The association between injected volume of local anesthetic and spread of epidural anesthesia: A hypothesis

BACKGROUND AND OBJECTIVES: Clinically, an increase of the injected volume of local anesthetic does not result in a linear increase in the spread of epidural anesthesia. This study was designed to evaluate this observation. METHODS: One-hundred twenty adult women undergoing abdominal gynecologic surgery were randomly assigned into 6 groups of 20 patients each. An epidural catheter was inserted in each patient via the L1-L2 interspace, and 2, 4, 6, 8, 10, and 12 mL of 2% mepivacaine was injected, respectively, in each group. Fifteen minutes later, the cephalad extent of hypesthesia above the second sacral dermatome was tested with an alcohol swab. RESULTS: The regression analysis between injected volume (x mL) and number of anesthetized dermatomes (y) approximates a cubic polynominal equation rather than a simple equation, as follows; y = 3.021 x - 0.274 x2 + 0.009673 x3 (R2 = 0.938, P = .0435). CONCLUSIONS: The present study demonstrates lack of a linear relationship between injected volume of local anesthetic and spread of epidural anesthesia. Whether the cubic equation developed is clinically useful remains unclear because of the variability of the data.