Programmed intermittent bolus erector spinae plane block exhibits progressive expansion of dermatomal spread but does not match the analgesic efficacy of paravertebral block: A sub-analysis of a randomized controlled trial

Background The erector spinae plane block (ESPB) demonstrates variable anterior thoracic dermatomal distribution compared to the paravertebral block (PVB), potentially limiting its clinical reliability. Programmed intermittent bolus (PIB) administration has been proposed to enhance the spread and uniformity of local anesthetic distribution. Objectives To compare dermatomal coverage and analgesic efficacy of PIB-based ESPB and PVB, and to examine determinants of sensory block extent and its relationship with postoperative analgesic outcomes. Methods Sixty adults undergoing lobectomy were included in this predefined sub-study of a randomized controlled trial comparing ultrasound-guided ESPB and PVB with catheter placement. Both groups received a 15-mL bolus of 0.5% ropivacaine followed by programmed intermittent boluses of 0.2% ropivacaine (15 mL every 6 hours). The primary outcomes were bidirectional anterior dermatomal extent along the midclavicular line and block success rate at 6 and 24 hours postoperatively. Secondary outcomes included postoperative morphine consumption, pain scores, and the proportion of opioid-free patients. Intergroup comparisons, correlation analyses, and mediation analyses were performed. Results At 6 hours, ESPB had a lower success rate (76.7% vs. 96.6%, P=0.048), shorter spread (3.8 vs. 5.0 levels, P=0.041), and reduced caudal extension (1.1 vs. 1.7 levels, P=0.009). These differences were no longer detected at 24 hours. ESPB spread correlated with female sex (P=0.006) and shorter height (ρ = –0.56, P=0.006). ESPB patients required more morphine (3.0 vs. 1.0 mg, P=0.004) and had fewer opioid-free recoveries (6.7% vs. 33.3%, P=0.014). Mediation analysis showed analgesic differences were not explained by dermatomal extent. Conclusions ESPB showed narrower initial sensory spread than PVB but comparable anterior dermatomal coverage at 24 hours. Female sex and shorter stature were associated with greater ESPB spread. Better analgesia with PVB despite similar dermatomal coverage suggests contributions from mechanisms beyond sensory mapping, including visceral or paravertebral blockade.