Sensory block levels during combined spinal-epidural for labor analgesia: influence of local anesthetic dose and lumbar spine dimensions-A randomized controlled trial.

Abstract Number: O2-06
Abstract Type: Original Research

Introduction: Lumbar cerebrospinal fluid volume measured by magnetic resonance imaging (MRI) bears an inverse correlation with the intrathecal spread of anesthetic solutions.[1] While MRI is a valuable research tool; it cannot be used at bedside to guide clinical practice. Ultrasound (US) is a practical bedside resource used to facilitate spinal and epidural anesthesia. A previous study using one standard dose of bupivacaine determined that US measurements contribute to predict the intrathecal spread during CSE analgesia for labor.[2] We hypothesized that the predictive model could be improved by studying a dose range of bupivacaine in conjunction with dural sac dimensions and patient’s characteristics.

Methods: In this randomized, double blind, controlled trial, we recruited women with singleton term pregnancies requesting neuraxial analgesia while in labor. US imaging was performed with a 5-2 MHz curved array probe in the left paramedian sagittal plane at levels L5-S1 to L1-L2. We measured the dural sac width (DSW) at each lumbar interspace; the lumbar dural sac length (DSL: distance between L5-S1 and L1-L2 interspaces); and the vertebral column length (VCL: distance from C7 prominence to L5-S1 interspace). The lumbar dural sac volume (DSV) was subsequently calculated, assuming the spinal canal being a cylinder with a diameter equal to the mean value of the five DSW measurements. CSE analgesia was induced with one of three doses of 0.25% bupivacaine: 1.5 mg, 2 mg or 2.5 mg – in association with 15μg fentanyl. Sensory block levels (SBL) to ice and pinprick (60g Von Frey filament) were assessed at 5, 10, 20, and 30 min. We used mixed effect models for repeated measures to examine the association of SBL to ice or pinprick with dose, time and patient characteristics. Multiple linear regression models were used to examine the association of peak SBL with dose, patient characteristics, and US measurements

Results: We recruited 60 women (20/dose group). Height, weight, and BMI: mean (SD) of 161.8(6.5) cm, 75.5(11.2) kg, and 28.8(3.8) kg/m2. Mean DSW, DSL, VCL and DSV were 1.2 cm, 11.4 cm, 51.5 cm and 14.6 cm3 respectively. The median peak SBL for 1.5, 2.0 and 2.5 mg were reached at 20 min: T6, T5, T4 (ice) and T8, T7, T6 (pinprick), respectively. We found that the peak SBL positively associated with dose, while inversely correlated with DSW. Side effects: hypotension 6.9% (only in 2.5 mg); uterine hypertonus 10.3%; fetal bradycardia 18.9%.

Discussion: Although, we obtained a new model that includes the dose and the DSW, this could not improve the predictive value. Nevertheless, we found that higher peak SBL were associated with larger doses of bupivacaine, lower DSW, and higher risk of fetal bradycardia. These findings may assist in predicting block levels in patients undergoing CSE for labor analgesia.

References:

[1] Anesthesiology 2004; 100:106-14

[2] Reg Anesth Pain Med 2012; 37:283–8

SOAP 2014