Background and Objectives The lumbosacral cerebrospinal fluid volume is a major determinant of the intrathecal spread of local anesthetics. Ultrasound imaging of the lumbar spine allows measurement of dural sac dimensions, which may potentially be used as a surrogate of cerebrospinal fluid volume. The purpose of this study was to investigate the correlation between lumbar dural sac diameter, dural sac length (DSL), and dural sac volume (DSV), measured by ultrasound, and the intrathecal spread of isobaric bupivacaine during combined spinal-epidural (CSE) analgesia for labor.

Methods We examined 41 women with singleton pregnancies requesting neuraxial analgesia for labor. Using a 5-2–MHz curved-array ultrasound probe in the paramedian sagittal plane, we measured the dural sac width at each lumbar interspace and the DSL from L1-2 to L5-S1 interspace and calculated the dural sac volume (DSV). Following CSE block with 0.25% isobaric bupivacaine 1.75 mg and fentanyl 15 μg, peak sensory levels (PSLs) were recorded using ice, cotton, and pinprick. Statistical correlation coefficients between dural sac dimensions and PSLs were assessed by Spearman rank correlation. In addition, multiple linear regression models were used to select important predictors of PSLs.

Results There was a moderate correlation between DSL and PSL to ice (ρ = −0.62; P < 0.0005) and to pinprick (ρ = −0.52; P = 0.017). Similarly, there was a moderate correlation between DSV and PSL to ice (ρ = −0.56; P = 0.004) and to pinprick (ρ = −0.61; P < 0.0008). Neither the DSL nor DSV correlated with PSL to cotton. Multiple linear regression analysis revealed that DSL, weight, and body mass index contributed to PSLs.

Conclusions The length of the lumbar spine determined by ultrasound, rather than the lumbar spine volume, combined with the weight or body mass index of the subject, is of particular value in predicting the intrathecal spread of isobaric bupivacaine during CSE analgesia for labor.