Abstract
Background: Thoracic paravertebral block (TPVB) entails injecting local anesthetic inside the paravertebral space (TPVS). Loss of resistance to air injection (air-LOR) was the first technique described in Humans to locate the TPVS. To date, no study has investigated the spread of any substance after injection into the TPVS using the air-LOR technique or described the cranial and caudal limits of the space.
Aim: To identify the boundaries of the TPVS, to determine whether the air-LOR technique is reliable for identifying the TPVS and to examine the relation between the volume of injectate and its spread.
Methods: After a preliminary phase, the thorax of five cat and five dog cadavers was accessed and eviscerated. After TPVS probing, the polyurethane foam was injected and after its maximum spread, the cranial and caudal borders were recorded. Different volumes of a mixture of new methylene blue and ioversol were injected in the TPVS after its localization with a Tuohy needle and air-LOR technique in fourteen dog and six cat cadavers. Lateral radiographs of the vertebral column were taken to document needle positioning, spread pattern and extension. The thorax of these subjects was then accessed and eviscerated to observe and record the spread of the mixture.
Results: Injecting a dye into the TPVS, localised by an air-LOR technique, resulted in multisegmental and often bilateral subpleural staining of paravertebral, intercostal, and dorsal mediastinal structures in dog and cat cadavers. The lateral radiographs most often showed a mixed cloud like-linear spread pattern, which could be a predictor of the longitudinal spread of the dye. The foam injected into the TPVS at the cranial and the caudal level revealed anatomical communication with the cervical, axillar, and lumbar paravertebral regions.
Conclusions: TPVS localization by air-LOR technique and injection results in longitudinal multisegmental spread in dogs and cat cadavers. The communication of the TPVS with the axillary and lumbar regions could be of clinical interest for the brachial plexus and the lumbar intercostal nerves blocks in a clinical setting.

Key words: cat,dog, loss of resistance, regional anesthesia, thoracic paravertebral block, thoracic paravertebral space