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Background and Aims The serratus anterior plane block (SAPB) is used for postoperative analgesia in thoracic surgery and minimally invasive cardiac surgery (MICS). Although a single injection of the SAPB 0.4 mL/kg local anesthetic reportedly affects T2–T9, the distribution of the infused local anesthetic from the SAPB catheter remains unexplored. Thus, this study aimed to use imaging to evaluate the distribution of contrast agents injected via SAPB catheters in patients undergoing MICS.

Methods This retrospective observational study included patients who underwent elective MICS. The SAPB catheters were preoperatively inserted into the caudal rib of the surgical site near the middle axillary line. Postoperatively, we conducted X-ray imaging sessions to assess catheter positioning and local anesthetic distribution. A mixture of 10 mL of iohexol and 10 mL of 0.75% ropivacaine was injected through the catheter, with four X-ray sessions conducted after each 5 mL injection of the mixture.

Results Twenty-seven patients were enrolled in this study; mitral valve surgery was the most common procedure (48%). The median (interquartile range) number of intercostal levels of contrast spread was 2 (2.0–3.0) at 5 mL, 2.5 (2.0–3.0) at 10 mL, 2.5 (2.3–3.0) at 15 mL, and 3 (2.5–3.3) at 20 mL. The contrast spread range was significantly larger at 20 than 5 mL (p=0.002).

Conclusions The longitudinal extent of contrast spread was greater after the injection of 20 versus 5 mL of SAPB. Based on these findings, the use of serial injections via a SAPB catheter may affect a relatively smaller area than the use of a single SAPB injection.