## 4 Discussion  
This study shows that the DSP block performed at the 5 th rib is a feasible technique in cats and that 0.4 mL kg\({}^{-1}\) of a mixture of methylene blue and iopromide regularly stains the RCL from RCL4 to RCL6. These findings are compatible with the supply of analgesia to the cranio-lateral thoracic area innervved by these nerves. Two approaches, superficial [13] and deep [14; 15], have been described to develop the ultrasound-guided Serratus plane block in dogs. Despite comparative studies between these two approaches being lacking, according to our observations in cat cadavers, we hypothesise that the DSP block would stain more efficiently the target RCL for two reasons. First, the interfascal plane between IE and SVT (target of the deep approach) seemed smaller than the one between SVT and LD (target of the superficial approach). This fact concentrates on the distribution of the injectate to a smaller area. Second, the RCL were greatly covered by fat in the superficial interfacial plane (SVT-LD), which could decrease the contact of the local anaesthetics with the target nerves. We decided to use the R5 as the anatomical landmark to administer the injectate because it is in the middle length of the SVT. Additionally, the results could easily be compared with previous studies performed in the superficial and DSP block approaches in dog cadavers [13; 14], a species anatomically close to cats. However, we have observed certain anatomical differences in this area between both species. In cat cadavers, the RCL do not pierce the SVT [3] but slide between the serrations of this muscle and continue laterally to reach the LD. Moreover, the SVT in cats extends more caudally (8 th to 9 th rib) [2; 24] than in dogs (7 th to 8 th rib) [13; 14]. Therefore, the DSP block could potentially cover more RCL in cats than in dogs. However, this hypothesis was not supported by our results, as no RCL beyond RCL7 was stained in any hemithorax. The volume of 0.4 mL kg\({}^{-1}\) per hemithorax was chosen as it is equivalent to a dose of 2 mg kg\({}^{-1}\) of bupivacaine or nopivacaine 0.5%. Further volume could exceed the recommended doses of these two commonly used local anaesthetics in cats [25; 26]. Dilution of a local anaesthetic is a very common practice when a wider area needs to be covered. However, it must be considered that this fact might negatively affect the duration and intensity of the analgesic action [27]. Further pharmacodynamic research is needed to determine what concentration of local anaesthetic would provide the best balance between the distribution, duration, and intensity of the block. When a fascial plane block is performed, the administration of a volume test is a common practice used to assess whether the tip of the needle is in the target anatomical structure [28]. Due to the small size of our cat cadavers, our volume test was 0.1 mL. This small amount of injectate was chosen because the total volume employed in our study was low (range 0.62-2.0 mL), and a higher volume test would dramatically reduce the remaining volume. The anechoic pocket generated during the volume test quickly disappeared, making it difficult to ensure the correct needle position. Even more, the anechoic pocket was not observed in some of the injections (26.09%). These findings could be due to the fast distribution of such a small volume in the DSP, helped by the pressure exerted by the probe on the thoracic wall and the reflux of injectate throughout the needle path. Despite these difficulties, the distribution of the injectate was performed in the target plane based on the CT images and the anatomical dissection of the cadavers. Computed tomography images revealed two distinct distribution patterns: a concentrated pool near the injection point and a widespread distribution towards the epaxial muscles. A continuous plane connecting the DSP and RDL areas was clearly identified. However, the precise reason why the injectate remained in the DSP or exhibited a dorsal distribution is unclear.