A novel extrafascial subscalene brachial plexus block in dogs: reducing hemidiaphragmatic paresis compared with the intrafascial technique.

Objective: To assess dye solution spread of an ultrasound-guided extrafascial subscalene approach (Esub) to the brachial plexus (BP) in canine cadavers and to compare the incidence of hemidiaphragmatic paresis between the Esub and intrafascial subscalene (Isub) approaches in Beagle dogs.

Study design: Descriptive anatomical study (Phase I) and prospective, randomized, blinded, experimental crossover trial (Phase II).

Animals: A total of six canine cadavers and seven adult Beagle dogs.

Methods: Phase I used six adult canine cadavers for anatomical assessment of dye spread after Esub. Phase II involved seven healthy adult Beagle dogs, each receiving Esub and Isub blocks with a 14 day washout in a randomized, crossover design. Both approaches used 0.4 mL kg^-1 of 2% lidocaine. Hemidiaphragmatic paresis was defined as ≥ 25% reduction in hemidiaphragmatic excursion compared with the preprocedural value, while < 25% was classified as no paresis. For the incidence of hemidiaphragmatic paresis (a dichotomous variable), exact binomial 95% confidence intervals (CIs) were calculated for each proportion, and the McNemar test was used for paired comparison. Statistical significance was set at p < 0.05.

Results: In Phase I, dissection confirmed dye accumulation in the lateral layer of the deep cervical fascia along the BP, with no dye in the visceral compartment; the recurrent laryngeal nerve, vagosympathetic trunk, phrenic nerve, and its branches were unstained. In Phase II, the rate of hemidiaphragmatic paresis was 0% (95% CI: 0.3-58%) and 100% (CI 95%: 59-100%) for Esub and Isub approaches, respectively (p = 0.015).

Conclusions: The ultrasound-guided Esub approach successfully confined injectate to the extrafascial plane in cadavers, avoiding spread to the phrenic nerve. In Beagle dogs, Esub significantly reduced the incidence of hemidiaphragmatic paresis compared with the Isub approach. Further studies in clinical populations are warranted to confirm these findings and assess long-term outcomes.