Peri-anesthetic pain management strategies (Proceedings)

There are a variety of options for providing analgesia to veterinary patients in the peri-anesthetic period. In the past, pain relief has not always been a priority in the veterinary field as some practitioners have not always believed that animals were capable of feeling pain. Today, pain management strategies in veterinary patients have improved drastically and the chance of decreasing patient suffering is greatly improved by providing appropriate analgesia.

Pre-emptive analgesia

Pre-emptive analgesia is the provision of an analgesic drug prior to the induction of a painful stimulus. Generally, if analgesia is provided before pain occurs it is easier to control. This occurs because administration of an analgesic before pain occurs prevents the occurrence of central sensitization, also called "wind-up". In surgical patients it is possible to administer an analgesic ahead of surgery to help achieve this, but it is not possible in every situation. Patients that come in after being hit by car, with intervertebral disk disease, fractures, or osteoarthritis cannot receive true pre-emptive analgesia. However, if these patients receive appropriate analgesics in a timely manner, the occurrence of central sensitization can be minimized as can the development of chronic pain states.

Multi-modal analgesia

When different classes of drugs are combined, they can have a synergistic effect. This means that the drugs when given together are more effective than any one drug administered alone. This technique reduces the total amount of each drug that is needed and therefore reduces the incidence of side effects from administering larger amounts of individual drugs. Combining different drug classes to provide analgesia also offers the advantage of being able to inhibit pain sensation at different points along the pain pathway.

     • NSAIDS- prevent pain and inflammation by interfering with prostaglandin synthesis (meloxicam, carprofen)

     • Opioids- bind to opioid receptors in the brain and dorsal horn of the spinal cord to produce sedation and analgesia (butorphanol, morphine, oxymorphone, hydromorphone, fentanyl, buprenorphine)

     • Tramadol- Centrally acting analgesic for treatment of moderate to severe pain. Acts as a weak agonist at mu opioid receptor. Acts to increase serotonin and norepinephrine.

     • Methadone- binds to the mu opioid receptor but also acts as a weak NMDA receptor antagonist

     • Gabapentin- helpful for management of intractable pain→ chronic pain, pain from nerve injury or extensive trauma.

     • Alpha 2 Agonists- provide sedation, muscle relaxation as well as analgesia by binding to receptors in the CNS. They also bind to other alpha 2 receptors throughout the body to exert their other effects (xylazine, medetomidine, dexmedetomidine).

     • Ketamine- should not be used alone for analgesia but excellent as an adjunctive analgesic at sub-anesthetic doses. Acts as an NMDA receptor antagonist.

     •Local Anesthetics- cause a reversible depression of nerve impulses by blocking voltage gated sodium channels→ loss of sensation as well as motor control (lidocaine, bupivicaine).

Epidural analgesia

Epidural analgesia (or anesthesia) is provided by injecting an analgesic drug into the space around the spinal cord. The drug will then diffuse out to reach the spinal nerves and produce analgesia. Epidurals are most commonly performed for surgeries that involve the rear limbs such as cranial cruciate ligament repair, rear limb amputations, and total hip replacements. Epidurals can also be used for abdominal surgeries such as cesarean sections and splenectomies. Epidural administration should be avoided in patients that are coagulopathic, have generalized sepsis, have a skin infection or pathology at the level of the L/S junction, or patient's with neurologic dysfunction. The main advantage of providing epidural analgesia is that the amount of inhalant and other analgesics needed to maintain a surgical plane of anesthesia can generally be greatly reduced, and post-operative analgesia is achieved with minimal sedation and other systemic effects.

Brachial plexus block

A brachial plexus block can be performed for surgeries distal to the elbow joint. Bupivicaine is most commonly used for this block due to its length of blockade (up to 6 hours), compared to lidocaine (1-2 hours). Bupivicaine has an onset time of about15-30 minutes. Drug doses should always be carefully calculated and the dose of bupivicaine should not exceed 1.5-2.0 mg/kg. If the volume is very small, the drug can be diluted with saline to provide an adequate volume for administration. The brachial plexus block is not difficult to perform but care must be taken to avoid IV administration of drug as well as penetration of the thoracic wall.

Intercostal Block

An intercostal local block can be performed for chest tubes, lateral thoracotomy, and rib fractures. Lidocaine or bupivicaine can be used but bupivicaine is preferred due to it longer duration. A small amount of local anesthetic, calculated based on patient weight, is injected caudally to the rib in order to avoid the nerves and blood vessels that run along the cranial aspect. About 2-3 ribs spaces cranially and caudally to the affected area should be blocked due to overlap of nerve supply.

Intra-pleural Analgesia

The simplest way to proved analgesia to a patient with a chest tube in place is to inject local anesthetic directly into the chest tube to provide intra-pleural analgesia. Bupivicane (1.5 mg/kg) can be instilled into the chest tube and flushed in with 5-6 ml sterile saline. The patient can then be placed with the affected side down so that the local anesthetic can contact nerves of the parietal pleura.

Radian/ Ulnar/ Median Nerve Block

Depending on the preference of the anesthetist, specific nerves can be blocked or a ring block can be performed to provide analgesia for declaws, toe amputations, or lacerations involving the foot. The injection site should be clipped and prepped as usual for surgery and then local anesthetic can be deposited at the site of nerves at the level of the carpus. Alternatively, local anesthetic can be deposited along the dorsal and palmar surfaces of the limb at the level of the carpus as a line or ring block.

Topical analgesics

When most local anesthetics are used topically, they must be applies to mucous membranes in order to me effective. There are some preparations of local anesthetics that allow the drug to permeate intact skin. EMLA cream is a eutectic mixture of local anesthetics (lidocaine and prilocaine) that can be used on intact skin to provide loss of sensation. This can be useful for IV catheter placement in sensitive animals or to decrease sensation for arterial catheter placement or arterial blood gas sampling. One disadvantage to the use of EMLA cream is that it requires 30-60 minutes to exert its effect.

"Wound soaker" catheters (Diffusion catheters)

These catheters work by infiltrating a surgical site or a wound with local anesthetic to provide analgesia. The diffusion catheter is a polypropylene catheter with many tiny perforations along its length. The catheter is sutured into the surgical site during closure and local anesthetic can be infused periodically or as a constant rate infusion to decrease or eliminate pain after surgery. These catheters can be very useful in painful surgeries such as limb amputation, large mass removal, mastectomy, thoracotomy, and a variety of other procedures. Lidocaine can be used as a CRI through the catheter at 2 mg/kg/hr. Alternatively, bupivicaine (1.5 mg/kg) can be administered as a bolus through the diffusion catheter every 6 hours.

Constant rate infusions (CRI)

The main advantage of using a CRI in the peri-anesthetic period is that it avoids the highs and lows that can occur from intermittent administration of analgesic drugs. The main disadvantage is that the CRI usually requires the use of a fluid or syringe pump to ensure accurate dosing.

     • Fentanyl CRI→ intra-op 0.1-0.7 ug/kg/minute, post-op 1-5 ug/kg/hr

     • Hydromorphone CRI→ 0.025 mg/kg/hr

     • Ketamine CRI→ 0.1-0.6 mg/kg/hr

     • Medetomidine CRI→ 1-3 ug/kg/hr

CRI calculation

Calculate CRI as part of maintenance fluids (60 ml/kg/day). Ex.- Fentanyl CRI at 5 ug/kg/hr:

     • Calc. fluid rate- 60 ml/kg/day x weight (kg). Divide by 24 hrs = mls/hr

     • Determine duration of infusion. Assume 500 ml bag used.

     • 500 ml/ (ml/hr) = hours of infusion

     • Determine how much analgesic to add.

     • 5 ug/kg/hr (of fentanyl) x weight in kgs x hrs of infusion = ug of fentanyl/ (50 ug/ml, concentration of fentanyl) = mls of fentanyl to add to bag.

MLK- Morphine/ lidocaine/ ketamine CRI

MLK is a multi-modal analgesic technique that can be used during surgery to provide analgesia as well as decrease inhalant requirements. Pre-determined amounts of each drug are added to a bag of regular replacement fluids to be used for maintenance and run at a rate of 10 ml/kg/hr (standard surgical maintenance rate). The advantages of this CRI over others is that it is simple to make up since every bag contains the same amount of drug, the fluid rate calculation is the same for each patient, and it is safe to administer without the use of a fluid pump.

     • Add to 500ml LRS

        o Morphine 12 mg (works out to 0.24 mg/kg/hr)

        o Lidocaine 150 mg (works out to 50 ug/kg/min)

        o Ketamine 30 mg (works out to 0.6 mg/kg/hr)

MLK can also be modified to suit the needs of the patient. LK (lidocaine, ketamine) can be used in the same manner if no opioid is desired. Lidocaine and morphine tend to be omitted with feline patients due to sensitivity to these drugs. Just a ketamine CRI can be administered in the same manner. This is a very versatile and effective analgesic strategy for a variety of patients.

Transdermal patches

Transdermal patches are another alternative for pain management in the peri-anesthetic period. They are easy to apply, generally require little specialized monitoring, and eliminate the need for bolus dosing or the need for a pump for CRI administration. The main disadvantage with the use of transdermal patches is that uptake of drug can be variable between patients and it often takes hours for the drug to reach therapeutic levels. The most commonly used analgesic patch is the fentanyl patch. If applied before surgery it is important to remember that it can take 12 hours or longer to reach therapeutic levels, so additional analgesics should be administered as needed.

References

Tranquilli et al. Pain Management for the Small Animal Practictioner. 2nd edition. Teton New Media, 2004.

Carrol, Gwendolyn DVM, DACVA. Small Animal Pain Management. AAHA Press, 1998.

Blaze, Glowaski. Veterinary Anesthesia Drug Quick Reference. Saunders, 2004.