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In regads to intermittent vs. continuous epidural analgesia during labor
- Davide Cattano, MD, Shawn Zeltwanger MD, PhD (4 January 2007)
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Davide Cattano, MD Washington University, Department of Anesthesiology, Shawn Zeltwanger MD, PhD
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We have read with interest the recent article by Fettes et al. comparing intermittent vs. continuous epidural analgesia during labor. This randomized, double-blinded study imparts new information on how patients respond to different modes of epidural infusion during the first stage of labor. The authors report that patients receiving intermittent boluses require significantly less rescue doses and less total drug than those receiving continuous infusion. However, there are a few points we would like to make. Firstly, it is known that that distribution of solution in the epidural space is nonuniform. Fascia that extends laterally from the posterior longitudinal ligament is a barrier to flow of epidural solutions (1). The authors theorize that more adequate analgesia is obtained by bolus administration because of the higher injection pressure at the catheter tip resulting in a more uniform spread of anesthetic in the epidural space. We wonder if a 10ml “bolus”, administered at 2 ml/min over 5 min in the study, is sufficient to generate this pressure. This seems to be a much slower bolus rate than is used in clinical practice. An informal survey among a handful of our faculty revealed that bolus rates of 10 to 20 ml/min are safely utilized for initial epidural dosing and rescue doses. Will using higher bolus rates of epidural anesthetics result in an even more optimal distribution in the space? Obviously this has to be balanced with safety, since an automated pump, and not a clinician would deliver the dose. Secondly, unlike another study comparing continuous and intermittent epidural analgesia after hysterectomy (Ducan et al), the current study showed no differences in sensory spread between patients receiving continuous and intermittent boluses. The authors state the difference between the two studies may be due to the relative brief duration of the current study. They also suggest that the additional rescue doses received by patients on a continuous infusion in their study may have masked any differences in sensory spread. The initial work by Ducan et al. actually utilized lower volumes (5ml vs 10ml) of a more concentrated (0.375% bupivacaine vs. 0.2% ropivicane) epidural solution (2). Thus, having a denser block at lower levels may also play a part in this difference. Lastly, we believe there were two flaws in the study protocol. 1) While the study was appropriately blinded, with n=20 in both the continuous and intermittent groups, 1/20 of the continuous infusion group required an extra priming bolus before t=0 (the start of maintenance analgesia) versus 5/20 in the study group. This subset of patients starts with 10 extra ml of a long acting local anesthetic, which could provide extra analgesia during the early portion of the study. Conversely, this subset could be classified as “more difficult to adequately comfort”. Either way, this group could skew the results and needs to be analyzed independently from the other subjects. Does their inclusion/exclusion for the data set change the results of this study? 2) Patients in the continuous infusion group start receiving an epidural infusion at t=0, while the study group is first bolused at t=30min. By our calculations, at t=35 min, the study group will have received 4.16 ml more of a slow-onset/long duration anesthetic than the control group. In Figure 3 of the paper, the authors compare the proportion of patients with sufficient analgesia versus time to the first rescue bolus. It is not surprising that none of the study group appears to have required a rescue bolus in the first hour of the protocol. The same is not true for the control group. Smaller boluses every 20 to 30 min, utilized in other studies comparing continuous versus bolus epidural infusions, may be a more effective alternative (3,4). References 1) Hogan Q. Distribution of solution in the epidural space: examination by surgery. Br J Anaesth 1998; 80:7–10.cryomicrotome section. Reg Anesth Pain Med 2002; 27:150–6. 2) Duncan LA, Fried MJ, Lee A, Wildsmith JAW. Comparison of continuous and intermittent administration of extradural bupivacaine for analgesia after lower abdominal surgery. Br J Anaesth 1998; 80:7–10. 3) Ueda K, Ueda W, Manabe M. A comparative study of sequential epidural bolous technique and continuous epidural infusion. Anesthesiology 2005; 103:126-9. 4) Lim Y, Sia ATH, Ocampo C. Automated regular boluses for epidural analgesia: comparison with continuous infusion. Int J Obstet Anaesth 2005; 14:305-9. Conflict of Interest:None declared |
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