As promised, I wanted to talk a bit about the technical aspects of epidural anesthesia. At some point I'll try to post pictures; they'll help quite a bit.
First a quick anatomy lesson is needed. So you have a brain and a spinal cord, as well as nerve roots that come out of each side of the spinal cord, between each of the 24 vertebrae that we all possess. This constitutes your central nervous system (CNS). The whole CNS is bathed in cerebrospinal fluid that is itself contained in a bag, as it were. That bag, that lining of the spinal cord and brain, is the dura mater, or just dura for short.
As I said before, the spinal cord has nerve roots that come off on each side at each vertebral level, all the way down to the sacrum, the middle bone of the pelvis. The spinal cord itself, though, doesn't extend all the way down to the pelvis, though; it stops at the L1 or L2 vertebra in most people, ie halfway down your lower back. The dural sac of fluid that envelops then whole CNS extends much further down. So past your L2 vertebra, the dural sac of fluid contains nothing but spinal fluid and nerve roots, just floating freely therein.
What's the point? The point is that when I (or any other anesthesia practitioner) is putting in an epidural, I put it in very low in the back, lower than the point where the spinal cord actually ends. The usual spot is between the 3rd and 4th lumbar vertebrae, or L3-4 for short. Sometimes due to technical considerations, I will go one level higher or lower, but never higher than L2. First of all, obviously, it's much safer to put in the epidural lower in the back, and second, the pain fibers activated during labor come from a wide range of nerve roots, and the L3-4 location is well in the middle of that.
To understand the epidural further we need to look at the name of the procedure - epi-dural. The prefix "epi" means "on" or "against". The epidural catheter place by the anesthesiologist does not rest on the spinal cord; instead it rests against the dural sac that surrounds the spinal cord. So it never actually touches nerves, and there is an added element of safety because of that.
To get to the epidural space just outside the dural sac requires using a big needle called a Tuohy needle, which is inserted (after numbing the skin with local anesthetic), in between the lumbar vertebrae, usually the L3-4 space as I said before. Then, once the skin is numb and the needle is in place in the back, I advance it slowly, while I have it hooked up to a syringe filled with air. What's this for? Well, the spaces between the vertebrae are populated by a number of very firm ligaments, and these ligaments help maintain the integrity of the spine. For the anesthesiologist, they provide resistance when the air filled syringe is pushed against them. As the needle is pushed inward, the resistance is felt when pushing on the air-filled syringe - until the tip of the needle enters the area right outside the dura. That's right, it's the epidural space. I guess technically anatomists don't consider it a "space" because there's really nothing in there except a few veins. This is key, because while I'm pushing on the air-filled syringe and advancing my needle, that epidural "space" feels completely different. Suddenly there's no resistance, and that's how I tell I'm in the right spot.
At that point that's where I can do one of two things, either placing the epidural catheter through the needle, or sticking a long, thin needle through the thicker needle to pierce the dura. The latter is called a combined spinal-epidural, which is how I was trained. Through this tiny needle, I put a small dose of narcotic and local anesthetic into the sac surrounding the CNS. Then I withdraw the small needle and thread the catheter through the larger Tuohy needle. The dural hole created by the spinal needle is small enough that the epidural catheter won't go through it.
The epidural doesn't go that far into the back. Usually it is about 4 or 5 centimeters deep to get to the epidural space, then the catheter is inserted another 4 to 5 centimeters into the space. So a total of 8 to 10 centimeters of catheter is left inside for the duration of labor. It's deep enough that it freaks some people out when I pull the catheter later and they see how deep it was, but not terribly deep.
The epidural catheter is hollow and wire reinforced, which is important because it means medication can be give through it, namely local anesthetic which can keep the woman comfortable throughout her labor and delivery. After giving a small dose of local anesthetic mixed with epinephrine through the catheter to make sure it hasn't inadvertently entered a vein (appropriately, this is called a "test dose"), the catheter is attached to a small pump that administers a continuous flow of local anesthetic up to and through the time the woman delivers.
So that's it. If things go smoothly, from the time I infiltrate the skin with local anesthesia to the time I thread the epidural catheter can be as short as a couple of minutes. Others may be even faster. Speed is key, not just because the woman is in such pain, but going fast makes it possible to (try to) time the epidural placement in between contractions. Now if there are technical difficulties, then things can take longer. A lot longer. That leads me to my next topic. Everyone knows the benefits to getting the epidural; how about the risks? It is empirically a very safe procedure, but it is still worthwhile to think about what could go wrong...
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