This technique combines the rapid onset and dense block of spinal anesthesia with the flexibility afforded by an epidural catheter.
After the peak spinal block height has been established, the injection of saline or a local anesthetic solution into the epidural space causes the block height to increase, presumably reflecting compression of the spinal meninges forcing CSF cephalad as well as a local anesthetic effect.
A potential risk of this technique is that the meningeal hole made by the spinal needle may allow high concentrations of subsequently administered epidural drugs to reach the subarachnoid space.