Assessment of ultrasound-guided external oblique intercostal block injections in dogs

To describe a technique for an ultrasound-guided external oblique intercostal (EOI) block and compare the anatomic spread of two different volumes of new methylene blue injectate in dogs.

Blinded, prospective, experimental cadaveric study.

A total of six Beagle dogs weighing 9.5 (8-12) kg [median (range)].

To determine technique feasibility, computed tomography-based visualization of iodinated contrast spread following EOI injections in a single dog was performed. Subsequently, ultrasound-guided injections were performed bilaterally in six anesthetized dogs (12 hemithoraces), in the fascial plane between the external abdominal oblique and intercostal muscles, at the T9-T10 thoracic space. We assessed two randomly assigned injectate volumes: low volume (LV; 0.25 mL kg^-1) and high volume (HV; 0.5 mL kg^-1). Animals were then euthanized and immediately dissected to assess the spread of methylene blue and nerves stained.

Total distance of injectate spread was 100.3 ± 8.8 mm (mean ± standard deviation) and 81.2 ± 12.0 mm in HV and LV groups, respectively. Cranial distribution of stain terminated at intercostal spaces T7-8, T8-9 and T9-10 in 5/12 (HV, n = 3; LV, n = 2), 6/12 (HV, n = 3; LV, n = 3) and 1/12 (LV, n = 1) injections, respectively. Caudal distribution of stain terminated at intercostal spaces T10-11, T11-12, and T12-13 in 1/12 (HV, n = 1), 9/12 (HV, n = 3; LV, n = 6) and 2/12 (HV, n = 2) injections, respectively. HV and LV injections stained 1.8 ± 0.8 and 1.0 ± 0.6 intercostal nerves, respectively.

The pattern of spread observed following ultrasound-guided EOI injections may be consistent with providing desensitization to nerves innervating the cranial and lateral abdomen. Further studies are needed to assess the clinical impact of these findings.