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

Background and Objectives: Whether patients titrate themselves to an indivi dualized blood or plasma opioid concentration (the so-called minimum effect ive concentration or [MEC]) has been debated extensively. Nevertheless, the re is consistent opinion that during patient controlled analgesia (PCA) pat ients balance acceptable pain relief against unacceptable side effects. Thi s study sought to characterize fentanyl used by PCA with respect to MEC and factors influencing PCA use. Methods: An intensive study of 25 patients with observations over the first 24 hours after orthopedic surgery was planned on the premise that this app roach would provide a measure of the fentanyl MEG. This necessitated repeat ed measurements of pain scores and plasma fentanyl concentrations before an d 10 minutes after every PCA demand. In addition, a battery of psychologica l tests was given before and approximately 48 hours after surgery. Results: Logistic difficulties of maintaining a 24-hour study design result ed in its termination after 5 patients. The patients had convincingly disti nct 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 i n concentration coincided with marked changes in pain relief. Despite preop erative 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. Unfortuna tely, the small sample size made systematic analysis of the psychological r ests impossible. Conclusions: This study found evidence to support the concepts of an indivi dual MEC and a therapeutic window of fentanyl used with PCA.