Ultrasound-guided rectus sheath block injections in miniature swine cadavers: technique description and distribution of two injectate volumes

Objective: To describe a technique for ultrasound-guided rectus sheath block in pigs and the distribution of two injectate volumes. Study design: Experimental study. Animals: A group of 11 Hanford miniature pig cadavers. Methods: The lateral border of each rectus abdominis muscle in 10 freshly euthanized pigs was visualized with a 6-15 MHz linear ultrasound probe. A spinal needle was inserted 1 cm cranial to the umbilicus, in-plane and medial to the probe, and advanced dorsal to lateral until the tip was ventral to the internal rectus sheath. Pigs were injected bilaterally with high volume (treatment HV; 0.8 mL kg^–1) or low volume (treatment LV; 0.5 mL kg^–1) of 1:1 solution of 1% methylene blue and 0.5% bupivacaine (1 mg kg^–1) diluted with 0.9% saline. Nerve staining ≥ 1 cm circumferentially was determined by dissection 15 minutes postinjection. The Clopper–Pearson method was used to calculate 95% confidence intervals (CIs) for proportions of stained nerves. In another pig, a 1:1 solution of 1% methylene blue and 74% ioversol contrast was injected, and computed tomography performed at 15 minute intervals after injection. Results: Nerve staining for thoracic (T) spinal nerves T9, T10, T11, T12, T13 and T14 occurred 20%, 60%, 90% 100%, 100% and 50%, and 0%, 20%, 90%, 100%, 100% and 50% of the time in treatments HV and LV, respectively. More nerves were stained in treatment HV in 4/10 animals (40%, 95% CI: 12%–74%) than in treatment LV (0%, 95% CI: 0%–31%). The greatest spread of injectate occurred within the first 15 minutes after injection. Conclusions and clinical relevance: Staining of T11–T14 nerves was the same in both treatments but the higher volume stained more T9–T10 nerves. Based on dye distribution, a rectus sheath block may only provide ventral abdominal analgesia cranial to the umbilicus in pigs.