The effect of body mass index on the spread of spinal block in patients with rheumatoid arthritis

Abstract

Body mass index (BMI) has a significant effect on the spread of sensory spinal block in rheumatoid patients. We tried to achieve the same spread of spinal block for patients in three different BMI groups and, on the basis of the results from a preliminary study, used a simple method feasible for clinical practice. We hypothesized that BMI-related inverse dosing of plain bupivacaine according to low, normal, and high BMI would result in no difference in block extent. Together 75 patients with seropositive rheumatoid arthritis were included in three equal-sized groups according to BMI: low (<23kg/m(2)), normal (23-28kg/m(2)), and high (>28kg/m(2)). Spinal anesthesia was induced with plain bupivacaine using doses 3.3, 3.0, and 2.7ml, respectively. The spread of sensory block was recorded 30min after injection of bupivacaine by use of a pin-prick test and a cold ice-filled container. Spreads of sensory block were different in low, normal, and high BMI groups (mean (SD); 14.0 (2.6), 14.5 (2.5), and 16.3 (2.5) dermatomes, respectively, P=0.006) because of greater block extent in the high-BMI group. Despite three-step dosing of plain bupivacaine inversely related to BMI (low, normal, or high), comparable block extent was not achieved because of greater spread in the high-BMI group. Adjustment of plain bupivacaine dose according to BMI could be used to achieve a more predictable spread of spinal block, but further reduction of dose is needed in patients with high BMI.