Background: Spinal ultrasound (US) in the transverse median (TM) plane underestimates the distance to the epidural space in obese pregnant women, most likely because of compression of the subcutaneous tissue during the assessment, often required to compensate for poor visibility. We tested whether scanning in the paramedian sagittal oblique (PSO) plane compared with the TM plane resulted in a more precise estimate of the actual skin-epidural space measurement in this population.
Methods: We recruited obese (World Health Organization classes I, II, and III) pregnant women at term requesting labor epidural analgesia or combined spinal-epidural anesthesia for cesarean delivery. US imaging was performed with a 5-2 MHz curved array probe to identify the insertion point and to estimate the distance from the skin to the epidural space (US-estimated depth, UD) in the PSO and TM planes. The measurements were performed with the least possible compression of the subcutaneous tissue by the US probe. All punctures were performed via the midline approach. An anesthesiologist performed the epidural/combined spinal-epidural procedure at the predetermined insertion point, and marked the actual needle distance from the skin to the epidural space (needle depth, ND). Bland-Altman analysis was used to determine the differences and 95% limits of agreement between US depth and ND.
Results: We studied 60 women. The mean (SD) body mass index was 39.6 (7.9) kg/m(2) (range 30.4-66.2 kg/m(2)). The US estimate in the PSO and TM planes, and the actual ND were 6.5 (1.2) cm, 6.5 (1.1) cm, and 6.6 (1.3) cm, respectively. The Bland-Altman analysis showed a mean difference of 0.05 cm and 95% limits of agreement of ±1 cm. The quality of imaging was rated as good in the PSO and TM planes in 86.7% and 68.3%, respectively (P = 0.028).
Conclusion: The estimates of the US-determined distance to the epidural space in the PSO are comparable to those in the TM plane. The ability to use both estimates interchangeably for midline punctures may prove useful in patients presenting with poor visibility in the TM plane.