with N O is greater than immediately after induction. This explains in part the discrepancy between the measured ED and the intubating dose.