A reduced dose of lidocaine may have advantages over a standard dose for detecting inadvertent intrathecal catheter placement during epidural anesthesia, but neither is wholly reliable, say researchers.
"The correct dose should balance the risk of missing an intrathecal catheter against the side effects caused by the test dose," say lead researcher Stephen Pratt (Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA) and team.
Their randomized trial, involving 96 women who were eligible for combined spinal and epidural anesthesia for elective cesarean delivery, suggests that a reduced dose remains an effective rule-in test for an intrathecal catheter. But a much larger study is needed to properly assess its rule-out ability and associated side effects, they say.
The patients were assigned to one of four groups, receiving lidocaine 30 or 45 mg as a spinal or epidural injection. When assessed 3 minutes later, all women given an intrathecal injection reported warmth and/or heaviness in their legs, compared with just 26% and 41% of patients given lidocaine 30 and 45 mg, respectively, as an epidural injection.
At this point, all women given an intrathecal injection had objective evidence of sensory block, compared with about a third of those who received an epidural injection. Also, 83% and 100% of patients given lidocaine 30 and 45 mg by intrathecal injection had evidence of motor block, compared with just two patients from the epidural group (both given 45 mg).
Based on these findings, given an accidental intrathecal catheter rate of one in 380, sensory block with either dose of lidocaine would have a 100% negative predictive value, say the researchers in Anesthesia and Analgesia.
However, the positive predictive values were very low, at less than 1% with both doses, suggesting a high false-positive rate, they report.
Also, reducing the lidocaine dose did not significantly reduce the rate of adverse effects. All women given lidocaine 45 mg developed a block above T8 by 10 minutes, with the block reaching T1 in four. All those given the 30 mg dose developed a block above T11, which reached T1 in two.
In an editorial accompanying the study, Jill Mhyre (The University of Michigan Health System, Ann Arbor, USA) writes: "Despite its imperfections, the epidural test dose prompts anesthesiologists to focus their attention on the early recognition of an intravascular or subarachnoid catheter."
She adds: "Careful attention to lower extremity strength testing both at the time the catheter is initially placed and tested, and throughout the course of the analgesic infusion, may improve recognition of the occasional intrathecal catheter."
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