Distribution of Epidural Saline Upon Injection and the Epidural Volume Effect in Pregnant Women

Abstract

How injected epidural solution is distributed and affects the epidural volume in pregnant women are unclear. Lumbar epidural catheters were placed using the loss-of-resistance technique with saline in eight full-term (39 weeks' gestation) parturients for labor and eight volunteer nonpregnant women. Lumbosacral cerebrospinal fluid volume was measured on thoracic and lumbosacral axial magnetic resonance images. Another image series was obtained after injecting 10 ml saline into the epidural space through the catheter to compare the saline distribution (dural sac coating and exit from foramina) and cerebrospinal fluid volume before and after epidural injection. Dural sac coating was based on observation of epidural saline in the anterior epidural space after injection in axial magnetic resonance images at the pedicle levels from T12 to L5. Saline leakage from the foramina was determined by the same method at six disc levels from T11-T12 to L4-L5. Significantly fewer images of pregnant women than nonpregnant women showed saline surrounding the dural sac (0 [0-0] vs. 3 [1-4], median [interquartile range]; P < 0.01) and saline leakage from the foramina (0 [0-1] vs. 6 [4-6]; P < 0.01). The mean reduction in cerebrospinal fluid volume was significantly greater in pregnant (8.4 ± 1.4 ml; mean ± SD) than in nonpregnant women (4.6 ± 1.1 ml; P < 0.001). Limited dural sac coating and decreased leakage from the foramina of saline injected into the epidural space may account for the facilitation of longitudinal spread of epidural analgesia in pregnant women. The epidural volume effect is greater in pregnant than in nonpregnant women.