Mechanistic insights into bupivacaine spread through anisotropic tissue planes and fascial barriers: experimental evidence for interfascial block dynamics
Simon Istenič, Luka Pušnik, Chiedozie Kenneth Ugwoke, Prof. dr. Tatjana Stopar Pintarič, Nejc Umek
Abstract
BACKGROUND: The mechanism by which local anesthetics spread and produce analgesia in interfascial plane blocks remains unclear. Clinical efficacy often exceeds the visible spread of injectate on imaging. This cadaveric study quantified the distribution of bupivacaine within skeletal muscle and across fascial barriers to explore diffusion as a potential contributor to interfascial plane block dynamics. METHODS: Fresh and frozen-thawed human soleus muscles were injected with a mixture of bupivacaine and methylene blue, and samples were collected along and perpendicular to fiber orientation. Fresh lower limbs were used to assess transfascial transfer across sequential crural fasciae after injection with bupivacaine, iodinated contrast, and methylene blue. The limbs were imaged with CT and samples were collected from crural compartments separated by fascial layers. Finally, simulated posteromedial quadratus lumborum block was performed in fresh cadavers, with samples harvested from the quadratus lumborum, psoas major, and diaphragm. Bupivacaine concentrations were quantified using high-performance liquid chromatography-mass spectrometry. RESULTS: In fresh muscle, bupivacaine exhibited an anisotropic pattern, with concentration gradients decreasing more slowly along muscle fiber orientation than in transverse direction (p=0.0008). This directional difference disappeared in frozen-thawed tissue. Across fascial barriers, bupivacaine decreased exponentially with the distance from injection site, compatible with diffusion-driven transport, whereas methylene blue and iodinated contrast were not detected beyond the injected compartments. After simulated quadratus lumborum block, bupivacaine was detected in the psoas major (0.7-487.6 µg/g), quadratus lumborum (0.2-266.1 µg/g), and diaphragm (9.0-27.3 µg/g) despite no evident methylene blue spread. CONCLUSIONS: Low-volume injectate of bupivacaine spread anisotropically within fresh muscle and permeated fascial barriers in ways not demonstrated by methylene blue or iodinated contrast. The exponential decrease of concentrations provides evidence that diffusion contributes to local anesthetic distribution and may help explain, at least in part, the broad and occasionally disproportionate sensory coverage observed clinically with interfascial plane blocks.