Ultrasound Imaging of the Lumbar Spine in the Transverse Plane: The Correlation Between Estimated and Actual Depth to the Epidural Space in Obese Parturients

BACKGROUND: Prepuncture lumbar ultrasound scanning is a reliable tool to facilitate labor epidural needle placement in nonobese parturients. In this study, we assessed prepuncture lumbar ultrasound scanning as a tool for estimating the depth to the epidural space and determining the optimal insertion point in obese parturients. METHODS: We studied 46 obese parturients, with prepregnancy body mass index (BMI) >30 kg/m(2), requesting labor epidural analgesia. Ultrasound imaging was done by one of the investigators to identify the midline, the intervertebral space, and the distance from the skin to the epidural space (ultrasound depth, UD) at the level of L3-4. Subsequently, an anesthesiologist blinded to the UD located the epidural space through the predetermined insertion point and marked the actual distance from the skin to the epidural space (needle depth, ND) on the needle with a sterile marker. The agreement between the UD and the ND was calculated using the Pearson correlation coefficient and a paired t-test. Bland-Altman analysis was used to determine the 95% limits of agreement between the UD and the ND. RESULTS: The prepregnancy BM I ranged from 30 to 79 kg/m(2), and the BMI at delivery was 33-86 kg/m2. The Pearson correlation coefficient between the UD and the ND was 0.85 (95% confidence interval: 0.75-0.91), and the concordance correlation coefficient was 0.79 (95% confidence interval: 0.77-0.88). The mean (isle) ND and UD were 6.6 +/- 1.0 cm and 6.3 +/- 0.8 cm, respectively (difference = 0.3 cm, P = 0.002). The 95% limits of agreement were 1.3 cm to -0.7 cm. Epidural needle placement using the predetermined insertion point was done without reinsertion at a different puncture site in 76.1% of parturients and without redirection in 67.4%. CONCLUSIONS: We found a strong correlation between the ultrasound-estimated distance to the epidural space and the actual measured needle distance in obese parturients. We suggest that prepuncture lumbar ultrasound may be a useful guide to facilitate the placement of epidural needles in obese parturients. (Anesth Analg 2009;108:1876-81)