## 4 Discussion
The freeze-thaw process, tissue hydration, and the absence of respiratory movements could have contributed to this heterogeneous distribution between the hemithoraces of the cat cadavers. It should be noted that these distribution patterns may not occur in live individuals. A discrepancy between the distribution (number of intercostal spaces) of the contrast media and the number of RCL stained was observed. This fact could be due to the dorsal distribution of the injectate observed in some hemithoraces. The RCLin the deep plane are localised in a small area in the ventral region of the SVT muscle near its serrations. The dorsal spread of the injectate could have avoided the staining of the RCL, but the RDL were dyed instead. [PERSON] et al. (1991) [29] reported that due to the overlap of innervation, a minimum of three consecutive RCL should be blocked to provide analgesia to the thoracic wall. However, five or more RCL are traditionally recommended to obtain more reliable analgesia for this region [29; 30; 31]. In the present study, we have registered a median of 3 (0-5) RCL stained when 0.4 mL kg\({}^{-1}\) of injectate was administered per hemithorax. Therefore, an increased volume or a greater number of injection points would be necessary to enhance the spread of injectate and nerve staining. Nevertheless, no studies comparing the analgesic effects of three versus five blocked nerves have been published. Cadaveric studies of the ultrasound-guided serratus plane block in dogs reported the distribution of injectate between R1-R9 and R1-R5 in the superficial [13] and deep plane [14], respectively. However, in these studies the number of RCL stained was not reported, making impossible the comparison with our results. We observed staining of the RCL in both planes, with more consistent dyeing in the targeted deep plane. The thoracicus longus nerve, which provides innervation to the SVT, was also stained in 17.39% of hemithoraces in our study, compared to 88.5% in superficial serratus plane blocks in dogs [13]. This nerve is involved in inspiratory movements [1], and its block could impair respiratory function in certain clinical scenarios. Thus, the DSP block could be advantageous in preserving respiratory function more efficiently. Our study revealed several unexpected findings. First, some RCL were stained in the superficial plane (between SVT and LD). We observed by ultrasound that some injectate volume could have flown backwards through the needle path and reached the superficial serratus plane, where it stained some RCL. Second, the DSP approach allowed an unexpected dorsal distribution of injectate in some hemithoraces, reaching the epaxial muscles. This extended distribution resulted in the staining of the RDL. This fact could have been assisted by the dorsal recumbency of the cats during CT and anatomical dissection. More studies increasing the administered volume of injectate would be needed to find out if the RDL could be more consistently stained. If so, the DSP block would be an interesting option to induce analgesia over a more extensive area, being useful in surgeries involving the skin of the cranio-dorsal aspect of the thorax. Third, in the cadaver number 4, no nerves were found stained. This could be due to the small size of this animal (2.56 kg) and the low BCS (3/9) compared with the needle level size, which could have impeded the correct distribution of the injectate [32]. Fourth, in cadaver number 3, more RCL nerves were found stained in the superficial plane than in the DSP. This may be due to an incomplete perforation of the SVT, despite appropriate ultrasonographic visualisation of the needle tip, leading to a more superficial distribution of the injectate. The DSP block has shown promising results as part of a multimodal analgesic plan in several case reports. In a cat submitted for lobectomy and pericardiectomy [19], the patient did not need rescue analgesia in the postoperative period.