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

Objective

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.

Study design

Blinded, prospective, experimental cadaveric study.

Animals

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

Methods

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.

Results

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.

Conclusions and clinical relevance

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.