Epidural Elegance: A Bit About Anatomy and Neuraxial Anesthesia

In medical school I wrote a poem in honor of my cadaver from anatomy class. I wish there were some way to tell her family that even now, years after her passing, her generous gift continues to be part of my daily life as a physician.

Anesthesiologists use a fair amount of anatomy. We have to know the larynx well, obviously - structure, nerve supply, possible distortions, etc. We have to know too, for example, about the interscalene muscles and the brachial plexus; the division of the sciatic nerve into tibial and common peroneal; where to apply P6 acupressure on the wrist to combat nausea and vomiting; where all the dermatomes are; how to tell the right upper lobe bronchus from the bronchus intermedius on internal examination of the lungs; how to place large IV lines into the great vessels of the thorax; and how to adjust needle angle and position when placing lumbar or thoracic epidurals.

I get this question a lot: "What's the difference between a spinal and and an epidural?"

First, location, location, location.

A spinal anesthetic is placed right into the fluid-filled spinal canal (A). An epidural is placed into the area just outside the spinal canal, in a circumferential space pocket surrounding it (B).

Secondly, technique.

A spinal is simply a shot. The skin on the back is numbed with local anesthetic; an introducer is inserted sometimes to help guide the spinal needle through; and the super-thin spinal needle is inserted through the introducer into the spinal canal (A) and used to inject a small dose of anesthetic, often with a little narcotic, to achieve the desired effect. Then the needle and introducer are removed.

An epidural needle is a much larger, 9-centimeter 17-gauge instrument that is inserted initially into the ligament between vertebral bones. An air- or saline-filled syringe is then attached to the end of the epidural needle, and the epidural needle is advanced while gentle, intermittent pressure is applied to the syringe to see if there is any ability to compress its contents. This little "give" or moment of compression should not occur until the needle traverses the ligament and enters the epidural space (B). Once this happens, the syringe is detached from the needle and a slender catheter is threaded through the epidural needle, to be used for injection of local anesthetic into the epidural space. The epidural needle is removed and backed out over the catheter.

Finally, timing. A single-shot spinal has a time limit, depending on the type of local anesthetic used. Its effects - pain relief, sensory block, lower blood pressure - appear more abruptly and with greater intensity than those of an epidural, but it will eventually run out, whereas an epidural can be extended for hours or even days. Epidurals are often used to administer post-operative pain relief after abdominal, thoracic, or orthopedic surgery.

Epidurals are fun to do. Lumbar epidurals, such as those done for women in labor, are somewhat different from thoracic epidurals. The vertebrae themselves are anatomically distinct, with the spinous processes on thoracic vertebrae (first illustration below) jutting out at a much steeper downward angle than those on the lumbar vertebrae (second illustration). Thoracic epidurals are often approached from off-center because of this steep angle; the epidural needle can use the bone as a guide to find a wide gap between the vertebrae that leads toward the epidural space. For lumbar epidurals it's usually no problem to insert the needle in the midline between the bony prominences. For some great answers to women's questions about epidurals for labor, see this page at Storknet.

When epidural catheters are in the right place, patients' pain can turn from being a scary, hard-to-control enemy to something much more manageable. We love 'em.

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Nowadays technology is reducing the necessity for "blindly" done procedures. Have a tough airway? Use a fiberoptic scope or video laryngoscope. Can't find the vein for your central line, or the nerve sheath for your peripheral block? Roll out the ultrasound machine. Maybe soon there will be hand-held fluoroscopy for those tough-to-place thoracic epidurals.

I'm glad my training occurred on the cusp of these new developments. We learned to use external landmarks and to train our sense of touch to do most procedures, but we got enough exposure to the new devices to be able to integrate them into our practice. I don't want to lose the ability to manage a tough airway safely without the convenience of direct visualization, or to adjust an epidural needle based on my analysis of the contact point with certain tissues or portions of the vertebrae, or try an alternate, off-midline approach for a spinal in a patient with scoliosis. I want my familiarity with human anatomy to be like a current flowing through my movements and skills and not dependent on the ability to see what I am aiming for.

That said, I have to admit that being able to watch the human body at work from within never ceases to amaze me. If you're interested in seeing a human heart beating in the chest (time index 1:30) while undergoing surgery in an AWAKE patient, click here or see the video below, which describes the bold, innovative use of thoracic epidurals in India in order to allow patients to undergo cardiac surgery without a general anesthetic. Creative, daring, and impressive.