Injectate spread following ultrasound-guided lateral to medial approach for dual transversus abdominis plane blocks

BackgroundBilateral dual transversus abdominis plane (BD-TAP) injections were devised to cover the T7-8 and L1 dermatomes, which are usually spared with classical and mid-axillary TAP injections. The purpose of this study was to delineate the vertical and lateral extent of injectate spread following a lateral to medial approach for TAP injections in embalmed cadavers. MethodsUltrasound-guided subcostal and lateral TAP injections were performed on nine embalmed cadavers using 30ml of 0.5% methylcellulose (20ml for subcostal and 10ml for lateral injections) with a 12-cm Tuohy needle in the first six cadavers (nine hemi-abdomens). Vertical extent and the medial to lateral extent of the dye spread were recorded after dissections of the abdominal wall. In a pilot of three cadavers not receiving TAP injections, anatomical impediments to proximal injectate spread were explored separately. ResultsThe vertical spread of injectate was T7-L1 (n=2/9), T8-L1 (n=5/9) and T9-L1 (n=2/9). None of the TAP injections extended beyond the mid-axillary line. No anatomical impediments for the flow of injectate to the T7 or T8 intercostal nerves were found at the level of the interdigitations of the transversus abdominis muscle and diaphragm. ConclusionA lateral to medial approach for TAP injection resulted in spread of the injectate ranging from T7/8-L1 dermatomes in the majority of the hemi-abdomens. Subcostal and lateral TAP injections do not cover the lateral cutaneous branches of the segmental nerves.