An epidural involves the delivery of medication into the epidural space, a small area just outside the membrane that protects the spinal cord. Understanding the specifications of the instruments involved can help alleviate apprehension. The overall process is designed for effectiveness and uses a multi-step approach to minimize patient discomfort during placement.
Deconstructing the Epidural Needle Size
The needle used to access the epidural space is a highly specialized instrument known as a Tuohy needle. It is designed with a specific curved, blunt tip that helps guide the fine plastic tube that delivers the medication and reduces the risk of accidental spinal membrane puncture. The size is described using a gauge measurement, where a lower gauge number indicates a larger needle diameter.
In most adult procedures, the epidural needle gauge falls within the 16- to 18-gauge range, with 17-gauge being standard. For perspective, a typical intravenous (IV) needle is 18- to 20-gauge, meaning the epidural needle has a slightly larger diameter. The length of the Tuohy needle is also important, as it must reach the specific anatomical target.
For an average-sized adult, the needle length is typically between 8 and 9 centimeters (3 to 3.5 inches). This length is necessary to pass through the skin, fat, muscle, and ligaments to reach the epidural space. In patients with a higher body mass index, longer needles (up to 12 to 15 centimeters) may be required for proper placement. The needle shaft includes markings that allow the anesthetist to track the depth of insertion precisely.
The Two-Step Insertion Process
The epidural procedure uses a carefully executed two-step process involving two different needles. This technique mitigates the pain associated with inserting the larger Tuohy needle. The first step involves administering a local anesthetic, such as lidocaine, directly to the skin and underlying soft tissue at the insertion site.
This initial injection is performed using a much smaller, fine needle, typically 25-gauge, comparable to those used for routine injections. Patients usually report a brief, sharp pinch or sting, which quickly subsides as the local anesthetic begins to work. This numbing agent anesthetizes the pathway that the larger instruments must travel through to reach the epidural space.
Once the local anesthetic has taken effect, the area is sufficiently numb to allow for the careful insertion of the larger Tuohy needle. The patient should not feel the sharp pain of the needle cutting through the skin and tissue. Instead, the sensation is commonly described as a feeling of deep pressure as the anesthetist guides the needle through the deeper layers of the back.
Beyond the Needle The Catheter System
The large Tuohy needle is only a temporary delivery tool and is not the device that remains in the patient’s back. Its primary function is to serve as a conduit to thread a much smaller, flexible tube into the epidural space. This thin tube, which stays in place for continuous pain relief, is called the epidural catheter.
The catheter is constructed from a thin, flexible plastic material and is significantly smaller than the needle used to insert it. Catheters are typically 20-gauge or even smaller in diameter, making them extremely fine and pliable. This small size and flexibility allow the catheter to remain in the epidural space for hours or even days, delivering continuous medication without causing tissue trauma.
Once the catheter is successfully threaded a few centimeters into the epidural space, the anesthetist carefully withdraws the large Tuohy needle. Only the soft, flexible catheter remains, secured to the skin with medical tape and a clear dressing. Medication is then administered through this tiny catheter, allowing for effective and long-lasting pain management.