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Epidural Stimulation Test (Tsui Test) and Local Anesthetic Spread in the Epidural Compartment We thank Muggleton et al for their interest in our article. We agree that the ability to predict appropriate spread of the anesthetic solution and symmetric bilateral blocks would be desirable in clinical practice. Intuitively, the epidural stimulation test (Tsui test) would be easier to interpret if a unilateral motor response predicted a unilateral block, whereas a bilateral response predicted a symmetric bilateral block. In clinical practice, however this has so far not been the case, indicating a discrepancy between the response to the Tsui test and the clinical response due to local anesthetic spread. In theory, this finding is not surprising, because symmetric epidural analgesia depends on the mechanical spread of local anesthetic in the epidural compartment, whereas the Tsui test stimulation depends on the shortest electrical circuit that can provide a conduction path to the spinal nerve root.1,2 Margarido et al2 were unable to demonstrate, in pregnant women, any correlation between the response pattern (unilateral versus bilateral) or the magnitude of the current required during the Tsui test and the epidural spread of the local anesthetic solution. The primary objective of the Tsui test is to confirm the correct catheter tip placement in the epidural space before the epidural administration of local anesthetic solutions.3 Generally, currents > 1 mA are needed to elicit muscle responses when the catheter tip is in the epidural space, whereas those observed at ≤ 1 mA usually are indicative of placement in the subarachnoid or subdural space or even migration out of the epidural space while remaining proximal to a nerve root.1,3 Mechanical spread of local anesthetic within the epidural compartment, however, depends not only on proper catheter placement, but it is also is influenced strongly by elements that are both extrinsic and intrinsic to the patient themselves. Extrinsic elements such as volume and force of local anesthetic injection and unique intrinsic anatomical elements such as the amount of epidural fat or presence of septa within the epidural compartment all can affect the mechanical spread of anesthetic. In contrast, those same elements have minimal effect on the conduction of electricity within the epidural compartment in terms of threshold current. In clinical practice, a lower stimulating current (eg, <2 mA) may be indicative of the greater proximity of the tip of the catheter to one of the nerve roots and may therefore suggest that a much larger volume of local anesthetic, or replacement of the catheter, would be needed as compensation to ensure adequate circumferential spreading to cover the nerve root on the opposite side of the compartment (ie, bilateral block).1 In such circumstances, there may be a risk of only achieving a unilateral epidural block when applying a small volume of local anesthetic. We agree with Muggleton et al that an objective physical measure, such as a bilateral response during the Tsui test that mirrors a bilateral epidural anesthesia block, would be much easier and preferable to interpret than the threshold current range. Despite the apparent lack of direct relationship between unilateral or bilateral electrical stimulation and unilateral or bilateral epidural anesthesia blocks, we currently are performing a study with the a priori hypothesis that the incidence of bilateral response to epidural stimulation will be greater with a 1.0-ms pulse width than with a 0.1-ms pulse width. This is based on the concept that increasing the pulse width allows peripheral nerves to be stimulated at a further distance from the source of a current.4 We believe that because the Tsui test uses the same mechanism (pulsed electrical current) to stimulate spinal roots, the effect of changing the pulse width also should be similar, resulting in the stimulation of spinal roots that are farther away from the source of the current. We hope that this will allow more consistent induction of a bilateral motor response when performing the Tsui test. Even if that proves to be true, however, it still remains to be seen whether a bilateral response to the Tsui test has any implication on the characteristics of the epidural block as compared with a unilateral response. In summary, the Tsui test is performed via stimulation of nerve roots rather than the spinal cord, and it allows improved identification of the spinal level of the catheter within the epidural compartment. It is important to remind the reader of the significance of the electrophysiologic mechanism of the Tsui test to help inform its appropriate evaluation. Ruchira Patel, MBBSCristian Arzola, MSc, MDVitali Petrounevitch, MSc, MDMrinalini Balki, MDKristi Downey, MScDepartment of Anesthesia and Pain ManagementMount Sinai Hospital, University of TorontoToronto, Canada Ban C. H. Tsui, MSc, MDDepartment of AnesthesiaStanford UniversityStanford, CA Jose C. A. Carvalho, MD, PhDDepartment of Anesthesia and Pain ManagementMount Sinai Hospital, University of TorontoToronto, Canada[email protected]

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