Continuous spinal anesthesia with hyperbaric bupivacaine: a dose-response analysis.

Continuous spinal anesthesia (CSA) with hyperbaric bupivacaine (0.75% bupivacaine in 8.25% dextrose and water) was administered to 27 adult males for transurethral endoscopic surgery. Patients were randomized to receive either 3.75, 7.5 or 10 mg hyperbaric bupivacaine in a double-blind fashion. A 20-gauge nylon catheter was inserted at the L3-4 interspace, via an 18-gauge Tuohy-Schliff needle, extending only 2 cm into the subarachnoid space. All patients were placed in the lithotomy position prior to administration of the hyperbaric bupivacaine. Sensory block was assessed by pinprick and perception of a 50-Hz tetanic stimulus at 5, 10 and 20 minutes, then every 20 minutes thereafter. Peak sensory and motor block were obtained by 20 minutes for all dose ranges. Peak sensory levels were T7.1, T5.6 and T3.9 for 3.75, 7.5 and 10 mg hyperbaric bupivacaine, respectively. Linear regression analysis demonstrated a significant correlation (r = -0.68) between peak sensory level and dose of bupivacaine. Peak sensory level as estimated by pinprick and 50-Hz tetanic stimulus showed excellent correlation (r = 0.99). The role of the subarachnoid catheter in the dispersion and distribution of bupivacaine within the subarachnoid space was studied in a model spinal canal system and compared with the distribution of bupivacaine administered via a standard 25-gauge spinal needle. No significant differences were found in the distribution of bupivacaine with either method of injection. The distribution of hyperbaric bupivacaine within the subarachnoid space appears to be related to baricity but is unrelated to administration via needle or catheter.