measured for 50% pain relief were 1–7 ng/ml. High responders to remifentanil had blood levels of the opioid in the range of 0.5–1.5 ng/ml, while normal responders required 2–7 ng/ml. In non-responders to remifentanil, the infusion had to be stopped because of sedation or side effects before any pain relief was obtained. There were 16 high responders, 44 normal responders and 20 non-responders.
