Pharmacodynamic evaluation of augmentation effect of isoflurane on mivacurium

This study evaluated the augmentation effect of isoflurane (ISO) given before or after the mivacurium (MIV) injection. Consented 33 adults (18-58 years), ASA I patients of both sexes were randomly assigned into three groups. In group 1 (no ISO) patients--IV propofol (0.5-1 mg/kg) for induction and (25-50 mg) aliquots every 2-5 minutes, N2O (60%) in O2 by mask, was followed by IV MIV (0.04 mg/kg). In group 2 (ISO before MIV) patients-IV propofol as in group 1 and ISO in N2O (60%) and O2 to achiever end tidal level 1% for 10 minutes was followed by same dose of IV MIV. In group 3 (ISO after MIV) patients-after propofol and IV MIV as above, ISO in N2O (60%) and O2 was given to get end tidal level 1% for 10 minutes. All patients were breathing spontaneously using tight fitting facemask and respiration was assisted to keep ETCO2 (35-40 mmHg), SaO2 100%. To monitor MIV effect, ulnar nerve was stimulated at wrist using supramaximal double burst stimuli (DBS). Adductor pollicis response was recorded on myograph-2000 (Biometer, Denmark). Twitch amplitude (D1) and tetanic fade DBSr (D2/D1) were calculated for each stimulus and recorded. Peak MIV effect (D1 suppression by approximately equal to 90% and abolished D2) was significantly (p < 0.05) more in group 2 patients (ISO before MIV) than in group 1 (control) or group 3 (ISO after MIV) patients. ISO given prior of MIV administration significantly augmented the effect of MIV. When ISO was started after MIV injection probably the quick hydrolysis of MIV limited the augmentation effect to tetanic fade only as compared to the patients, who did not get ISO at any point of study. Prior administration of ISO causes intense MIV block at adductor pollicis.