A dose-response study of bupivacaine for spinal anesthesia.

A randomized double-blind study was performed to elucidate the interrelationships among volume, concentration, and dosage of bupivacaine administered intrathecally. Sixty male patients between the ages of 40 and 80 years having transurethral surgery in the lithotomy position were studied using 10-, 15-, and 20-mg doses of glucose-free bupivacaine as either a 0.5 or a 0.75% solution. Success rate, time of onset and duration of anaglesia and motor block, and cardiovascular responses were assessed. It was found that both 15 and 20 mg of either concentration of bupivacaine provide satisfactory spinal anesthesia for transurethral urologic procedures. However, three of 20 patients receiving the 10-mg dose required supplementation with general anesthesia. Comparison of various volumes and concentrations of bupivacaine indicates that total dosage of bupivacaine is more important than volume or concentration. In several patients with sensory block involving cervical dermatomes, there was no significant hypotension or bradycardia, which suggests that cardiac output and venous return were maintained, perhaps because of the use of lithotomy position.