Cp50 of propofol with and without nitrous oxide 67%

The target concentration of propofol required to prevent response to surgic al incision was determined in 60 unpremedicated ASA I or II patients, who b reathed either oxygen-enriched air or nitrous oxide 67% in oxygen. Propofol was infused using a target-controlled infusion system incorporating the st andard 'Diprifusor' pharmacokinetic model, with the target concentration fo r each patient decided by up/down sequential allocation. Presence or absenc e of movement in response to a groin incision was determined by the surgeon . The calculated blood concentration at which 50% of patients responded (Cp 50(calc)), determined by probit analysis, was 6.8 mu g ml(-1) for patients who breathed oxygen-enriched air and 4.9 mu g ml(-1) for those who breathed nitrous oxide 67% in oxygen.