Ultrasound-guided proximal intercostal block for cervicothoracic sympathetic blockade: a proof-of-concept cadaveric study

Background: The ultrasound-guided proximal intercostal block (PICB) is a regional anesthesia technique used for thoracic surgeries and sympathetic blockade in refractory ventricular arrhythmias. However, the anatomical extent of injectate spread to specific targets when PICB is performed at the upper thoracic levels has not been studied comprehensively. This cadaveric study aims to evaluate the spread of moderate-volume injections at the upper thoracic level, focusing on the cervicothoracic sympathetic chain and proposing possible mechanisms of the PICB effects in patients with refractory ventricular arrhythmias. Methods: Seven PICBs were performed in five soft-embalmed cadavers. Radiographic contrast mixed with methylene blue was injected at the proximal segment of the first intercostal space (ICS-1). Fluoroscopy and dissection were performed to examine the injectate spread. Results: In the six successful PICBs, both fluoroscopy and dissection revealed injectate spread within the ipsilateral ICS-1 (T1-2) with direct anteromedial fascial spread and consistent coverage of the upper thoracic and/or inferior cervical sympathetic chain in all specimens. Injectate spread to the adjacent paravertebral space was observed in 67% of specimens by fluoroscopy (2-6 segments) and 83% by dissection (1-4 segments). Limited spread to the epidural space was observed in 33% of specimens by fluoroscopy (2-5 segments) and 67% by dissection (2-6 segments). Conclusions: Ultrasound-guided PICB at PICS-1 produced consistent injectate spread to the adjacent cervicothoracic sympathetic chain in all specimens. The inconsistent paravertebral and epidural spread suggests that direct interfascial spread to the sympathetic chain is likely to be the main mechanism of sympathetic block for refractory ventricular arrhythmia.