Failure of a lidocaine test dose to identify subdural placement of an epidural catheter.

We report the failure of a test dose of 3 ml lidocaine 1.5 per cent with 15 micrograms epinephrine to identify subdural placement of an epidural catheter in a parturient. Thirty-five minutes after injection of 13 ml lidocaine 1.5 per cent, intended to provide epidural analgesia, the patient developed an extensive sensory neural blockade. Some motor control was maintained and sympathetic block was incomplete. Blood pressure and oxygenation were easily supported with optimum positioning, fluids, ephedrine and oxygen by mask. The patient remained alert. The duration of neural blockade was approximately two hours. The patient underwent a second epidural for labour analgesia that was uneventful. There were no sequelae. Subdural injections are uncommon and unpredictable in their occurrence. Test doses do not consistently identify misplaced catheters. A negative response to a test dose does not guarantee that extensive neural blockade will not occur during epidural analgesia.