Ultrasound-Guided Deep Serratus Plane Block in Cat Cadavers (Felis catus): A Description of Dye and Contrast Media Distribution

The serratus plane block is an ultrasound-guided anaesthetic technique that aims to provide analgesia to the lateral thoracic wall cranial to the 8th rib. This block can be performed in a superficial (between the latissimus dorsi and the serratus ventralis thoracis (SVT) muscles) or deep plane (between the intercostales externi and the SVT muscles). This study aimed to assess the distribution and nerve staining of a mixture of 0.4 mL kg^-1 of methylene blue and iopromide 50:50 performing a deep serratus plane (DSP) block at the level of the 5th rib. We hypothesise that this technique would be feasible and could stain the rami cutanei laterales (RCL) of the intercostales nerves cranial to the 8th rib in cat cadavers. This study was divided into two phases. Phase 1 consisted of an anatomical study of the thoracic wall (2 cadavers). Phase 2 consisted of the ultrasound-guided injection of the aforementioned mixture and the assessment of its distribution by computed tomography and anatomical dissection (12 cadavers). Contrast media spread a median of 5.5 (2.5-7.5) intercostal spaces. The dye stained a median of 3 (0-5) RCL, affecting RCL 2 (17.39%), RCL 3 (57.17%), RCL 4 (78.26%), RCL 5 (91.30%), RCL 6 (78.26%), and RCL 7 (8.70%) within the DSP. Occasionally, the rami dorsales laterales and the thoracicus longus nerve were stained. Based on these findings, the DSP block performed with a volume of 0.4 mL kg^-1 of an anaesthetic could provide analgesia in the area innervated from T4 to T6.