Superficial and Deep Parasternal Intercostal Plane Block: Anatomical Evidence of Dye Spread

Objectives: To compare injectate spread between superficial parasternal intercostal plane (SPIP) and deep parasternal intercostal plane (DPIP) blocks, and to evaluate anatomical factors relevant to procedural safety. We hypothesized that DPIP blocks provide greater cranio-caudal distribution than SPIP blocks. Design: An anatomical cadaveric study. Setting: Secondary-level academic anatomy facility in northern Italy, within a body donation program. Participants: Five fresh-frozen human cadavers (10 hemithoraces). Interventions: Ultrasound-guided SPIP and DPIP blocks were performed bilaterally at the third intercostal level using a 22-gauge, 50 mm needle. Each injection delivered 20 mL of diluted black tissue-marking dye (1:5 ratio with saline). Measurements and main results: Gross dissection assessed the cranio-caudal spread of dye by the number of intercostal spaces. Histology measured the distance between the internal thoracic artery and the fourth rib. DPIP blocks spread across more intercostal spaces than SPIP blocks (median 4, IQR 4-5 vs median 2, IQR 2-2; p < 0.001). Histological analysis showed the internal thoracic artery was a median 1.9 mm (IQR 1.7-2.2) from the fourth rib, separated only by soft tissue. Conclusions: DPIP blocks produce a wider craniocaudal spread than SPIP blocks, which may enhance clinical efficacy. However, their close proximity to the internal thoracic artery and pleura underscores safety considerations.