Epidural Anesthesia in Small Mammals

Video

Video produced by M. Scott Echols, DVM, DABVP (Avian Practice) and narrated by Marla Lichtenberger, DVM, DACVECC.

What is epidural anesthesia?

The epidural is a neuroaxial technique that provides preemptive analgesia by injecting drug into the epidural space surrounding the spine (Fig 1).

Figure 1. Epidurals are performed in small mammals to provide preemptive analgesia. Click image to enlarge.

Indications

Epidurals can be used for abdominal surgeries, perineal surgeries, and orthopedic procedures involving the pelvic limb or spine. Some opioids can also travel cranially to provide supplemental analgesia for chest and thoracic limb procedures. Epidurals serve as an adjunct to systemic analgesia, and epidural analgesia also reduces the amount of anesthetic drug needed. Epidurals can also shorten recovery time (Sladky 2000).

Despite these many benefits, clinicians are often hesitant to perform epidurals in small mammals, however the technique is relatively safe, easy and effective.

Potential adverse effects

One possible adverse effect of epidural analgesia is transient paresis or paralysis. Additional rare side effects with epidural opioids can include pruritus, vomiting or nausea, respiratory depression, and urinary retention.

Contraindications

Epidural injections are contraindicated in patients with coagulopathy, sepsis, skin infection, hypovolemia, or when anatomy has been distorted by pelvic or sacral fractures.

Equipment needed

To perform an epidural you will need:

• Pre-warmed surgical scrub
• An appropriately sized needle (Box 1)

  Box 1. Recommended needle size for epidural injection in small mammals
  Species	Needle size
  Ferret or rabbit	22-gauge 1.5-in spinal needle 24-gauge spinal needle 25-gauge hypodermic needle
  Chinchilla or guinea pig	25-gauge hypodermic needle 24-gauge spinal needle

  Use of a spinal needle prevents creation of a skin plug that could block the flow of cerebrospinal fluid or serve as a nidus of infection.
• An assistant to keep the needle stabilized while switching syringes
• Saline
• Epidural drug:  Select an agent to provide anesthesia, analgesia, or both. The drug selected should be preservative-free to prevent neurotoxic or allergic responses. Morphine is the most commonly used opioid because it is highly potent and long lasting (Box 2). Other opioid agents for use may include buprenorphine, oxymorphone, and hydromorphone.

  Box 2. Epidural morphine dose in select small mammals
  Species	Epidural morphine dose (mg/kg)
  Ferret	0.1
  Rabbit	0.22

  Local anesthetics can be administered by epidural injection alone or with an opioid. Bupivacaine (0.1 mg/kg in rabbits) may be the best choice because it appears to have less motor effects while producing good sensory blockade. Small mammals, particularly rabbits, can get very stressed during recovery if they are unable to move their rear limbs. Epidural blocks develop from caudal to cranial. The volume of drug administered will influence how far cranially the drugs migrate.The total volume of drug given should always be less than or equal to 0.33 ml/kg.

Performing an epidural injection

1. To perform an epidural injection, the patient should be completely anesthetized.
2. Place the patient in ventral recumbency with the hips flexed cranially to fully open the lumbosacral space. (Figs 2-3).
   

   Figure 2. Restrain the rabbit in ventral recumbency with the legs pulled cranially. Click image to enlarge.

   

   Figure 3. Restrain the patient in ventral recumbency. Note the ferret’s legs are pulled caudally. Click image to enlarge.

3. Identify the cranial aspects of the wings of the ilium 3. (Fig 4).
   

   Figure 4. Identify the cranial aspects of the iliac wings. Click image to enlarge.

4. Visualize a transverse line connecting these two points. The lumbosacral junction sits very near this line on midline (Fig 5).
   

   Figure 5. Visualize a horizontal line connecting the wings of the ilium. Click image to enlarge.

5. Palpate the vertebral spinous processes and the lumbosacral space (Fig 6).
   

   Figure 6. Palpate the vertebral spinous processes and the lumbosacral space. Click image to enlarge.

6. Shave fur over the lumbosacral space. The area should be large enough to easily view bony landmarks and maintain sterility (Fig 7).
   

   Figure 7. Clip a square or rectangle over the lumbosacral space. Click image to enlarge.

7. Aseptically prepare the skin.
8. Holding the needle perpendicularly with the bevel facing cranially, insert the needle exactly on midline caudal to the last lumbar vertebrae. The needle should drop smoothly through the lumbosacral space until it encounters bone on the floor of the spinal canal (Fig 8). The distinctive popping sensation felt in dogs is usually not appreciated in ferrets, particularly when a hypodermic needle is used, however the ferret’s tail will frequently twitch as the needle engages nerve tissue.
   

   Figure 8. Insert the needle exactly on midline caudal to the last lumbar vertebrae. Click image to enlarge.

9. To confirm correct needle placement: Attach an empty 1 ml syringe and slowly inject 0.1 ml of air OR Inject 0.2 ml of sterile saline combined with 0.2 ml of air. There will be a lack of resistance to this injection of air if the needle is within the epidural space. Also when the syringe is gently aspirated, no blood or cerebrospinal fluid should appear within the needle hub. If there is resistance to injection of air or if the skin bulges, advance the needle until it fully penetrates the ligamentum flavum, then check for resistance again.
10. Once needle position is verified, replace the empty syringe with one containing the anesthetic agent. Slowly administer drug into the epidural space. Caution: Rapid injection can cause uneven drug distribution within the epidural space and even an increase in intracranial pressure.
11. Be sure to empty the patient’s urinary bladder before it rouses from general anesthesia. Bladder and sphincter tone will hopefully return to normal before the bladder fills.

Did the epidural injection work?

A successful epidural is associated with loss of anal sphincter tone and loss of tail tone as well as a drop in systemic blood pressure. It can take 15-30 minutes for full analgesic effects to develop. Effects may persist anywhere from 8 to 24 hours.

References