Ultrasound-guided interfascial rectus sheath-associated plane block in sheep: a cadaveric study

To evaluate ovine anatomy to determine the feasibility of an ultrasound-guided interfascial rectus sheath (RS)-associated plane block in sheep cadavers.

Prospective, experimental, randomized cadaveric study.

A group of 22 fresh cadavers (median 52 kg, range 47-62 kg).

In phase I, six cadavers underwent anatomical (n = 2) and ultrasonographic (n = 4) examination of the ventral abdominal wall and RS. Based on these findings, two lateral (one- or two-injection) approaches were defined. In phase II, 14 cadavers were randomly injected bilaterally using the one-injection approach under ultrasound guidance with 0.1% methylene blue at either high (HV, 0.5 mL kg^-1) or low (LV, 0.25 mL kg^-1) volume, targeting the plane between the rectus abdominis muscle (RAM) and its internal sheath. Dye distribution and nerve staining were evaluated by dissections.

The one-injection approach provided better visualization and consistent nerve staining. The eleventh thoracic nerve was stained in all cases, whereas the twelfth (HV = 93%; LV = 79%) and thirteenth thoracic nerves were stained only in two cadavers with HV. Dye spread was limited, suggesting compartmentalization of the interfascial plane. Further investigation in two additional cadavers included injections of methylene blue and a neutral red dye (LV each) between RAM and its internal sheath between alternate muscle bellies. The latter injections were within the same unilateral RAM, separated by tendinous intersections. Dissection showed distinct dye accumulation without mixing.

The described approach produced a limited spread and nerve staining, possibly owing to ovine-specific anatomy and the technique used, which differs from the standard rectus sheath block (RSB) technique. Targeting individual muscular compartments could be an alternative. Further studies using the traditional RSB approach are needed to confirm its applicability in this species.