Effective concentration 50 for propofol with and without 67% nitrous oxide.

The Effective Blood Concentration (EC) of propofol required to prevent response to surgical incision was determined in 65 ASA I or II female patients breathing either 100% oxygen or 67% N2O in oxygen. Propofol was administered via a microcomputer-controlled infusion system programmed to maintain the blood propofol concentration at predetermined target values. The blood propofol concentrations predicted by the micro-computer were validated by measurement of whole blood propofol concentration. Predicted and measured concentrations differed during infusion of propofol, but became similar after discontinuing the infusion for at least 90 s, suggesting that equilibration within the central compartment was incomplete during infusion. The response to the initial incision was observed and probit analysis used to determine the predicted blood concentration at which 50% of patients responded. The predicted EC50 for propofol/N2O/O2 and propofol/O2 was 4.5 micrograms ml-1 and 6.0 micrograms ml-1 respectively, and the measured EC50 propofol/N2O/O2 and propofol/O2 was 5.36 micrograms ml-1 and 8.1 micrograms ml-1, 67% nitrous oxide in oxygen reducing the predicted EC50 by 25% and the measured EC50 of propofol by 33%. The predicted EC may be more representative of the equilibrated concentration in the central compartment and thus reflective of tissue propofol concentrations.