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 (R ² = 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.