Introduction Erector spinae plane (ESP) blocks can be used to provide analgesia following thoracoabdominal and lumbar spine surgical procedures. However, the influence of injectate volume and injection location on the spread of anesthetic with ESP blocks remain unclear.

Methods Ultrasound-guided ESP injections were performed on two fresh cadavers using a solution of iopamidol radiographic contrast, indocyanine green or methylene blue dye, and saline. The relationship between injectate volume and cephalocaudal spread was assessed using real-time fluoroscopic recordings after incremental ESP injections to the lumbar and thoracic region. Cadavers were then dissected to expose tissue staining and document the precise disposition of the dye within the ES muscle, paravertebral space, dorsal and ventral rami, and other relevant structures.

Results Larger injection volumes resulted in more extensive cephalocaudal spread in most cases, with fluoroscopic images revealing a small but direct relationship between injectate volume and contrast spread. Dissection reinforced the radiographic findings, with staining ventral to the ES muscle ranging from 4 to 7 paravertebral levels with injections of 30–40 mL vs 12–13 levels following injections of 60–80 mL. No spread of dye to the lamina, transverse processes, paravertebral space, epidural space, or pleura was observed following any injection.

Conclusions Increased ESP injection volumes resulted in more extensive cephalocaudal spread, resulting in anesthetic spread to the dorsal rami and ventral ES muscle without involvement of the ventral rami or other anterior structures. Injection volumes of 30 mL may be optimal for ESP blocks requiring analgesia across 4–7 levels.