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Epidural Anesthesia: A Step by Step Guide

Charlotte R. Marquis, BVM&S (Hons.) DACVAA MRCVS


Epidural & spinal anesthesia can be rewarding for both you and your patients. If there’s one thing anesthesiologists are famous for, this is it!





Epidural anesthesia is the administration of drugs into the epidural aka extradural space. The administration of drug into the subarachnoid space is known as spinal aka intrathecal anesthesia. Local anesthetics and opioids are commonly used for these techniques, but dissociative anesthetics like ketamine or sedatives like a-2 agonists can be used too. Epidural or spinally-administered drugs target nerve roots where they leave spinal cord from intervertebral foraminae. When local anesthetics are used, afferent and efferent nociceptive transmission in the dorsal and ventral spinal nerve roots is interrupted. When opioids are used, descending inhibition of pain occurs. Epidural anesthesia provides effective pain relief for procedures involving the front & hind limbs, thorax and abdomen in veterinary species. Epidural anesthesia can provide several benefits including:


  • Complete, long-lasting analgesia…anywhere from 1 hr to 5 days, depending on drugs selected and if you placed an epidural catheter

  • Decreased inhalant anesthetic requirements…0.1 mg/kg PF morphine is 30% isoflurane sparing in cats

  • Decreased intra and post-operative opioid consumption

  • Decreased post-surgical stress markers in dogs

  • Quicker time to 1st meal post-surgery (in goats)

  • Improved muscle relaxation

  • Decreased mortality in human patients, compared to those not getting neuraxial anesthesia


Epidural anesthesia can be helpful in providing multimodal pain relief & balanced anesthesia. That being said, all drugs have side effects… some of which aren’t desirable. Maybe you shouldn’t do the epidural if:


  • There’s significant departure from normal anatomy…landmarks are difficult to palpate

  • Skin abnormalities at the site of puncture…tracking of infection into epidural space

  • Neurologic disease or deficits…unable to tell if recovery is due to drugs or worsening disease

  • On-going urinary retention…PF morphine is assocated with 3% incidence in urinary retention…about the same or lower incidence compared to systemic opioids

  • Coagulopathies…bleeding in epidural space could lead to permanent damage

  • Aggressive big dogs…motor blockade in hindlimbs means lifting the dog every couple of hours…frustrating or dangerous for the hospital staff


Anatomy:



Diagram of the epidural space in a dog courtesy of Lumb & Jones

The epidural space is located between the dura, which covers the spinal cord, and the boundaries of the vertebral canal. The epidural space is filled with semi-liquid fat and has a vertebral venous plexus. When local anesthetics or morphine are deposited into the epidural space, they migrate through the space to block spinal nerve roots. This is how analgesia of the limbs, trunk and thorax can be achieved from an injection at a single site. The conus medullaris is the termination of the meninges covering the spinal cord. In dogs, the conus ends at L6/7. In cats, the conus ends at L7/S1. This means you have a higher chance of hitting CSF when puncturing the lumbosacral (LS) space in a cat. Don’t worry if you do, you can still deposit drugs here, just reduce the volume of each of drug by half. Cranial spread and level of sensory block are determined by the baricity (specific gravity) of the local anesthetic, total drug volume and the patient's position. Large volumes of iso or hypobaric local anesthetic solutions will spread more cranially. In the CSF, spread of local anesthetic to the T5-L3 level can result in “total” or “high” spinal, causing nausea, vasodilation, hypotension and neurotoxicity.


Drug & dilution options:


Drugs

Class

Pros

Cons

Duration of action

Dose in LS space

2% Lidocaine

Local anesthetic

​Complete analegsia; sensory and motor blockade

Cranial migration → Blockade of T5-L3 → vasodilation & hypotension (aka total or high spinal), + Horner’s syndrome

1 Hour

0.1 ml/kg

0.5% Bupivacaine

Local anesthetic

As for lidocaine

​As for lidocaine; cardiotoxicity if injected intravascularly

​6-8 hours

​0.1 mL/kg

​0.75% Ropivicaine

​Local anesthetic

As for lidocaine

As for lidocaine

​4-6 hours

​0.1 mL/kg

Preservative free (PF) morphine 1mg/ml

Opioid

​Significant MAC-sparing effect, travels cranially to cisterna magna within 24 hours

​Small incidence of urinary retention has been described in dogs

​24 hours

​0.1 mg/kg

Dexmedetomidine 500mcg/mL

​A2-agonist

​Prolongs local anesthetic analgesia & hyperpolarizes C fibres

​Can result in sedation & decrease in cardiac output

​2 hours

​1-2 mcg/kg

​Ketamine 100mg/mL

​Dissociative anesthetic

​Prolongs local anesthetic analgesia

​Concerns for potential neurotoxicity & presence of preservative

​15 minutes

​1 mg/kg

Pro tip : For LS epidurals using 1mg/ml PF morphine, I use 0.2 mL/kg total drug volume and split it 50/50 between local and PF morphine. For sacrococcygeal (SC), I use 0.1 mL/kg. Here’s an example of how I’d calculate an LS epidural for a 20kg dog:


20 kg dog x 0.2mL/kg = 4 mLs total, draw up 2 mls 0.5% bupivicaine and 2 mls PF morphine


If you don’t want or need motor blockade for abdominal/thoracic limb procedures, you can use just PF morphine in the epidural. Do the 0.1 mg/kg dose and dilute to an appropriate volume with saine depending on your desired level of cranial spread.



Sensory dermatomes in dogs courtesy of Lumb & Jones


Total Volume of Injection

Anticipated cranial spread

​0.2 mLs/kg

L3

0.36 mLs/kg

L1-T10

0.4 mLs/kg

T9

Pro tip: Dilute with normal sterile saline. Limit drugs & dilutions to 6 mLs total volume in any dog or 1.5 mLs in any cat.


Equipment you’ll need:


  • A buffet of drugs from NEW bottles opened that day only

  • Appropriate anesthetic monitoring

  • Sedated or anesthetized patient

  • Spinal, Tuohy or nerve stimulator needles sizes 1.5-3.5 inches ins → provide better feedback than hypodermic when puncturing spinal tissues

  • Clippers, surgical scrub, alcohol

  • Sterile gloves

  • Sterile saline flush syringe

  • Small surgical drap (if desired)

  • Epidural catheter (if desired)

  • Nerve stimulator (if desired)


A selection of supplies that may be used for the placement of a "single shot" epidural


How to perform an L7/S1 lumbosacral epidural in a dog or cat using the hanging drop method:


1. Sedate or anesthetize the animal. Intrument them with monitoring equipment. Position the animal in either sternal or lateral recumbency with the hind limbs extended cranially. Take care to make the patient nice and straight, it’ll make finding the epidural space so much easier.


2. Before you clip and scrub, palpate the landmarks. If you’re right handed, stand on the animal’s left side, palpate landmarks with your left hand and place needle you’re your right hand. If you’re left handed, do the opposite. Palpate the wings of the ilium with your thumb and middle finger. It’ll feel like you’re rolling over a small hill. Using your pointer finger, palpate midline for the dorsal spinous process of L6 then L7. Roll caudally to identify the lumbosacral space, which feels like a dip after L7.


3. Clip and aseptically prepare a small square of skin overlying this area. Don sterile gloves and get your spinal needle out. Drape your puncture site, if desired. Loosen the stylet from the needle now, it’ll make separation of these easier later on.


4. Palpate landmarks as before. Where you feel the dip aka LS space, place your needle at a 90 degree angle to the skin and puncture just through the skin, then stop. Do not let go of the needle.


5. Grasp the hub of the needle with your palpating hand, and use your needle-advancing hand to remove the stylet. Ask an assistant to place drops of sterile saline into the hub for the hanging drop test. Replace your needle-advancing hand on the hub. Your palpating hand can return to the patient’s skin.


6. Slowly advance your needle through the skin, SQ tissue, supraspinous ligament and interspinous ligaments. As you meet the interarcuate ligament, you’ll feel slight resistance. Advance the needle even further and a slight give or “pop” is felt. If you’re wondering what this is like, it feels like puncturing a Capri Sun with a straw . If you’re teaching someone to do this for the 1st time, fold a sterile glove on itself twice and have them poke through with the needle- this will simulate the popping sensation.


7A. The pressure in the epidural space is usually -6 to 15 mmHg. When saline is placed on the hub and the needle punctures the epidural space, the drop usually gets sucked in. Destination: epidural space! Fair warning though…in lateral recumbency, obese patients and on 22g needles, the saline drop probably won’t go in. Go to step 8 to confirm placement.


7B. You get the popping sensation, but clear fluid starts coming out. Destination: subarachnoid space! This is especially common in cats, where the conus medullaries (termination of the meninges covering the spinal cord) ends more caudally (around S1) in cats compared to dogs (L6/7). If you see spinal fluid, reduce the volume of each of your chosen drugs by half.


8. To confirm you’re in the right spot, aspirate some air into your saline syringe and attach it carefully to your needle. Aspirate, then try and inject 0.5ml of saline solution. If there’s resistance, blood or your plunger bounces back on the air, try withdrawing the needle a couple millimeters and adjusting cranially or caudally. If the injection goes in smoothly, you’re in the right place.


Glass and loss of resistance syringes can be used to facilitate needle placement in the epidural space. When the needle is not yet in the epidural space, the air inside the syringe bounces back against the plunger. Once the epidural space has been reached, the air inside the syringe injects easily. Some concerns have been raised over whether or not this technique increases the likelihood of air embolism development, thus manual palpation of ligament puncture, electrolocation and the hanging drop technique have been favored for determining correct needle placement in recent years.


8. Combine your epidural drugs and attach the syringe to the needle. Aspirate, then inject the drugs over 30-60s. If you meet resistance, stop injecting and withdraw the needle. Congratulations on the epidural!


How to perform a S3/Co1 or Co1/2 sacroccygeal (aka caudal) epidural in a dog or cat:


1. Follow the general instructions for patient positioning, prep, injection technique as for LS epidural.


2. Ask an assistant to move the tail up and down slowly. Palpate the 1st moveable joint at the base of the tail- it’ll either be S3/Co1 or Co1/Co2. This is the spot where your needle will enter.


3. The hanging drop technique is not generally effective back here. Use the popping sensation, successful test injection, or nerve stimulation to confirm needle placement.


How to perform a LS or SC epidural with a nerve stimulator:


Insulated nerve stimulator needles are coated with non‐conducting material except for a small area at the needle tip. This insulation prevents the electrical current from “leaking” along the needle shaft into surrounding tissues. As a result, the current travels down the length of the needle and is concentrated at the tip. When low intensity currents (0.3-0.5 mAs) are applied to motor nerves, you elicit the motor effect of that nerve, thus identifying the correct target areas for blocking.


1. Follow the general instructions for patient positioning, prep, injection technique as for LS/SC epidural.


2. Connect the negative electrode (black) to the nerve stim needle. Connect the positive electrode (red) to the patient. Connect the drug syringe to the nerve stim needle and push the drugs all the way through. Put an air bubble into the syringe for test injection later on.


3. Turn on the nerve stimulator and set the current to 1.0 mAs, 2Hz and 0.1ms.


4. Place and advance the needle as you would for a LS/SC epidural. As you get closer to the target nerves, you’ll feel or see muscle contraction. LS space: twitching of tail and hind limbs. SC space: tail twitch.


5. Once the correct motor response is obtained, turn down the current to 0.7 mAs. The closer you are to 0.7 while getting a motor response, the closer you are to the target nerves. Try to aim for this.


6. If you don’t see any motor responses, withdraw the needle gradually and redirect it.


7. Aspirate using the syringe. If blood is aspirated, reposition the needle.


8. Slowly inject the drugs. As soon as the solution is injected, motor twitches will disappear when the conductive area is expanded by the local anesthetic. If any resistance is encountered, stop injecting and withdraw the needle . You can always try postoperatively if you’re unsuccessful…better late than never!


Selected citations:



Golder, F., Pascoe, P., Bailey, C., Ilkiw, J. and Tripp, L. (1998). The effect of epidural morphine on the minimum alveolar concentration of isoflurane in cats. Journal of Veterinary Anesthesia, 25(1), pp. 52-56. https://www.sciencedirect.com/science/article/pii/S1351657416300249


Guay, J., Choi, P., Suresh, S., Albert, N., Kopp, S. and Pace, N. (2014). Neuraxial blockade for the prevention of postoperative mortality and major morbidity: an overview of Cochrane systematic reviews. Cochrane Database of Systematic Reviews 2014, 1, pp. 1-2. doi: 10.1002/14651858.CD010108.pub2.

Martin-Flores, M. (2019). Epidural and Spinal Anesthesia. Veterinary Clinics of North America: Small Animal, 49, pp. 1095-1108.


Moraca, R., Sheldon, D. and Thirlby, R. (2003). The role of epidural anesthesia and analgesia in surgical practice. Annals of Surgery, 238(5), pp. 663-673.


Otero, P., Verdier, N., Zaccagnini, A., Fuensalida, S., Tarragona, L. and Portela, D. (2015). The use of a nerve stimulation test to confirm sacrococcygeal epidural needle placement in cats. Veterinary Anesthesia & Analgesia, 42(1), pp. 115-118. doi: 10.1111/vaa.12173.


Peterson NW, Buote NJ, Bergman P. Effect of epidural analgesia with opioids on the prevalence of urinary retention in dogs undergoing surgery for cranial cruciate ligament rupture. J Am Vet Med Assoc (2014) 244:940–3. doi:10.2460/javma.244.8.940

Steagall, P., Simon, B., Teixeira, F. and Luna, S. (2017) An Update on Drugs Used for Lumbosacral Epidural Anesthesia and Analgesia in Dogs. Frontiers in Veterinary Science, 4(68). doi: 10.3389/fvets.2017.00068


Read, M. (2005). Confirmation of epidural needle placement using nerve stimulation in dogs. Veterinary Anesthesia & Analgesia, 32(4), p. 13. doi.org/10.1111/j.1467-2995.2005.00232a_26.x


Troncy E, Junot S, Keroack S, Sammut V, Pibarot P, Genevois JP, et al. Results of preemptive epidural administration of morphine with or without bupivacaine in dogs and cats undergoing surgery: 265 cases (1997-1999). J Am Med Assoc (2002) 221:666–72. doi:10.2460/javma.2002.221.666


Verdier, N., Martinez-Taboada, F., Otero, P., Redondo Garcia, J., Zaccagnini, A., Costoya, A., Tarragona, L. and Portela, D. (2021). Evaluation of electrical nerve stimulation to confirm sacroccyggeal epidural needle placement in dogs. Veterinary Anesthesia & Analgesia, 48(4), pp. 612-616. https://doi.org/10.1016/j.vaa.2020.12.008




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