# Assessment of an Ultrasound-Guided Longitudinal Approach to the Thoracic Erector Spinae Plane Block in Cat Cadavers: Description of Dye and Contrast Medium Distribution

## TL;DR

This study explores the feasibility of a thoracic erector spinae plane block in cats using ultrasound guidance and dye distribution analysis.

## Contribution

This is the first cadaveric study evaluating a thoracic ultrasound-guided erector spinae plane block in cats.

## Key findings

- Contrast medium was distributed in the erector spinae musculature and thoracolumbar fascia in all 24 sides.
- Dye staining was observed in dorsal branches of thoracic spinal nerves, with occasional staining of ventral branches and sympathetic trunk.
- The technique shows potential for analgesia in the dorsal thoracic region but requires further validation for ventral nerve and sympathetic effects.

## Abstract

The erector spinae plane (ESP) block is an ultrasound-guided locoregional anaesthetic technique that primarily provides analgesia to the vertebral laminae, spinous process, epaxial musculature, and overlying skin. However, several clinical studies in humans have demonstrated the potential of the ESP block to provide analgesia for both thoracic and abdominal surgeries. While this block has been extensively studied in dogs, to the authors’ knowledge, this is the first cadaveric study of a thoracic approach to the ESP block in cats. The objective of this research was to evaluate the feasibility of an ultrasound-guided longitudinal approach to the thoracic ESP block in feline cadavers. Fifteen cat cadavers were used, three for an anatomical study, and twelve to assess the ultrasound-guided ESP blocks. A mixture of methylene blue, lidocaine, and iopromide (0.4 mL kg−1) was bilaterally administered at the level of the transverse process of the seventh thoracic vertebra (T7). Then, a computed tomography (CT) scan and anatomical dissection were performed to assess the distribution of the mixture. The CT images revealed the distribution of contrast medium withing the epaxial musculature and the middle layer of the thoracolumbar fascia. Anatomical dissection frequently showed staining of the dorsal branches of the thoracic spinal nerves. In some cadavers, the ventral branches of T7 and the sympathetic trunk were also stained. These findings support the feasibility of the thoracic ESP block in cats and its potential to provide analgesia for surgical procedures involving the dorsal thoracic area.

The ultrasound-guided erector spinae plane (ESP) block is a locoregional anaesthesia technique primarily used to provide analgesia to anatomical structures innervated by the dorsal branches (DB) of the spinal nerves. However, several clinical studies in humans have demonstrated its analgesic efficacy in areas innervated by the ventral branches (VB) of the spinal nerves, such as thoracic and abdominal surgeries. In veterinary medicine, the ESP block has been extensively studied in dogs at the thoracic, thoracolumbar, and lumbar regions. In contrast, only one cadaveric study, focused on the lumbar area, has been published in cats. The objective of this research was to evaluate the feasibility of an ultrasound-guided longitudinal approach to the thoracic ESP block in feline cadavers. A total of 15 feline cadavers were used for this purpose: 3 for the anatomical study, and 12 for the ultrasound-guided approach (24 sides). The needle tip was ultrasound-guided to the transverse process of the seventh thoracic vertebra (T7), and a mixture of methylene blue (0.5%), lidocaine, and iopromide (0.4 mL kg−1) was administered. A computed tomography (CT) scan was subsequently performed to assess the distribution of the contrast medium, followed by anatomical dissection to evaluate the distribution of dye. Contrast medium was observed within the erector spinae musculature, between T1 and T13 (24 out of 24 sides), with a distribution spanning 8 (4–11) vertebral bodies. Anatomical dissection showed staining of two (0–9) DB, mainly T6, T7, and T8, and 0.5 (0–1) VB, with T7 the only one stained. Additionally, staining of the sympathetic trunk was observed in 7 out of 24 sides. Based on these findings, this technique may be indicated in surgeries involving the epaxial region. However, further studies are needed to confirm its effectiveness in providing analgesia to structures associated with the VB of the spinal nerves and the sympathetic trunk.