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Correspondence  |   September 2006
Efficacy of Epidural Block during General Anesthesia
Author Affiliations & Notes
  • Merlin D. Larson, M.D.
    *
  • *University of California, San Francisco, California.
Article Information
Correspondence
Correspondence   |   September 2006
Efficacy of Epidural Block during General Anesthesia
Anesthesiology 9 2006, Vol.105, 632-633. doi:
Anesthesiology 9 2006, Vol.105, 632-633. doi:
To the Editor:—
When searching for possible advantages of epidural–general anesthesia compared with general anesthesia as has been attempted recently,1,2 it 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 method3 to 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.4 After 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)5 connected 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).6 We 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.
Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
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Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
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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,7,8 and 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.
References
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Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
Fig. 1. The face of the pupillometer displays the 3-s scan, the percent dilation, and the time and date. The 800-ms stimulus is delivered during the square wave, shown below the x-axis. Pupillary reflex dilation is locked to the stimulus with a long latency. 
×
Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
Fig. 2. Average pupillary reflex dilations from L4 and C5 stimulation sites for all 20 subjects.  Black squares  are dilations after C5 stimulation, and  black circles  are dilations after L4 stimulation. The data point for −1 min represents measures taken before injection. With one subject, the L4 dilations were not blocked. *  P  < 0.001 compared with percent dilation at C5, two-way analysis of variance. 
×