Spinal anesthesia. Volume or concentration--what matters?

An investigation was made of the effects of volume and concentration of a constant dose of subarachnoid lidocaine on the extent and duration of sensory and motor anesthesia produced, as well as of the lidocaine concentration of the cerebrospinal fluid (CSF) as a function of time.

In a prospective study, 40 American Society of Anesthesiologists (ASA) status 2 or 3 patients were assigned to one of five groups, who received a 70-mg subarachnoid dose of lidocaine hydrochloride as a 0.5, 1, 2, 5, or 10% solution. Dural puncture was performed at the L3-L4 interspace with a 19.5-gauge Periquick needle (Pajunk, Germany), and a 24-gauge catheter was inserted 3-4 cm into the subarachnoid space. The patient remained in the lateral position during injection of the local anesthetic and was then turned to the supine horizontal position. The level of anesthesia and the motor block were measured at 5, 10, 15, 20, 30, 40, 50, and 60 minutes and then at 15-minute intervals until the effect of the anesthesia had ceased. Samples of CSF were collected at the same times that the pinprick and motor block measurements were made.

Five minutes after injection, a median sensory block height of T4 or T5 was observed in all groups. The range of mean total times to complete recovery of the sensory blocks was 139-152 minutes, while that for the motor blocks of the lower extremities was 100-122 minutes. The values were similar in all groups (P > .05). The motor block was complete in all patients 10 minutes after the lidocaine injection. Five minutes after injection, the mean CSF lidocaine concentration was highest in the 10% group (P < .001 vs. the other four groups). At 15 minutes, the only statistical difference was found between the 0.5% and the 10% group (P = .026). At 20 minutes, the CSF lidocaine concentrations were similar in all groups (P > .05).

A constant 70-mg dose of subarachnoid lidocaine produced the same pinprick level of analgesia, degree of motor block, and duration of spinal anesthesia in spite of being injected over an extremely broad range of concentrations and volumes. Despite the fact that all patients received the same dose of lidocaine, the CSF concentrations at 5, 10, and 15 minutes were different and directly related to the concentration of the solution injected. at 20 minutes, the CSF concentrations were similar in all groups. These results indicate a relatively uniform distribution of lidocaine in the CSF for all solutions tested.