BACKGROUND: Spread of local anaesthetic solution in the paravertebral space after erector spinae plane block (ESPB) is variable. We evaluated whether paravertebral spread of local anaesthetic is affected by patient position after ESPB. METHODS: We randomised 84 patients to receive ESPB at T RESULTS: Local anaesthetic-contrast mix reached the paravertebral space, intercostal space, and neural foramina in 96.5%, 94.2%, and 77.9% of individuals, respectively. Epidural space spread occurred in 20 cases. Prone positioning consistently allowed paravertebral and intercostal spread in all patients, with more thoracic level spread compared with supine positioning (5.0 [1.9] vs 3.1 [1.7], difference [95% confidence interval, CI]: 1.9 [0.8-3.0] levels, P<
0.001 for paravertebral space spread
2.8 [1.9] vs 1.4 [1.4], difference [95% CI] levels: 1.4 [0.4-2.5], P=0.004 for neural foramina spread
4.3 [1.3] vs 3.2 [1.5], difference [95% CI] levels: 1.0 [0.1-1.9], P=0.019 for intercostal space spread). Local anaesthetic-contrast extended to the intercostal space further in the prone than in the lateral position group (4.3 [1.3] vs 2.6 [1.5] thoracic levels, difference [95% CI]: 1.7 [0.8-2.6], P<
0.001). Sensory block in ventral dermatomes was variable in all participants. CONCLUSIONS: Prone positioning after ESPB significantly enhanced local anaesthetic-contrast spread to the paravertebral space, intercostal space, and neural foramina, suggesting that gravity plays a substantial role in spread. CLINICAL TRIAL REGISTRATION: Clinical Trials.gov (NCT06142630).