Background and Aims The erector spinae plane (ESP) block is widely used as alternative to more complex neuraxial and para-neuraxial blocks. However, the extent and reproducibility of the injectate spread remain unclear. The aim of this study was to investigate the pattern, extent, and variation in the spread of injectate during simulated ESP blocks.

Methods Bilateral ultrasound-guided ESP blocks were performed at T5 transverse process using iodised-contrast solution in fresh cadavers (20ml x 2). Computed Tomography (CT) imagining was performed 30 minutes after block administration. Two cadavers were dissected after injectate containing dye was administered.

Results The study included 20 sides on 10 cadavers; mean age 58years and mean height 163.6cm. Table 1 shows the frequency of spread across intramuscular planes and neural structures. There was a consistent spread to the dorsal ramus, while spread to neural structures and lateral spread was less predictable (table 2). There was greater spread cephalad than in the caudal direction. An inverse relationship was observed between the extent of LA spread and height. This pattern remained consistent after controlling for confounding variables (table 3).

Conclusions The spread of injectate during ESP blocks varied widely and was inversely proportional to the height of the specimen. Consistent spread was observed to the dorsal ramus. Further studies should be conducted on live subjects. Overall, the study provides valuable insights into the pattern and extent of injectate spread in simulated ESP blocks.