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

To describe a technique for ultrasound-guided rectus sheath block in pigs and the distribution of two injectate volumes.Experimental study.A group of 11 Hanford miniature pig cadavers.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.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.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.