Background and aim:
Psoas compartment block, useful in hip surgery, in combination with sciatic nerve block needs rather large volume of local anesthetic. Our aim was to compare the spread of 20 ml vs 40 ml of aqueous solution injected into psoas compartment.
Methods:
In 12 fresh adult cadavers, 0.01% methylene blue aqueous solution was injected after Capdevila et al [1] into psoas compartment, 20 ml at one side and 40 ml at the opposite side. Then the dye distribution along the vertebral bodies was analysed.
Results:
The distance between the injection point and the caudal limit of the dye spread (M ± SD) was 9.3 ± 1.5 cm after 20 ml and 10.9 ± 2.1 cm after 40 ml, the difference being statistically significant (unpaired one-tailed t-test, p = 0.021), but clinically irrelevant. The upper (L1 vertebral body) and the lower (S1) limits of the dye spread were identical with both volumes, although these limits were reached in one cadaver (8%) only. L3 and L4 levels were stained densely with both volumes in all cadavers, L5 in nine (75%) with 20 ml and in ten (83%) with 40 ml, L2 in two (17%) and three (25%), respectively (non-significant difference, Fisher exact p = 0.5). 
Conclusions:
With both 40 ml and 20 ml injected into psoas compartment, the solution always spreads along L3 and L4, frequently along L5, and rarely along L1, L2, and S1 levels.

Reference:
1. Capdevila X., Macaire P., Dadure C., et al. Continuous psoas compartment block for postoperative analgesia after total hip arthroplasty: new landmarks, technical guidelines, and clinical evaluation. Anesth Analg 2002; 94: 1606-1613.

The spread of 20 ml vs 40 ml of dye injected into psoas compartment in cadavers