Factors affecting the spread and duration of epidural anesthesia with ropivacaine.

Background: Epidural anesthesia has an unpredictable extent and duration. Differences in the surface area of the lumbosacral dura, epidural fat volume, and epidural venous plexus velocity might explain the variability in the extent and duration of epidural anesthesia with ropivacaine.

Methods: Twenty-six healthy patients, aged 18-45 y, undergoing peripheral orthopedic surgery were enrolled. Dural surface area and posterior epidural fat volume were calculated from low thoracic, lumbar, and sacral axial magnetic resonance images obtained at 8-mm increments. Epidural venous plexus velocity at the L3-L4 disk level was derived from phase-contrast magnetic resonance images. The patients received 100 mg ropivacaine (1.0%) epidurally. The spread and duration of sensory anesthesia was assessed by pinprick, and that of motor block was assessed using a modified Bromage scale. Statistical correlation coefficients (rho) between magnetic resonance imaging and epidural anesthesia measurements were assessed by Spearman rank correlation. Stepwise multiple linear regression models were used to select important predictors of measures of epidural anesthesia.

Results: Dural surface area correlated with peak sensory block level (rho= -0.73, P = 0.0003) and onset time of caudal and cephalad block (rho = 0.62, P = 0.002; rho = -0.63, P = 0.002). Fat volume correlated with the regression to L5-S3 (rho = -0.44.44 to -0.54, P = 0.029 to 0.007). Epidural venous plexus velocity was significantly correlated with the regression to L3 (rho = -0.42, P = 0.038) and L4 (rho = -0.48, P = 0.017). Multiple regression analysis revealed that dural surface area was a significant predictive variable for the peak sensory block level (R = 0.61, P < 0.0001).

Conclusions: These findings indicate that dural surface area influences the spread of epidural anesthesia with ropivacaine and posterior fat volume influences the duration of epidural anesthesia in healthy patients within a narrow age range. Epidural venous plexus velocity might also influence the duration of epidural anesthesia with ropivacaine.