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

Abstract

S olution injected into the epidural space spreads freely, but not necessarily uniformly. In pregnant women, the dural sac is narrowed, and this narrowing may facilitate the spread of neuraxial block. The distribution of injected epidural solution may differ in pregnant and nonpregnant women. This study investigated the distribution of epidural saline injection and the extent of cerebrospinal fluid (CSF) reduction in pregnant women.Eight healthy women with a full-term singleton pregnancy and 8 nonpregnant nulliparous volunteers were studied. A day before exogenous oxytocin administration to augment labor, an epidural puncture was done at the L3-L4 level using loss-ofresistance technique with saline, without injecting more than 1 mL saline. An epidural catheter was advanced 3 to 5 cm into the epidural space. Low thoracic and lumbosacral axial magnetic resonance (MR) imaging was performed to measure CSF volume after catheter insertion. Sagittal MR images of the lower thoracic lumbosacral column were obtained to determine the level of the disk between T11 and 12 vertebrae. Low thoracic, lumbar, and spinal axial MR images caudal from this site were obtained at 8-mm increments. After the images were obtained, 10 mL saline was injected into the epidural space, and immediately thereafter, MR images were obtained in the same order as before the epidural saline injection. Cerebrospinal fluid volumes before and after epidural saline injection were compared. Dural sac coating was based on the observation of epidural saline in the anterior epidural space after injection in axial MR images at the pedicle levels from T12 to L5. Saline leakage from the foramina was determined using the same procedure at 6 disk levels from T11YT12 to L4YL5. Leakage of saline from the foramina was determined by counting the foramina that had saline on either side of the criteria line, defined as a straight line passing through the center of the intervertebral joint and the point of contact with the vertebral body at the disk level.Compared with nonpregnant women, the anterior and lateral epidural venous plexuses in the pregnant women were enlarged in association with dural sac narrowing, with the result that CSF volume before saline injection in the pregnant women was notably less than that in the nonpregnant women (42.0 T 4.2 and 33.6 T 6.5 mL, respectively; P G 0.05). In nonpregnant women, saline spread freely through the epidural space and coated the cylindrical dural sac with partial leaking through the foramina with no pattern. In pregnant women, saline did not leak through the intervertebral foramina, and the enlarged epidural venous plexus interfered with the dural sac coating, leading to only posterior accumulation of saline at the level of the pedicles of the vertebral bodies. Dural sac coating was not seen in any of the images from the pregnant women and in 3 of those from nonpregnant women. Saline leakage from the foramina was seen in images from 6 nonpregnant women and in none of the pregnant women. Mean CSF volume decreased in both g...