Sensitivity, specificity and predictive value of the sensation of warmth as a method of detecting inadvertent subarachnoid injection of local anaesthetic when performing extradural blocks.

In order to test if the rate of onset of sensation of warmth in the legs after the injection of 0.5% bupivacaine might discriminate between subarachnoid and extradural injection, 150 urological patients were allocated randomly to receive either spinal anaesthesia with isobaric (IS) or hyperbaric (HS) 0.5% bupivacaine, or extradural anaesthesia with isobaric 0.5% bupivacaine. The volume of the local anaesthetic for spinal anaesthesia and for the extradural test dose was 3-4 ml. The patients were asked to report at once if they had a sensation of warmth in the legs during or after injection of local anaesthetic. The mean time to the sensation of warmth was significantly shorter in the spinal groups (80 (SEM 10) s in IS and 76 (8.0) s in HS) than in the extradural group (558 (38) s). However, six patients in the IS and two in the HS group had no sensation of warmth.