Quantitative assessment of differential sensory nerve block after lidocaine spinal anesthesia

Recent technology allows for quantitative and selective measurement of A beta, A delta, and C fiber nerve transmission. To gain further insight into the physiology of differential block after lidocaine spinal anesthesia, the function of these different fibers was quantitatively measured over time, and these measurements were correlated with regression of anesthesia to pinprick, touch, cold, and tolerance of tetanic electrical current (equivalent to surgical incision).

Six volunteers received lidocaine spinal anesthesia with 50 mg lidocaine (5% in dextrose). Cutaneous current perception thresholds at 2,000, 250, and 5 Hz, which stimulate A beta, A delta, and C fibers, respectively, were determined at L2-L3 (medial aspect above knee) before and every 10 min after spinal anesthesia. Dermatomal levels to pinprick, touch, and cold were assessed every 5 min after spinal anesthesia. Tolerance to tetanic electrical stimulus was assessed at L2-L3 every 10 min after spinal anesthesia.

Differential block was demonstrated by the sequential return of sensation to touch, pinprick, and cold at L2-L3. Recovery of function of A beta, A delta, and C fibers correlated with return of sensation to touch (R2 = 0.7, p = 0.03), pinprick (R2 = 0.75, p = 0.02), and cold (R2 = 0.67, p = 0.04) respectively. Loss of tolerance of surgical anesthesia corresponded to return of A beta current perception thresholds to baseline, whereas current perception thresholds for A delta and C fibers were still increased to greater than baseline (p = 0.025).

Differential sensory block during spinal anesthesia is due to different recovery profiles of A beta, A delta, and C fibers. Return of A beta current perception thresholds to baseline correlated with duration of surgical anesthesia as assessed with an electrical stimulation model.