Ultrasound imaging of the thoracic spine in paramedian sagittal oblique plane: the correlation between estimated and actual depth to the epidural space

Abstract

Background: Ultrasound (US) imaging of the spine has been shown to be a reliable tool to facilitate lumbar epidural needle placement; however, its feasibility in thoracic epidural placement is still unknown. The objective of this study was to assess the accuracy and reliability of prepuncture US imaging in the paramedian sagittal oblique plane to estimate the depth to the epidural space and optimum insertion point for guiding epidural needle placement at the mid-low thoracic level.

Methods: This prospective study included 35 healthy adult patients who requested thoracic epidural analgesia before their upper abdominal surgeries. Ultrasound imaging was done in the paramedian sagittal oblique plane at the desired thoracic level to identify the intervertebral space, the distance from the skin to the epidural space (US depth [UD]) and the needle insertion point. Subsequently, a staff anesthesiologist 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 and concordance correlation coefficients and Bland-Altman analysis with 95% limits of agreement.

Results: The average patient age was 56 (SD, 14) years, and body mass index was 28 (SD, 6) kg/m2. The precision of the agreement between UD and ND estimated by Pearson correlation coefficient was 0.75, and the accuracy was 0.80, whereas the concordance correlation coefficient was 0.60 (confidence interval, 0.43-0.78). The mean UD and ND were 4.3 (SD, 0.96) and 5.0 (SD, 1.2) cm, respectively. The Bland-Altman analysis showed a mean difference of -0.71 cm (95% limits of agreement, 0.8 to -2.2 cm). There was a significant direct correlation of the ND with the body mass index (r2 = 0.27, P = 0.008). The mean number of attempts was 1 (p25-p75 = 1-2), and the epidural space was identified with 2 or less redirections in 88% of the cases.

Conclusions: We found a good correlation between the US-estimated distance to the epidural space and the actual measured needle distance in our patients. We suggest that our proposed prepuncture US method, using the paramedian sagittal oblique approach, can be a useful guide to facilitate the placement of epidural needles at mid-low thoracic levels. A randomized controlled trial is necessary to confirm the utility of prepuncture US in thoracic epidural placement.