The assessment of dermatomal level of surgical anesthesia after spinal tetracaine

Transcutaneous electrical stimulation (TES), a 60-mA, 50-Hz continuous squa re wave, has been considered equivalent to surgical incision. We examined w hether TES at a smaller current (10 mA) can be used to predict surgical ane sthesia and compare the results with sensory block to cold, pinprick, and t ouch after the administration of spinal tetracaine. Two groups of 40 consec utive patients, 17-69 yr old and 70 yr old or older received a subarachnoid injection of 0.5% tetracaine in 10% glucose or saline according to the typ e of surgery. Patients undergoing abdominal surgery received glucose soluti on, and those scheduled for lower extremities surgery received saline solut ion, and thus, the resultant four groups of patients were studied. Neural b lock was assessed by the loss of sensation to cold, pinprick, touch, and TE S at 10 mA (T10s), and tolerance (i.e., the loss of pain or discomfort) to TES at 10 (T10p) and 60 (T60) mA. Dermatomal levels of sensory block to col d, pinprick, and touch that were cephalad to T60 varied widely. In contrast , dermatomal levels of T10s and T10p cephalad to T60 were less variable, an d the difference between T10s and T60 was the smallest among all the differ ences in any groups. Our results demonstrate that, regardless of patient ag e and baricity of a local anesthetic solution, T10s is a good predictor of T60 equivalent to the dermatomal level of surgical anesthesia.