Sites of action of propofol and isoflurane on somatosensory pathway

Objective To determine the sites of action of propofol and isoflurane on somatosensory pathway using median nerve somatosensory evoked potential(MnSSEP) .Methods Twenty-six ASA I - II patients aged 20-50 yrs were randomly divided into two groups: I propofol group( n = 13) and H isoflurane group( n = 13) .In propofol group patients received propofol infusion at a rate of 10 mg·kg-1·h-1 .Oxygen was administered via mask and respiration was assisted or controlled to maintain SpO2 at 96%-100% and PETCO2 at 35-45 mm Hg. The propofol infusion was continued until the patient failed to respond to verbal command. Six minutes later if the patient was still breathing spontaneously, the rate of propofol infusion was increased to 20-40 mg·kg-1·h-1 . In isoflurane group anesthesia was induced with propofol 2 mg · kg-1 and intubation was facilitated by succinylcholine and anesthesia was maintained with isoflurane inhalation and intermittent iv boluses of vecuronium. The end-tidal isoflurane concentration was maintained at 0.5,1.0 and 1.5 MAC. Each concentration was maintained for 15 minutes. MAP, HR, SpO2,PCTCO2, T0(naso-pharyngeal) and BIS were continuously monitored. MnSSEP( Viking IV D type) was measured and recorded before induction of anesthesia(baseline) and in propofol group when patients became unconscious and apneic; in isoflurane group when end-tidal isoflurane reached 0.5,1.0 and 1.5 MAC.Results In group I there was no change in both latencies and amplitudes of N9 and N13'.With increasing infusion rate, propofol gradually prolonged the latencies and decreased the amplitudes of N60, P45, N35, N20 and P25 waves. In isoflurane group there was no change in the latencies and amplitudes of N9. There was no change in the latencies of N13', but the amplitudes decreased at 1.0 and 1.5 MAC. With increasing concentration, isoflurane gradually prolonged the latencies and decreased the amplitudes of P45, N60, N35, N20 and P25 waves. At 1.5 MAC the inter-peak latencies between N13'-N20 and N13'-P25 were prolonged. Conclusions The sites of action of different infusion rates of propofol and different concentrations of isoflurane are different on somatosensory pathway. The higher the doses, more widespread are the sites of action.