Peri and Post-operative Pain Management
Preservative free morphine and Bupivacaine (Marcain® 0.5% Steripack) are required if an epidural is to be administered.
• Epidural injection requires lower drug doses than systemic injection, resulting in fewer side effects.
• Less inhalant and/or injectable anaesthetic is required in surgical patients, leading to improved cardiorespiratory function.
• Use of lower doses and the long duration of action make epidural injection convenient and economical.
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 (7 out of 242 dogs) and pruritus (2 out of 242 dogs) have been reported after morphine administration, and there is a report of a dog developing an epidural abscess leading to diskospondylitis. Prolonged motor dysfunction (lasting longer than the expected duration of the injected local anaesthetic) is occasionally reported, and it is possible that direct trauma caused by the epidural needle could damage the spinal cord and nerve roots. Conscious animals may struggle and resent injection of local anaesthetics, and morphine can cause vomiting in conscious animals. 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. For elective surgery fentanyl patches should ideally be placed 20 to 24 hours prior to surgery in dogs (8 to 12 hours is sufficient 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.
Fentanyl patches must not be cut. It is acceptable to leave part of the patch covered so that it does not contact the skin, however as patch adhesion can be problematic this is not recommended. Fentanyl patches must be disposed of safely. A used patch may still contain sufficient fentanyl to be lethal in ingested by the patient or a child.
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 removal of fat from the skin will affect absorption. The site must be thoroughly dried before patch application; once patch is applied maintain firm pressure for a minute or two before covering with a dressing (and a bandage for lower limb application).
Dogs: approx. 4 µg/kg/h (2 – 5 µg/kg/h) e.g. 100 µg/h patch for a 25 kg dog
Cats 3 – 6kg: 25 µg/h patch
Cats < 3kg: 12.5 µg/h patch
Duration of Action:
Dogs: 72 hours (3 days) from application
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:
1) A more stable plane of analgesia with less incidence of break-through pain (which can be difficult to treat);
2) A lower drug dosage delivered at any given time, resulting in a lower incidence of dose-related side effects;
3) Greater control over drug administration (easy to change the dose);
4) Decreased need for stimulation of resting patients to administer drugs;
5) 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.
It is essential to be able to vary total fluid volume administered to an anaesthetised patient independent of drug administration. In an anaesthetic crisis the emergency IV fluid bolus rate for a dog may be as high as 90ml/kg in 15 minutes (50-55ml/kg in cats). Increasing an MLK CRI to this rate would rapidly deliver a potentially fatal cocktail of drugs. It is ideal to set up a separate IV line for a CRI i.e. one IV line for fluid requirements and one IV line for CRI. Alternatively the CRI may be set up as a “piggyback” onto anaesthetic maintenance fluids.
Loading doses should precede CRIs. Loading doses are needed prior to the initiation of the CRI in order 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 MLK CRI Recipe
Never use a CRI as the sole source of intravenous fluids to an anaesthetised patient (see above).
Only use lignocaine in Dogs – in cats use Morphine and Ketamine as below but exclude lignocaine:
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
• Use 500 ml bag of saline and remove a volume equal to the total volume of agents to be added (round up/down to nearest ml)
• 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
• Morphine and lignocaine are light-sensitive; if bag is protected from light (foil &/or Vet-Wrap) solution should remain stable for 4 days
• 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.
Example MLK CRI:
From a 500 ml bag remove (2 + 25 + 0.6) ml = 27.6 ml, (round up to 28 ml) from the bag then add 2 mls of 30mg/ml morphine, 25 ml of 20mg/ml lignocaine and 0.6 ml of 100mg/ml ketamine. Wrap bag in foil and/or Vet-Wrap, label bag clearly, attach to a separate IV line, set rate at (weight in kg) mls/hr and use for up to 4 days.