DIAGNOSTIC-ACCURACY OF AN INTRATHECAL TEST DOSE IN EPIDURAL ANALGESIA

The authors evaluated the accuracy of an intrathecal test dose (TD) to rule out unintentional subarachnoid injection in a prospective and do uble-blind study. Twenty-nine unmedicated patients scheduled to receiv e continuous spinal analgesia for their surgical procedures entered th e study. After placement of noninvasive monitoring (ECG, BP and SpO(2) ), an intrathecal catheter was placed in the lumbar area. In the supin e position 15 patients in the study group received lidocaine 45 mg + e pinephrine 15 mg and 14 patients in the control group received normal saline 3 ml intrathecally. After the injection, a blinded observer ass essed the presence of the following signs at one minute intervals for five minutes: sensory loss by pin-prick (P) over the lumbar and sacral dermatomes, ability to raise legs (F), and subjective feeling of warm th (W) and heaviness (H) over the lower limbs. For each sign the sensi tivity (SN), specificity (SP), and negative (-) and positive (+) predi ctive value (PV) were calculated. At four minutes SN reached 100% (CI 78-100%) for R and H, while only R obtained a SP of 93% (CI 66-100%). Using R as detector of intrathecal injection the -PV was 100% and the +PV 25% (assuming a 2% prevalence of unintentional spinal). We conclud e that, with our test dose; (a) four minutes are needed to recognize s igns of intrathecal injection; (b) leg-raising is a reliable sign; and (c) inquiring about other signs may only decrease the diagnostic accu racy of this test.