The distribution of solutions injected into the epidural space has not been determined. The author therefore examined the site of catheter tips and the spread of contrast material in the epidural space using computed tomographic (CT) imaging in patients receiving successful epidural analgesia. Lumbar epidural catheters were placed in 20 female patients by a midline technique. Anesthetic effect was determined by motor and sensory examinations during analgesic infusion. CT images were obtained for identification of the catheter tip and after radiographic contrast injection of 4 ml and then an additional 10 ml. Catheter tips were most often found lateral to the dura in the intervertebral foramen. In these subjects with normally functioning epidural analgesia, there was remarkable interindividual variability in patterns of spread, including various amounts of anterior passage, layering along the dura, and compression of the dura creating a posterior fold. Accumulation becomes more symmetric with increasing injectate volume. Spread through the intervertebral foramina was seen in all subjects. Air and fat in the region of the catheter interfered with solution spread in three subjects, but only over a limited area. Asymmetry in anesthetic effect was attributable to catheter position. No substantial barriers to solution spread were observed. A variety of catheter tip positions and patterns of solution spread underlie normal epidural anesthesia. Nonuniform distribution of injectate is common and is compatible with uniform anesthesia. Posterior midline structures play a minimal role in impeding distribution of injectate. A far lateral catheter position is a more common cause of asymmetric block than anatomic barriers to solution spread.
