The Epidural Test Dose in Obstetric Patients

The unintentional IV injection of local anesthetic is one of the most serious potential complications of epidural anesthesia in pregnant women [1]. The epinephrine-containing test dose is the most commonly used method for detecting an intravascular catheter. Many anesthesiologists consider the epinephrine test dose a sensitive and specific method of identifying an intravascular epidural catheter [2]. Others have questioned its usefulness in obstetric patients [3]. A recent study [4] evaluated the use of an epidural test dose of lidocaine 45 mg with epinephrine 15 [micro sign]g in laboring women; the authors noted no false-negative and 14 true-positive responses, thus demonstrating the usefulness of the epinephrine-containing test dose in laboring women. However, in this issue of Anesthesia & Analgesia, Norris et al. [5] present data that they interpret as justification for abandoning the epidural test dose. Does this study justify a conclusion that the epidural test dose has outlived its usefulness? To answer this question, several areas should be explored. 1. Have changes in technique (e.g., use of a multiorifice epidural catheter, administration of a dilute solution of local anesthetic, and use of a continuous epidural infusion) made the epidural test dose unnecessary? 2. Do the data presented by Norris et al. justify their conclusions? 3. Does the "perfect" test dose exist? It has been 20 yr since Albright [6] reported several cases of bupivacaine-induced cardiotoxicity and maternal mortality. These cases not only prompted the Food and Drug Administration to forbid the epidural administration of 0.75% bupivacaine in obstetric patients, they also resulted in several changes in the technique for administration of epidural analgesia in laboring women (e.g., use of an epinephrine-containing test dose, followed by incremental injections of a dilute solution of local anesthetic). Few anesthesiologists continue to give large bolus doses of a concentrated solution of local anesthetic through the epidural needle or catheter. Further, during the administration of combined spinal-epidural analgesia, some anesthesiologists initiate an epidural infusion of local anesthetic without giving any bolus dose of local anesthetic. Moreover, the widespread use of a continuous epidural infusion of local anesthetic-with or without an opioid-has reduced the need for subsequent bolus injections of a local anesthetic. More recently, at least one new local anesthetic (ropivacaine) has been introduced, and another (levobupivacaine) will soon follow. Both drugs seem to be less cardiotoxic than bupivacaine. Epidural analgesia seems to be safer today than it was 20 yr ago, but cannulation of an epidural vein is an indisputable problem in obstetric patients [7]. Cases of unintentional IV injection of a local anesthetic-with consequent morbidity and mortality-continue to occur. Aspiration may allow the anesthesiologist to identify most cases of IV placement of a multiorifice epidural catheter, but many anesthesiologists continue to use a single-orifice epidural catheter. Further, aspiration does not identify all cases of IV cannulation of a multiorifice catheter. In the study by Norris et al. [5], at least two (and probably three) IV catheters were not detected by aspiration alone (see their Table 3. These data do not support abandonment of the epinephrine-containing test dose. Some anesthesiologists argue that the unintentional IV administration of a dilute solution of local anesthetic (e.g., 0.0625% bupivacaine with fentanyl) could not possibly harm a patient. They argue that a continuous IV infusion of a dilute solution of local anesthetic would result in regression of analgesia long before the occurrence of systemic local anesthetic toxicity. Perhaps this is true, but this argument fails to consider the patient who requires an emergency cesarean section before the recognition of an intravascular catheter. The administration of a large IV dose of a more concentrated solution of local anesthetic can cause systemic toxicity, including seizures, cardiovascular collapse, and death. Because any laboring patient may require an emergency cesarean section at any time, it seems prudent to confirm the location of an epidural catheter early rather than late. We acknowledge that using an epinephrine-containing test dose may occasionally result in a false-positive response that might prompt the unnecessary removal and replacement of an appropriately sited epidural catheter. However, the incidence of false positives may be reduced by careful attention to detail, such as injecting the test dose immediately after a uterine contraction, which will decrease the likelihood of confusing epinephrine-induced tachycardia with pain-induced tachycardia. In patients with an equivocal response, the epinephrine test dose may be repeated, or another test, e.g., the air test [8], may be performed. The inconvenience resulting from the occasional removal of an appropriately sited catheter pales in comparison with the morbidity or mortality that may result from an unintentional IV injection of a large dose of local anesthetic. Further, delayed recognition of an IV catheter will undoubtedly result in unnecessary pain while both the patient and the anesthesiologist wait for analgesia that will never come (because the catheter is IV rather than epidural in its location). The data presented by Norris et al. [5] do not support the abandonment of the epidural test dose in laboring women. However, their observations cast serious doubt on the practice of withdrawing an intravascular catheter until aspiration is negative. The present study suggests that aspiration is a less sensitive test during and after partial withdrawal of an intravascular epidural catheter and that such a catheter may remain intravascular despite partial withdrawal and subsequent negative aspiration for blood. No epidural test dose is perfect. One report documented the failure of injection of lidocaine, epinephrine, and fentanyl to signal an intravascular catheter. This patient subsequently experienced a generalized tonic-clonic seizure [9]. However, the epinephrine-containing test dose provides a sensitive, objective marker of intravascular injection. Careful use of an appropriate test dose also facilitates the detection of a subarachnoid catheter and thus improves the safety of epidural analgesia in laboring women. Just as the Apgar score prompts healthcare providers to focus their attention on the newborn infant, the epidural test dose prompts anesthesiologists to focus their attention on the early recognition of an intravascular or subarachnoid catheter. The administration of a test dose is especially important in patients who may require the administration of a large dose of local anesthetic for an emergency cesarean section. In other words, a test dose is important in all laboring women who receive epidural analgesia. There is no substitute for careful observation, vigilance, and sound clinical judgment. Careful aspiration, followed by an appropriate test dose, increases the likelihood that an intravascular catheter will be detected and that a systemic reaction to local anesthetic will be avoided. We believe that continued use of an epidural test dose should be embraced, not abandoned.