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Tuesday, December 18, 2007

The World of Epidurals

By Courtney L. Baetge, DVM

If I have learned anything during my time as an anesthesiologist, it is that for every topic there are a multitude of opinions. But I do believe most anesthesiologists are strong advocates of epidurals. The benefits of an improved anesthetic plane, smoother recovery and superior post-operative pain control well outweigh the additional time and effort involved in placing an epidural. However, for those who don’t do epidurals on a regular basis, they can seem a bit intimidating. I feel the skill is a small portion of this intimidation. Most practitioners with just a little practice become adept quite quickly. Most of our fourth year veterinary students can perfect the skill during the 2 week rotation. Having said this, why are they so scary? There are a lot of “other things” involved with an epidural than just the technique. Knowing when, where, who, and what are just a few. So let’s try and address a few of these (and how).

Most patients that will have a painful procedure done anywhere caudal to the umbilicus will appreciate an epidural. Obvious ones are rear limb surgeries and perineal hernias. But the less obvious cases include exploratory laparotomies, mastectomies, anal sac carcinomas, and preputial surgeries, just to name a few.

The subarachnoid space stops at L7 (can go further in young or very small dogs). So attempting epidurals at L7-S1, helps decrease the chance of causing damage to the spinal cord and performing a spinal rather than an epidural. However, if you end up getting CSF coming out of your needle simple reduce your dose by half and keep going. This will speed your onset of action but decrease your duration as well. There are times when the pelvis anatomy is not ideal and going to the L6-L7 space is easier. That’s fine!

More importantly is who NOT to give epidurals. Do not use an epidural if there is infection at the injection site, the patient is hypovolemic, the patient has a bleeding disorder or is on anticoagulants, axonal disease, severe anatomical abnormalities, or neurologic disorders. You do not want to spread any infections into or out of the spinal canal.

If there is a drug created, some researcher has tried putting it in some poor animal’s epidural space. BUT, there are a few drugs that most anesthesiologists consider staples. The drugs should be preservative free (the preservative in typical morphine is formaldehyde). The total volume should be less than 0.2 ml/kg [if the patient is very obese or pregnant the space will be smaller due to venous congestion and the dose should be reduced (0.15ml/kg)]. I tend to max out at 6 ml, especially if using a local, so there is less risk of respiratory or sympathetic blockade.

Opioids: Morphine [0.1 mg/kg] has a nice long duration of analgesia (12-24 hrs) without motor blockade or sympathetic nervous system blockade and will migrate well cranially to increase your area of analgesia. However, it can take up to 1 hr to begin working and can cause respiratory depression, urinary retention, decreased GI motility, vomiting and itching. Fentanyl is another opioid that can be used. It has a duration of 2-6 hrs, with a smaller area of analgesia and less systemic effects.

Local anesthetics: Lidocaine has a fast onset of action (10-15 minutes) but will only last about 60-120 minutes. Lidocaine can also cause respiratory depression and sympathetic nervous system blockade that can lead to hypotension. Bupivicaine has a longer onset of action (20-30 minutes) and a longer duration of action (4-6 hours).

Combining a local and opiod can give you the best of both worlds. I usually just reduce by opioid volume by 25% and replace that with local anesthetic. For instance, in a 10 kg dog at 0.1 mg/kg of morphine, you would use 1 mg of morphine which, if the 0.5 mg/ml Duramorph is used, equals 2 ml of morphine. Therefore, I would reduce my dose to 1.5 ml of morphine and add 0.5 ml of a preservative-free local anesthetic like bupivicaine for a total of 2 mL.

It is usually helpful for most people to have the patient in sternal recumbency with the rear legs pulled as far cranially as possible. This opens the L7-S1 space up and gives a larger target for your needle. The area is clipped and sterilely prepped. The thumb and middle finger rest on the wings of the pelvis which should direct the first finger to fall into the L7-S1 space directly between the thumb and middle finger. I tend to use a 20-22g 2 ½ to 3 ½ inch epidural needle, depending on the size and amount of fat at the injection site. An epidural needle is always preferred since the bevel is not a cutting edge and is less likely to damage the spinal cord if accidentally nicked. I turn the bevel to which ever direction needs the greatest analgesia (i.e. caudally for hernia repair, left for left stifle surgery, cranial for mastectomy). Once past the skin and majority of the fat, I remove the stylet and attach a test syringe of sterile saline or place a “hanging drop” so that I can see when I get negative pressure. The negative pressure may be gone in some patients but a test injection of saline should flow very easily (like an IV injection). Once in place, I inject my drug. If you get blood, stop and pull the needle out – it’s not the day for an epidural.

Hopefully this will help anyone trying to ease into the epidural world. Practice, be sterile, and good luck!