Postoperative Epidural Fentanyl Analgesia

Abstract

In Response: Dr. dos Santos expresses two concerns in his letter: "methodological considerations … which may bring into question the conclusions" and available literature that was not quoted in the area of concern, i.e., the mechanisms of postoperative analgesia after epidural or intravenous fentanyl administration. Regarding the methodological concern, it would have been helpful if Dr. dos Santos had been more specific regarding which of the studies he thought had problems with the number of the patients enrolled, bias from inappropriate blinding methods and pain scoring, diluent of a given dose of fentanyl, and the position of the epidural catheter in relation to the area of nociception. Moreover, references to support his observations in this area are needed. Although the power of analysis may be a problem in some studies, this may not be the case in the five studies cited to support our observation that analgesia after epidural administration for postoperative analgesia is primarily supraspinal. Moreover, the authors of these studies used blinding techniques so that bias could be avoided. Regarding his observation of "diluent volume," there are no data in the literature that suggest that the volume in which a dose of fentanyl is administered in the epidural space will affect the vascular absorption or the arachnoid penetration. Birnbach et al. [1] showed that increasing diluent volumes of fentanyl given epidurally reduced the onset time of analgesia without changing the duration after bolus doses. They concluded that one should expect both a shorter onset and a shorter duration of analgesia when large volumes of diluted fentanyl were given epidurally. As discussed in the "Physical and Chemical Properties of Opioids in the Epidural Space" section of the article [2], vascular absorption, arachnoid penetration, and dosing are the three factors that appear to determine the spinal and supraspinal effects. Finally, Dr. dos Santos comments on the "site of the epidural catheter" as another factor that may affect the results. As discussed in the article, Guinard et al. [3] clearly showed that the position of the catheter in relation to the area of nociception (surgical incision) or the route of administration (epidural versus intravenous) had no influence on the quality of analgesia, incidence of side effects, or opioid consumption in patients who underwent thoracic surgery and received fentanyl for postoperative analgesia. Thus, the belief that the position of the catheter will influence the quality of analgesia after epidural fentanyl is not founded. The results of the studies that Dr. dos Santos believes demonstrated clinical advantages of epidural fentanyl need to be taken with reservations. Two of the cited studies addressed intraoperative variables [4,5]. Thus, their results are not applicable to postoperative analgesia. Two others evaluated analgesia either during labor [6] or after cesarean delivery [7]. The results of studies in these two groups should not be extrapolated to the postoperative population due to the analgesic effects of progesterone that will reduce opioid consumption [8]. Moreover, the study by Cooper et al. [7] can be criticized because the protocol design did not allow for titration of the fentanyl doses during the study. This shortcoming is reflected in the analysis of pain scores during movement between the epidural and the intravenous groups. If patient-controlled analgesia had been adequately used by allowing patients to titrate their doses, there should not be differences in the pain scores between the two groups; the differences should be seen in opioid consumption. Nevertheless, analysis of their results suggest that contrary to their conclusion, there were no differences in opioid consumption. The cumulative doses of fentanyl (the best indicator) in the epidural and the intravenous groups were similar, and there was only a single difference in the mean hourly doses of fentanyl used at the 4- to 8-h interval. Fentanyl consumption during the rest of the intervals was similar between the two groups. Thus, this study does not support the notion that there are advantages in using epidural fentanyl over intravenous fentanyl for postcesarean delivery analgesia. We concur with Dr. dos Santos' observation that a meta analysis may help to address the spinal versus supraspinal question. However, in the end, clinicians must ask themselves whether epidural use of fentanyl alone is justified for postoperative epidural analgesia in view of the results of the studies reported. Oscar A. de Leon-Casasola, MD Mark J. Lema, PhD, MD Department of Anesthesiology; Roswell Park Cancer Institute; Buffalo, NY 14263