Efficacy of Epidural Block during General Anesthesia

When searching for possible advantages of epidural–general anesthesia compared with general anesthesia as has been attempted recently,1,2it is important to know whether antinociception and blood pressure control during the procedure are accomplished by the epidural local anesthetic or by other adjuvants such as β-adrenergic blockers, opioids, or sodium nitroprusside. Epidural dose requirements vary widely and are unpredictable. When reviewing this variability, it becomes apparent that weak local anesthetic solutions, set infusion rates, and infusions based on the patient’s weight will result in occasional inadequate blocks during prolonged procedures.We simplified a previously described method3to assess the adequacy of epidural block during general anesthesia using pupillometry while stimulating selected dermatomes. Our study population consisted of children aged 3.5 ± 2.2 yr, a group that others have found difficult to assess with pupillometry.4After institutional approval, we induced anesthesia with sevoflurane. Tracheal intubation was performed after administration of rocuronium bromide and fentanyl, and sevoflurane concentrations were set at 1.7–2% end-tidal. Three-second pupillary scans were alternatively measured (fig. 1) after tetanic stimulations (800 ms duration, 100 Hz, 140 mA) of the L4 and C5 dermatomes with a handheld Neuroptics pupillometer (Neuroptics Inc., Irving, CA)5connected to a Fisher-Paykel nerve stimulator (Fisher and Paykel Healthcare Inc., Panmure, Auckland 6, New Zealand) and surface electrocardiograph pad electrodes. Injection of 1 ml/kg bupivacaine, 0.25%, was administered either caudally or into the lumbar epidural space at time zero. As shown in figure 2, dilations were abolished after stimulation at the L4 dermatome but not after stimulation at the C5 dermatome. The latency of block onset was highly variable but averaged 8.7 ± 2.0 min, slightly longer than previously reported with 0.5% bupivacaine in awake subjects (5.8 min).6We conclude that in this group of subjects, in whom the epidural is placed after induction of general anesthesia and is often difficult to assess even after emergence, loss of pupillary dilation after dermatomal stimulation indicated the onset of epidural block.Pupillary dilation during general anesthesia is a parasympathetic reflex and is not confounded by antihypertensives that alter sympathetic function. This method avoids the previously described cumbersome technique of using steel needles and a laptop computer to synchronize the pupillary measurement to the stimulus. It works just as effectively in adults but should not be used in unparalyzed patients and is unreliable if dopamine 2 antagonists or large doses of opioids have been administered. In actual practice, it is not necessary to repeatedly stimulate and assess different dermatomes; absence of dilation two segments above the anticipated skin incision predicts an ongoing effective local anesthetic block. A neutral stimulation site (such as C5, or a trigeminal location) serves as a control to confirm that the pupil will dilate.With strict research protocols, many studies have failed to observe short-term advantages of epidural–general over general anesthesia, and this may be a valid conclusion for many operations. However, other studies, using stronger local anesthetic concentrations, have demonstrated reduced pain and improved functional capacity several weeks after hospital discharge,7,8and this benefit might outweigh the added risks involved in placement and maintenance of epidural catheters. Furthermore, an effective epidural block before emergence from general anesthesia assures, with few exceptions, a pain-free emergence without the need for neuraxial or intravenous opioids and their related side effects.*University of California, San Francisco, California. larsonm@anesthesia.ucsf.edu