When Is Testing the Test Dose the Wrong Thing to Do?

Abstract

T he article by Schoenwald et al. in this issue (1) is one of many in the anesthesia literature that report on the optimal test dose before epidural anesthesia to determine whether the catheter has been misplaced in a surgical patient. Like the others, this article seems to dismiss an important question: Is a test dose necessary? If so, what are its relative risks and benefits? The benefits and risks for any test must be evaluated, at least theoretically, before we consider the best conditions for performing the test. Let us explore the risks and benefits of test doses in vascular surgical patients to justify the search for an optimal test dose in that population. Epidural catheters can be misplaced into either the intravenous or subarachnoid space. A misplaced catheter can be identified by the return of blood or cerebrospinal fluid spontaneously or with aspiration; by the use of sequential divided doses, rather than a single dose; and by the use of an additional drug, commonly epinephrine, to elicit a physiologic sign of misplacement (2,3). In epidural anesthesia for obstetrics, misplaced catheters not detected by blood return or aspiration and subsequent incremental dosing are very rareless than 1% (4,5). But obstetric patients may belong to a different risk group than vascular patients. In vascular surgery patients, the frequency with which epidural catheters are misplaced, and whether misplacement can be detected by the first two methods described above, is not known. We need more information before we can determine whether a significant problem exists and, if so, whether we should focus on a test dose to solve it. We strongly recommend that prevalence studies be undertaken to quantify the likelihood that a misplaced catheter will remain undetected in vascular surgical patients even after aspiration and sequential dosing. If we find a problem that can be resolved by a test dose, we can assess the value of the test by clinical