Peri and Postoperative Pain Management
Clinical Guidelines and Recipes
Epidural
Requirements:
- Bupivacaine 0.5% (1mg/kg) in a single-use vial (can be used alone or in combination with an opioid e.g. morphine, bupivacaine).
- Preservative-free morphine (0.1 mg/kg) or buprenorphine (0.004 mg/kg) in a single-use vial.
Advantages:
- Analgesia for up to 24 hours post-injection (when including an opioid).
- Saper, "smoother" anaesthesia; improved intra-operative analgesia allows the use of less inhalant and/or injectable anaesthetic (maintenance of a lower plane of anaesthesia), leading to improved cardiorespiratory function.
- Epidural injection requires significantly lower drug doses than systemic injection of the same drugs, resulting in fewer side effects.
- Use of lower doses and the long duration of action make epidural injection convenient and economical.
Complications:
The incidence of reported complications associated with epidural injections is low, but problems can arise and may range from minor to serious. Most complications can be avoided by careful patient selection. Urine retention and pruritus have been occasionally reported after morphine administration, and there is a report of a dog developing an epidural abscess leading to discospondylitis. Prolonged motor dysfunction (lasting longer than the expected duration of the injected local anaesthetic) is occasionally reported, and it is possible direct trauma caused by the epidural needle could damage the spinal cord and nerve roots. Although of no medical consequence, owners should be warned about delayed and abnormal hair regrowth at the clipped site over the lumbosacral space. The reasons for this phenomenon are not well understood.
Fentanyl (Duragesic) Patches
Fentanyl (Duragesic) patches are likely to benefit any orthopaedic patient; they are particularly advised for poly-trauma patients (especially useful in fractious cats) and animals undergoing spinal surgery and should be placed as soon as possible for these cases. Onset of duration is 12-24 hours in dogs and 4-12 hours in cats. For elective surgery, fentanyl patches should ideally be placed 20 to 24 hours prior to surgery in dogs, and 8 to 12 hours in cats.
Whilst a patch may allow for continuous transdermal drug delivery over an extended period there is tremendous variability in the drug delivery through this route. A patch is best used as a component in a more balanced strategy including additional drug classes; it should not be relied upon as the sole source of analgesia.
Placement:
Sites commonly used include behind the neck, flank, lower antebrachium, metacarpal & metatarsal regions. Lower limb application may be preferable as the skin is less mobile and bandaging helps secure the patch in place, thus maintaining good skin contact. However, as the patch is easily accessible by the animal an Elizabethan collar is essential. For prolonged use you should not use the same site continuously, however, it is acceptable to rotate between two sites.
The site should be carefully clipped and gently cleansed with warm water. Do not use soaps or alcohol as the removal of fat from the skin will affect absorption. The site must be thoroughly dried before patch application; once the patch is applied maintain firm pressure for a minute or two before covering it with a dressing (and a bandage for lower limb application).
Dose:
Dogs: approx. 4 µg/kg/h (2 – 5 µg/kg/h)
Example dosing:
- 25 µg/h patch for a 6 kg dog
- 50 µg/h patch for a 15 kg dog
- 75 - 100 µg/h patch for a 25 kg dog
- 100 - 150 µg/h patch for a 40 kg dog
Cats:
- Cats > 4 kg: 25 µg/h patch
- Cats < 4 kg: 12.5 µg/h patch
Duration of Action:
- Dogs: 72 hours (3 days) from application (some reports suggest up to 100 hours).
- Cats: up to 5 days from application.
Constant Rate Infusion (CRI)
Constant rate infusions (CRI) of analgesic drugs are an excellent way to manage pain in both dogs and cats. A CRI of analgesic agents has several advantages over multiple repeated injections for pain relief, including:
- A more stable plane of analgesia with less incidence of breakthrough pain (which can be difficult to treat).
- A lower drug dosage delivered at any given time, resulting in a lower incidence of dose-related side effects.
- Greater control over drug administration (easy to change the dose).
- Decreased need for stimulation of resting patients to administer drugs.
- Decreased cost (when compared to staff time, needles, and syringes required for repeat injections).
Drugs that are useful for CRIs include fentanyl, hydromorphone, morphine, butorphanol, ketamine, lidocaine, and various combinations of these drugs.
- Dogs: Morphine, Ketamine and Lignocaine (Lidocaine) may be used in combination for highly effective analgesia (MLK).
- Cats: Morphine and Ketamine (MK) may be used together, however, Lignocaine is best avoided due to toxicity concerns.
Loading doses should precede CRIs. Loading doses are needed prior to the initiation of the CRI to achieve initial therapeutic blood levels. Otherwise, it would take 3 to 5 half-lives of each drug to reach steady-state drug levels.
Simple Ketamine CRI Recipe
Ketamine: 75 mg per 500ml bag of fluid (i.e. 0.75 ml of 100mg/ml Ketamine*)
Examples:
- Add 0.75 ml of Ketamine* to a 500 ml bag of saline
- Add 1.5 mls of Ketamine* to a 1-litre bag of saline
Administration:
- Loading Dose: Give Ketamine 0.25–0.5 mg/kg IV prior to commencement of CRI.
- Administer fluid at surgical rate during anaesthesia, and reduce to maintenance rate in recovery. Can continue overnight if required.
- Try to avoid sudden discontinuation; wean over several hours.
* Ketamine 100 mg/ml
Simple MLK CRI Recipe
Drug Volumes:
- Morphine: 60mg per 500ml bag of fluid (i.e. 2ml of 30mg/ml morphine into a 500 ml bag)
- Lignocaine: 500mg per 500ml bag of fluid (i.e. 25ml of 20mg/ml lignocaine without epinephrine)
- Ketamine: 60mg per 500ml bag of fluid (i.e. 0.6ml of 100mg/ml ketamine)
Instructions (Example MLK CRI):
- From a 500 ml bag remove (2 + 25 + 0.6) ml = 27.6 ml, (round up to 28 ml).
- Add 2 mls of 30mg/ml morphine, 25 ml of 20mg/ml lignocaine and 0.6 ml of 100mg/ml ketamine to the bag.
- Wrap the bag in foil and/or Vet-Wrap to protect from light (Morphine and Lignocaine are light-sensitive). If protected, the solution should remain stable for 4 days.
- Label the bag clearly.
- Attach it to a separate IV line.
- Set the rate at (weight in kg) mls/hr and use for up to 4 days.
Important Notes:
- Dose is 1ml/kg/hour (i.e. enter the patient weight into the infusion pump).
- In dogs the dose can be increased up to 3ml/kg/hour; be aware the incidence of dose-related side effects will be higher at this dose.
- Any combination of the above agents can be used in dogs at the above doses; typically all three agents are used together.
- As Morphine is a controlled drug you cannot legally reuse bags on other patients.
- It is best practice to gradually wean the patient off the infusion over (at least) several hours.
- As with any opioid usage the patient should be regularly reassessed for signs of opioid-related side effects; however the incidence of such is lower compared to other forms of administration.