Specific Techniques for Epidural Anesthesia
Using the midline or paramedian approaches detailed previously, the epidural needle is passed through the skin and the ligamentum flavum. The needle must stop short of piercing the dura. Two techniques make it possible to determine when the tip of the needle has entered the potential (epidural) space: the “loss of resistance” and “hanging drop” techniques.
The loss of resistance technique is preferred by most clinicians. The needle is advanced through the subcutaneous tissues with the stylet in place until the interspinous ligament is entered, as noted by an increase in tissue resistance. The stylet or introducer is removed, and a glass syringe filled with approxi-mately 2 mL of saline or air is attached to the hub of the needle. If the tip of the needle is within the ligament, gentle attempts at injection are met with resistance, and injection is not possible. The needle is then slowly advanced, millimeter by millimeter, with either continuous or rapidly repeating attempts at injection. As the tip of the needle just enters the epidural space, there is a sudden loss of resistance, and injection is easy.
Once the interspinous ligament has been entered and the stylet has been removed, the hang-ing drop technique requires that the hub of the needle be filled with solution so that a drop hangs from its outside opening. The needle is then slowly advanced deeper. As long as the tip of the needle remains within the ligamentous structures, the drop remains “hanging.” However, as the tip of the needle enters the epidural space, it creates negative pres-sure, and the drop of fluid is sucked into the needle. If the needle becomes plugged, the drop will not be drawn into the hub of the needle, and inadvertent dural puncture may occur. Some clinicians prefer to use this technique for the paramedian approach and cervical epidurals. Successful “epiduralists” rely on either the loss of resistance or hanging drop asconfirmation (rather than as the primary test) that the needle has entered the epidural space. Successful “epiduralists” will generally have sensed the “give” in their hands as the epidural needle tip passes through the ligamentum flavum.