Background and Objectives The dependence of unilateral spinal anesthesia on injection flow is controversial. We hypothesized that it is possible to achieve strictly unilateral sympathetic block (as assessed by temperature measurements of the limbs) and unilateral sensory and motor block, respectively, during spinal anesthesia by a slow and steady injection of a hyperbaric local anesthetic solution.

Methods Forty-four patients (American Society of Anesthesiologists [ASA] physical status I-III) undergoing surgery of one lower extremity were randomly assigned to one of two groups. Dependent on the patients' height, 1.4 to 1.7 mL hyperbaric bupivacaine 0.5% was injected manually with the patient in the lateral decubitus position, which was maintained for 30 minutes after injection. Injection flow was approximately 0.5 mL/min in group I (“air-buffered” injections performed by 4 mL air between the local anesthetic and the syringe's plunger, n = 25) and approximately 7.5 mL/min in group II (“conventional” injections, n = 19). Sympathetic block was defined as a temperature increase of more than 0.5°C at the foot. Any reduction in the ability to move the hip, knee, or ankle as well as loss of temperature discrimination and/or pinprick even in one dermatome on the nondependent side was considered as a bilateral block.

Results Before surgery, significant differences (P < .05) were observed for unilateral motor paralysis (92% in group I v 68.4% in group II), unilateral sensory block (48.0% v 10.5%), and unilateral sympathetic block (72% v 42.1%). Strictly unilateral spinal anesthesia was found to be significantly more frequent in group I (40% v 5.3%). Significant hemodynamic differences between the groups were not detected.

Conclusions For hyperbaric spinal anesthesia, the injection flow is an important factor in achieving unilateral sympathetic block. A slow injection proves useful to restrict spinal anesthesia to the side of surgery.