The minimum effective concentration of opioids: a revisitation with patient controlled analgesia fentanyl

Whether patients titrate themselves to an individualized blood or plasma opioid concentration (the so-called minimum effective concentration or [MEC]) has been debated extensively. Nevertheless, there is consistent opinion that during patient controlled analgesia (PCA) patients balance acceptable pain relief against unacceptable side effects. This study sought to characterize fentanyl used by PCA with respect to MEC and factors influencing PCA use.

An intensive study of 25 patients with observations over the first 24 hours after orthopedic surgery was planned on the premise that this approach would provide a measure of the fentanyl MEC. This necessitated repeated measurements of pain scores and plasma fentanyl concentrations before and 10 minutes after every PCA demand. In addition, a battery of psychological tests was given before and approximately 48 hours after surgery.

Logistic difficulties of maintaining a 24-hour study design resulted in its termination after 5 patients. The patients had convincingly distinct MECs (ranging from 0.23 to 0.99 ng/mL). The relationship between plasma fentanyl concentration and pain score was steep, such that small changes in concentration coincided with marked changes in pain relief. Despite preoperative expectations of achieving satisfaction in postoperative analgesia, not all patients titrated themselves to a pain-free state; all but one were satisfied with PCA. Surprisingly few side effects were reported. Unfortunately, the small sample size made systematic analysis of the psychological tests impossible.

This study found evidence to support the concepts of an individual MEC and a therapeutic window of fentanyl used with PCA.