Background and Aims Parasternal intercostal plane (PIP) block provides effective analgesia for the anteromedial thorax; however, the optimal technique has not been elucidated. This cadaveric study aims to describe the anatomy of the transversus thoracis muscle (TTM) and internal mamillary artery (IMA), and to investigate dye spread after superficial and deep approaches of PIP block.

Methods The study was approved by the hospital ethics committee (ANE-256307685). After sonographic examination, superficial or deep PIP blocks with single or multiple injections were randomly applied in 24 hemithoraces. Twenty mL of dye was injected for each block. On dissection, dye distribution over the intercostal spaces (ICS) was recorded.

Results The TTM was consistently found at the 2nd-6thICS on both sides of the thorax. Agreement between the sonographic identification and presence of the TTM was higher than 80% at the 2nd-5thICS. Along the 2nd-6thICS, the IMA located medial to the halfway between lateral border of the sternum and the costochondral junction, and the mean distance from the sternal border was 9.3 (4.8) mm. For both superficial and deep approaches of PIP block, the more numbers of injections rendered the wider dye distribution. The numbers of stained ICS after superficial PIP block at 2nd, 4th, and 5thICS, and deep PIP block at 3rd and 5thICS were 5.3 (1.2) and 5.7 (0.6), respectively.

Conclusions Triple injections at the 2nd, 4th, and 5thICS for superficial approach and double injections at the 3rdand 5thICS for deep approach were optimal techniques of PIP block.