Local Anesthetic Test Dose as a Predictor of Effective Epidural Opioid Analgesia

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Abstract
Background: When local anesthetic is used to produce epidural anesthesia intraoperatively, epidural catheter placement is confirmed. However, when epidural catheters are placed intraoperatively only to provide postoperative opioid analgesia, correct catheter placement may not be confirmed by administration of a local anesthetic. The current study tests the hypothesis that the extent of sensory blockade produced by a 10-ml dose of 1.5% lidocaine can be used to predict the adequacy of epidural opioid analgesia. Methods: Forty-nine patients undergoing major abdominal surgery in whom a lumbar epidural catheter was placed intraoperatively were studied, but no more than 3 ml 1% lidocaine had been injected. Placement of the epidural catheter was assessed in the postanesthesia care unit by administration of a 10-ml dose of 1.5% lidocaine. The extent of sensory blockade was determined using the pinprick technique: All dermatomes, T2 and below, were assessed and scored using 1 point per dermatome per side from L1 to T2 to a maximum of 24 points. Scores were arbitrarily divided into three groups, where group 1, 0-7 points; group 2, 8-15 points; and group 3, 16-24 points. Epidural morphine infusion was initiated independently of the extent of the sensory blockade and adjusted using predetermined guidelines. Adequacy of opioid-induced analgesia was determined using the visual analog scale. Results: Significantly lower visual analog scale scores for pain at rest and with movement from epidural morphine infusion were associated with sensory blockade score of 16-24 points. Seven patients failed to obtain a detectable sensory block. No patient requested alternative analgesia. None of the epidural catheters was removed because of inadequate pain relief, even in patients who failed to obtain a detectable sensory block. Conclusions: Extensive sensory block from 10 ml 1.5% lidocaine was associated with excellent epidural opioid analgesia. Extent of analgesia after a 10-ml test dose of 1.5% lidocaine can be used to predict the adequacy of analgesia resulting from an epidural opioid infusion. The failure of a local anesthetic dose to produce sensory blockade does not necessarily predict a failure to produce analgesia from an epidural opioid infusion, as indicated by the presence of analgesia in several patients without detectable sensory block.