A catheter positioned properly in the epidural space can provide excellent surgical anesthesia, postoperative analgesia, and labor analgesia. Inadvertent placement of the catheter into the cerebrospinal fluid (CSF) (intrathecal) or an epidural vein (intravascular) could lead to catastrophic complications. Positive aspiration of blood or CSF from the catheter confirms catheter misplacement. However, the absence of an aspirate cannot rule out whether or not the catheter is actually in the epidural space. The incidence of false negative aspiration is lower for multiorifice epidural catheters (<1%) compared to single-hole catheters (2%). Aspiration of fluid may fail due to low epidural venous pressure, air locking within a filter, mechanical obstruction due to tissue or blood, or simply incorrect identification of the aspirate. For these reasons, a “test dose” should be administered subsequent to epidural catheter placement and prior to incremental dosing of small volumes of local anesthetic.
An epidural test dose involves injecting local anesthetic to determine accidental intravenous or intrathecal catheter placement. The most popular and effective test dose is 3 mL of lidocaine 1.5% with epinephrine 1:200 000. From a practical standpoint, an ideal test dose should be a single solution that produces objective evidence of intravascular or intrathecal injection within several minutes of administration. A test dose should be safe for a parturient and her fetus, and should not increase the risk of complications for all patients. It should not significantly delay the onset of epidural anesthesia.
The ideal epidural test dose would have both high sensitivity and specificity. As sensitivity increases, more intravascular catheters would be detected. A high false-positive rate (low specificity) would lead to unnecessary manipulations or replacements of correctly positioned epidural catheters. In general, the epidural test dose has high (>90%) sensitivity but poor specificity (around 50%). Therefore, a negative test dose does not guarantee—it only decreases the probability—that the catheter is not in the intravascular or intrathecal space. A negative test dose also does not ensure proper placement in the epidural space.
The epidural test dose should always be injected rapidly. Slow administration may cause the drugs (both epinephrine and local anesthetic) to undergo redistribution and metabolism before a sufficient mass could bind to its receptors. Furthermore, most anesthesiologists use closed-tip multiorifice epidural catheters. Any number of the three orifices could be positioned in blood or CSF while the other is in the proper space. Slow administration may mean that the epidural test dose exits the proximal orifice and does not reach the most distal orifice. As a result, part of the catheter may remain undetected within the intravascular or intrathecal space.
Testing for Intrathecal Placement
The test dose for accidental intrathecal catheter placement should produce relatively rapid sensory changes to allow for easy identification. The intrathecal component of the test dose should not cause cardiovascular compromise, high or total spinal anesthesia, or neurotoxicity. For these purposes, 3 mL of lidocaine 1.5% is the ideal local anesthetic. Lidocaine allows for reliable detection of intrathecal injection within a short period of time. If the catheter is placed in the CSF, 45 mg lidocaine produces detectable sensory block (leg warmth and sensory loss to pinprick) within 1-2 minutes and a motor block (leg weakness and ...