Association Between Initial Injection Volume and Radiographic Catheter Alignment in Continuous Deep Serratus Anterior...

Background Continuous serratus anterior plane block (SAPB) is used for analgesia after thoracic and cardiac surgery. However, no standardized catheter placement method exists. This study investigated the association between initial injection volume and radiographic catheter alignment after minimally invasive cardiac surgery (MICS). Methods This retrospective cohort study included patients who underwent continuous SAPB after MICS between April 2021 and September 2024. Patients were divided into three groups based on initial bolus volume: low (≤ 30 mL), medium (40 mL), and high (≥ 50 mL). The primary outcome was radiographic catheter alignment assessed on frontal chest radiographs, categorized as favorable or unfavorable using predefined criteria. Secondary outcomes included fentanyl consumption and rescue analgesic administrations within 48 hours. Exploratory analysis evaluated associations between catheter alignment and analgesic use. Results The low-, medium-, and high-volume groups included 20, 33, and 45 patients, respectively. Favorable radiographic alignment was observed in 14 of 20 patients (70.0%) in the low-volume group, 32 of 33 patients (97.0%) in the medium-volume group, and 40 of 45 patients (88.9%) in the high-volume group (p = 0.015). Fentanyl consumption and rescue analgesic use did not differ significantly among groups. However, in an exploratory subgroup analysis, patients with favorable alignment required fewer rescue analgesic administrations than those with unfavorable alignment. Conclusion During continuous SAPB for MICS, an initial bolus volume of 40 mL or greater was associated with favorable radiographic alignment. The clinical relevance of these findings for postoperative analgesic outcomes warrants further prospective investigation.