Epidural Anesthesia and Analgesia
Epidurals are commonly used to provide anesthesia and analgesia for many surgical and pain relieving procedures in all species including dog, cat, equine, bovine and select exotic patients . This technique is very useful for surgical procedures involving the hind limbs, pelvis, and perineal regions. Depending on what agent is used, desired results can be achieved with local anesthestic and analgesic agents. Some of the more common surgical procedures epidurals are used for include:
- Equine or Bovine: Standing surgeries including ovarioectomies, laparoscopic cryptorchidectomy, flank laparotomy, caslicks, perineal hernias, cervical tears, rectovaginal tears, mass removal from anal or perineal region, fetotomies, perineal urethrostomies, tail amputations
- Small Animals (canine, feline): Hindlimb surgeries including femur, tibia, metatarsal bones, cranial cruciate ligament repairs, MPL disease repairs, femoral head and neck ostectomy (FHNO), perineal hernias, tail amputations and many others.
For those interested, here is a link to a small animal Epidural dose calculator which allows you to choose a calculator for any of the following combinations: Morphine, Morphine/Bupivicaine, or Hydromorphone/Bupivicaine. http://www.vasg.org/resources_and_support_material.htm#Item10
Anatomy of the epidural space:
The epidural space has a large amount of epidural fat that is inside the vertebral bone column. Even with a short period of weight loss the amount of epidural fat will not be affected. The spinal cord is surrounded by the meninges with the pia mater adhered to the spinal cord. The arachnoid and the dura mater are a combined layer, placing the cerebrospinal fluid between the pia mater and the arachnoid/dural layer. There is a venous plexus on the ventral floor of the vertebral column that will occasionally be entered.
The vertebral level at which the cord and the dural sac (meningeal layers) end is variable in the dog, cat and horse. In the dog the cord ends at L6-L7 and the dural sac at L7-S2. In the cat, they usually end further caudally (about one more vertebra). In the horse, they end at the location of S2.
The indication is to provide analgesia and anesthesia caudal to the diaphragm. It is an excellent means to provide anesthesia and analgesia to the pelvis, pelvic limbs and perineum. In all species, anesthesia is utilized with local anesthetics administered in such volumes that they inactivate action potentials in the spinal cord caudal to the branching of the sciatic and femoral nerves. Analgesia, on the other hand, may include local anesthesia, but it has a much broader application.
The main contraindications are coagulopathies, sepsis, skin infections over the injection site, and pelvic/sacral fractures. With coagulopathies, if the ventral venous plexus should be entered, it could cause excess bleeding, which would put pressure on the spinal cord.
The animal is either heavily sedated or anesthetized. There are two ways to administer an epidural injection. The easiest generally is to place the patient in sternal recumbency with the hind limbs frog-legged cranially to open the L-S space. The hair is clipped and the skin prepped with a surgical scrub. It is recommended that gloves be worn while performing this procedure.
Palpate both wings of the ileum with your thumb and middle finger and find the highest point. There is now an imaginary line between the thumb and middle finger. With your index finger, palpate the spine of L7, which is even with, or just cranial to the line between thumb and middle finger. Insert the needle just caudal to the L7 dorsal spinal process. Alternatively, the animal can be placed in lateral recumbency with its legs pulled cranially. The landmarks are the same.
Generally, 18-22 gauge spinal needles are used for the procedure, 22 gauge being the most common. It is small enough for all patients yet stiff enough not to alter course while going through the tissue. Smaller needles can be used but on occasion will deviate due to the small diameter. Some veterinarians prefer 20 gauge needles for larger patients. The increased stiffness allows for more directional control in these patients.
It is recommended that a spinal needle be used for two reasons. One, the bevel is shorter than a hypodermic needle allowing you to better sense the pop when the needle passes through the ligamentum flavum. The other is that the stylus prevents a skin plug from clogging the needle. This would prevent you from seeing CSF fluid and, if a skin plug were injected into the epidural space, it could serve as a nidus for infection and inflammation.
The needle is advanced until a distinct pop is felt or resistance is met. Occasionally the animal’s tail will twitch. If the needle is advanced too far you could enter the ventral plexus or the dorsal annulus of the disc space.
If in the epidural space, there should be little to no resistance when injecting. Initially a small amount of air or saline can be injected as a test. There should be no resistance. The plunger should always advance and not retract so as to refill the syringe. An alternative method is called the hanging drop technique. With this technique the needle hub is filled with saline after initial penetration of the skin. When the needle enters the epidural space, the saline is drawn into the epidural space due to the subatmospheric pressure within the spinal canal. If resistance is met is it usually due to the needle entering the ventral tissue and it should be withdrawn slightly. Once in the epidural space it is important to check for CSF fluid. This would affect the volume of drug injected as described below.
To see a video presentation on epidural technique in small animal patients, refer to the link below:
Caudal (low) epidural is the most common technique performed in equine patients. Crainal (high) epidurals are contraindicated. The volume of fluid to be injected like in small animals, should be kept to a minimum, in order to minimize effects of motor control of the patient. The technique is similar to the technique in the bovine patient.
Injection is through the space between the first and second coccygeal vertebrae or the sacrocaudal (first intercoccygeal) vertebral space. In general, this is usually cranial to the long tail hairs. A divot can be palpated in this region with your thumb or index finger while moving the tail up and down with the opposite hand. After preparing the area in routine sterile fashion, a skin bleb is made to allow location and entry of the needle. The hanging drop technique can be used in equine patients as well when performing an epidural block. An 18-20 gauge, 3 inch spinal needle is preferred. The needle can be inserted at a 30-60 degree angle from the perpendicular line through the vertebrae. Generally, a volume of 3-7 mls of local anesthetic/analgesic agent is infused.
Additionally, a continuous epidural catheter can be placed in similar format using a commercial epidural catheter kit for treatment of long-standing or chronic pain conditions in the hindlimbs.
Performing an caudal epidural in the bovine patient is similar to the equine technique. The location between the first and second coccygeal vertebrae is the preferable site, however the sacrococcygeal space can also be used.
Drugs commonly used to for epidural:
Ideally the drugs should be preservative free, although the methylparaben type of preservative can be used without causing obvious clinical problems.
Four drugs have been used in epidurals:
Drugs Dose (ml/kg) Duration Time of onset(minutes)
Lidocaine 2.0% 0.2 45-90 5
Mepivacaine 2.0% 0.2 60-90 5
Bupivacaine 0.5% 0.2 120-360 20
Ropivacaine 0.5% 0.2 90-420 15
Generally a dose of 1ml per 4.5 kg (canine) or 5-7 mls per 500kg (equine), with a maximum dose of 6ml total regardless of the dog or horse’s size, is recommended. Bovine analgesic dose can range from 0.5-1 mls per 100lbs. If you are trying to effect a blockade of the cranial abdomen then a dose of 1 ml per 3-5 kg may be needed but must be injected slowly. If CSF fluid is obtained during epidural administration then this dose needs to be reduced by 50%. If this is not done you may cause a block further cranially than anticipated. The drug should be administered slowly to prevent a patchy block and to prevent the drug from moving too far cranially. If the animal is pregnant the block needs to be reduced 50-75% due to engorgement of the venous plexus, which makes the epidural space smaller. Obese animals should have their dose base on an estimated lean body weight.
Using local anesthetics will result in sensory, motor and autonomic blockade. If the blockade extends to the thoracic region, autonomic effects may be significant, potentially causing bradycardia. Respiratory function will not be impaired unless a motor block of the phrenic nerves occurs (C3-C5). Initially, after an epidural there may be a decrease in blood pressure and an increase in heart rate. No significant changes in cardiovascular function, respiratory rate or arterial blood gases are expected with local anesthetics.
Side effects of epidural local anesthetics:
The most common side effect is prolonged paresis. This is self-limiting but may require the animal to stay in the hospital until normal function returns. Hindlimb paresis can cause the equine patient to become extremely agitated and result in catastrophic compliations, therefore it is generally recommended epidurals be performed in stocks when working on the standing equine patient. Rarely, respiratory paralysis may occur, requiring ventilation until the effect of the block dissipates.
Opioids are probably the most commonly used class of epidural drugs. Opioids can be used alone or they can be combined with local anesthetics.
Opioids used in epidurals Dosage (mg/kg)
The most commonly used opioid for epidurals is morphine. It is the least lipid soluble so it travels the farthest cranially and has the slowest onset of action. Because of these qualities the duration of action is longer. In humans, the analgesia range is from 4 to 51 hours. In canine and equine patients, the duration of effect is 10 to 24 hours. Spinally administered opioids in small animal patients have been shown to decrease inhalant requirements by up to 50% of the normally expected levels.
There is a debate about the epidural use of morphine products containing formaldehyde as the preservative. Some anesthesiologists are concerned about neurotoxicity from this preservative while other says it poses no problems. I have used both morphine with formaldehyde and preservative free morphine. While the neurotoxicity is rare, when it occurs it produces very painful myoclonus of the affected muscles that is considered an emergency situation requiring treatment. This may requires general anesthesia. What I prefer to do is to administer another bupivacaine dose epidurally until the myoclonus is eliminated. Once the latter epidural has worn off the myoclonus is usually gone. Because of the severity of this reaction, I recommend preservative free morphine (Astromorph 1mg/ml or Duramorph 1mg/ml).
Hydromorphone and oxymorphone are more lipid soluble and may produce more segmental effects. They will not move as far cranially and may provide more pronounced analgesia to the hindquarters. The more lipid soluble the drug, the closer the epidural dose will be to the dose required systemically. Hydromorphone and oxymorphone both contain methylparaben as a preservative. Methylparaben is considered an acceptable preservative for epidural use.
Fentanyl is very lipid soluble and is of short duration and unless given via epidural catheter offers little advantage over morphine, hydromorphone, or oxymorphone.
Buprenorphine is lipid soluble and a partial mu agonist that can be used but I see little advantage or need for using buprenorphine instead of the previous drugs.
Side effects of epidural opioids:
With the use of epidural opioids, particularly morphine, pruritus and urinary retention are not uncommonly seen. Occasionally urinary catheterization will be necessary. In humans vomiting, nausea and respiratory depression can be seen. The reasoning is that due to morphine’s low lipid solubility, it travels rostrally and binds to supraspinal opioid receptors in the brain causing respiratory depression. This respiratory depression does not appear to be clinically significant.
Important points for epidural use:
- Local anesthesia dose is 1ml /4.5kg (maximum 6ml)
- With pregnancy decrease the dose of local anesthesia 50-75%
- With obese patients use lean estimated body weight
- Old age decreases the dose and they may have stenosis of the intervetebral foramen causing increased cranial movement and drug effect
- Increased intra-abdominal pressure will engorge venous plexus reducing the volume needed for block
- If you obtain CSF fluid reduce the does of local anesthetics by 50%. If using a opioid/saline then no reduction is needed
- Infuse slowly, as this can cause patchy block or to far cranial placement of local anesthesia
Other drugs used in the epidural space:
In equine patients, alpha-2 adrenergic agonists such as xylazine and detomidine hydrochloride provide profound analgesia without complications of ataxia. Xylazine dose in the equine, is 0.17 mg/kg. Onset of action is 10-30 minutes and duration of analgesia is 2.5-4 hours. This volume should be diluted to a total volume of 6-10 mls. Epidural administration of detomidine at 30-60 micrograms/kg provides analgesia up to 2-3 hours, however produces sedation and ataxia. Often, xylazine or detomidine or commonly administered in combination with local anesthetic agents.
In small animal patients, ketamine, alpha-2 drugs (xylazine and medetomidine) and NSAIDs have been used epidurally. Medetomidine at a dose of 5ug/kg did not show evidence of analgesia but it did appear to have a supra-additive effect when combined with morphine, prolonging the duration of overall analgesia. Xylazine has been used at a dose of 0.02 mg/kg in combination with morphine and was not associated with any cardiovascular side effects.
Information contained in this article was taken from the www.vasg.org website – authored by Gary Old, DVM; Auer and Stick – Equine Surgery 3rd ed; Turner and McIlwraith – Techniques in Large Animal Surgery 2nd ed.