Anatomical and radiological study of the thoracic paravertebral space in dogs: iohexol distribution pattern and use of the nerve stimulator.

To describe the landmarks and methodology to approach the thoracic paravertebral space in dogs; to evaluate if intercostal muscular response could be evoked by a nerve-stimulator; to radiographically assess the distribution pattern of a radio-opaque contrast medium after thoracic paravertebral injections. Randomized, controlled, experimental trial. Two mongrel dog cadavers (anatomical study) and 24 mongrel dogs (experimental study). For the anatomic study 0.2 mL kg(-1) of new methylene blue (NMB) was injected at the 5th thoracic paravertebral space; for the experimental study dogs were divided into three groups and received 1 (T(5)), 2 (T(4) and T(6)) or 4 (T(4), T(5), T(6) and T(7)) paravertebral injections of iohexol. The paravertebral approach was performed with insulated needles using landmarks and a blind technique. When the needle tip reached the respective thoracic paravertebral space, the nerve-stimulator was switched-on and the presence/absence of intercostal muscular twitch was registered, thus a total volume of 0.2 mL kg(-1) of iohexol, divided into equal parts for each injection point, was administered. Radiological studies were performed with two orthogonal projections at different times. Positive injection was confirmed when the paravertebral space was occupied by iohexol in both projections. NMB was distributed in the T(5) paraverterbal space. In the experimental study, when the needle tip reached the respective paravertebral space, intercostal twitching was obtained in 80% of the total injections with a stimulating current of 0.5 mA. The incidence of positive cases when the intercostal twitch was obtained with 0.5 mA was 83.3%. The main distribution pattern observed was cloud like without longitudinal diffusion. Intercostal muscular responses obtained with a stimulating current of 0.5 mA could be useful to locate thoracic spinal nerves in dogs and in our study the injected solution was confined to one thoracic paravertebral space.