Pain recognition and management (Proceedings)

What is pain? Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. All potential and actual tissue damage in animals should be considered to cause pain. Pain can be experienced with or without accompanying signs of stress (e.g., tachycardia, hypertension). The first step in treating pain is to recognize the signs and symptoms.

Pain commonly arises from activation of a physiological process that results in nociception. This process involves receptors and neural pathways that carry a noxious stimulus from transduction, transmission, modulation and perception.

Physiologically, pain is divided into two classifications, nociceptive or neuropathic. Nociceptive pain is further divided in tow two categories, somatic and visceral. Neuropathic pain is either central or peripheral, depending on the origin of the stimulus (i.e. direct damage to peripheral nerves or the spinal cord).

Peripheral nociceptors respond to noxious stimuli, the impulse is altered from physical energy to chemical (transduction), and is transmitted along peripheral neurons (transmission) to the dorsal horn of the spinal cord. Here the neurons synapse with neurons in the spinothalamic tract (modulation), which carry the impulse to the brain resulting in conscious recognition (perception) of pain.

Somatic pain, which in humans, has been described a localized, sharp, aching, or throbbing, originates from skin, connective tissue, muscle, joint and bone.

Visceral pain, described as dull or hard to localize, originate from receptors in the heart, lungs, kidneys, liver, gastrointestinal tract, uterus or bladder.

If pain is left untreated or under treated, animals can become depressed, lethargic, withdrawn, and eventually immobile. Early recognition and aggressive treatment is necessary to prevent these experiences. Patients being treated for pain during the perioperative period return to normal function sooner that those individuals who are left untreated.

Recognition of pain in veterinary patients is dependent on behavioral and physiologic responses. Behavioral signs of pain can very widely among patients. Some individuals and certain breeds are known for their stoic behavior while other patients seem to more readily present behavioral changes.

Behavioral signs can include:

     1. Vocalization

     2. Silence

     3. Guarding of the affected area/surgical site

     4. Changes in facial expression

     5. Self-mutilation

     6. Muscle rigidity or weakness

     7. Restlessness

     8. Reluctance to move

     9. Personality changes

     10. Dullness- slow to respond to handling

     11. Loss or decrease in food and/or water intake

     12. Failure to groom

Physiological responses may include:

     1. Cardiopulmonary changes:

          a. increased heart rate

          b. b increased blood pressure

          c. cardiac dysrrhythmias

          d. increased respiratory rate

          e. shallow breathing

          f. pale mucous membranes due to catecholamine-mediated vasoconstriction

     2. Dilated pupils

     3. Excessive salivation

     4. Hyperglycemia

Physiological response to and consequences of pain are identical to those attributed to shock (sympathetic stimulation, atelectesis,, hypoxemia, vasospasm, localized tissue ischemia, decreased gut motility, increased myocardial oxygen consumption, increased metabolic rate, increased insulin resistance, sodium retention, decreased urine output, reduced socialization and increased anxiety.

Mild pain usually presents as a nuisance type pain that is easily tolerated and manipulation of the affected part appears to be an annoyance to the patient, but does not seem to be painful when not being stimulated.

Moderate pain is thought to be present when a patient has a disease or has undergone a surgical procedure that is known to cause pain in human patients.

Severe pain is such that a patient howls cries or screams unprovoked. Self mutilation may be seen, as may be violent thrashing around in the cage.

It may not be possible to totally eliminate all pain but rather the goal should be to reduce or eliminate the pathological pain.

Nursing care can greatly improve the pain tolerance of most of our patients.

Remember to clean all blood from the surgical area and if warranted, express the bladder of post-operative patients. Provide external heat sources to prevent hypothermia or to aid in active rewarming. This can be done by utilizing forced warm air blankets, circulating water pads or in-line fluid warmers if the patient is to be on IV fluids. Warm, dry bedding to cushion and support the patient will bring some relief. If possible, during the immediate re-emergency phase of anesthesia, a quiet room with dimmed lighting can reduce anxiety. The human touch is greatly reassuring to the majority of our patients.

It is important that all members of the health care team evaluation pain response in the same manner. It is recommended that each care facility adopt a pain scoring method. There are many scoring systems available. The following is an example of the pain/analgesia scoring method used in the Texas A&M Small Animal Clinic.

While general anesthesia prevents the conscious perception of pain, it does not stop the nociception of pain during surgery. Pre-emptive analgesia reduces the input from the peripheral nerves during anesthesia and may reduce the amount of drug required to provide patient comfort in the post-operative area. Additionally, pre-emptive analgesics have been shown to be advantageous in being able to decrease the amount of inhalant anesthetic necessary to achieve surgical anesthesia.

Pharmacological intervention can be delivered in multiple routes of administration. As previously stated one can be pre-emptive and utilize local or regional blocks inaddition to systemic administration. This multi-modal approach allows for a more balanced analgesia. In addition to analgesics, phenothiazine tranquilizers, benzodiazepine tranquilizers and alpha2-agonists can be used in combination to provide relief.

Phenothiazine tranquilizers have no analgesic properties but will enhance the effect of an opioid analgesic.

Benzodiazepine tranquilizers may have some limited analgesic properties and will also enhance the analgesic affects of an opioid.

Alpha2-agonists have analgesic as well as sedative properties but must be used cautiously when combined with opioids due to the cardiopulmonary effects of the alpha-2 agonist.

Local blocks and/or regional blocks are more effective if used prior to surgical stimulation. The local anesthesia decreases the intensity of postoperative pain. It is not always possible to provide local anesthetics prior to surgical stimulation but you can incorporate their use intra-operatively.

     1. infiltration of a wound

          a. nerve root stumps

          b. wound edges

          c. ear ablations

     2. line blocks

     3. intercostals neural blocks

     4. intrapleural analgesia

     5. brachial plexus blocks

     6. epidurals

Lidocaine has a rapid onset (5-10 minutes) and is of short duration (1-2 hours). The total dose should not exceed 4-7 mg/kg. Toxic effects can be seen with a dose of 11mg/kg. These side effects include: restlessness, muscle tremors, seizures, cardiopulmonary depression, coma and death.

Bupivicaine has a longer onset of action (20-30 minutes) and a longer duration of action (4-6 hours). The dose should not exceed 2.2 mg/kg. Toxic side effects can be seen with a dose of 4-5 mg/kg and are similar to those of Lidocaine.

Epidurals should not be administered to patients with pre-existing neurological dysfunction, coagulopathies, sepsis or affected skin areas involved with the epidural site.

Epidural administration of local anesthetics, but not opioids, is contraindicated in those patients suffering from hypovolemia. Local anesthetic administration produces no or very mild sedation, minimal nausea and vomiting and occasionally urinary retention.

Local anesthetic administration may produce a decreased heart rate, cardiac output and hypotension, but does not appear to impair the respiratory system. Opioid administration may cause marked sedation, nausea, vomiting, urinary retention and or pruritis. At appropriate doses, it may also cause minimal change in heart rate, cardiac output, or blood pressure but may cause early and late respiratory depression. The respiratory depression can be antagonized. Total volume to be injected into the epidural space should not exceed 6 mls in the canine or 1.5mls in the feline patient.

Opioids are frequently given pre-operatively to assist in sedation and analgesia for IV catheter placement with the added benefit that giving pre-emptive analgesics can decrease the amount of analgesics required post-operatively as well as potentially decreasing the intra-operative vaporizer setting.

Chronic pain management for the post-operative patient

Use of nonsteroid anti-inflammatory (NSAIDS) drugs should included in the post-operative period provided that the patient is adequately hydrated and remains normotensive during the anesthetic event to ensure protection of the most commonly affected body systems. The most commonly associated adverse reactions seen with the use of NSAIDS are related to the gastrointestinal tract, renal system and liver.

With the advent of multi-modal analgesia, pre-emptive use of NSAIDS will help prevent central nervous system "windup". Any surgical procedure performed will result in tissue inflammation. The following is by no means a complete listing of NSAIDS available for use.

The addition of gabapentin to the arsenal of medications available for neurogenic pain is another tool that is becoming more widely used. Gabapentin has been used in human medicine to treat a variety of painful conditions ranging from diabetic neuropathy, central pain, and inflammatory pain. It has been used in veterinary medicine in both canine and feline patients and appears to be mediated via calcium ion channels. This medication should not be stopped abruptly but weaned from the patient as seizure activity has been reported in human medicine as well as a rebound phenomenon.

Physical therapy and rehabilitation is becoming more recognized and more widely available to the veterinary patient.

     • Cryotherapy

     • Moist heat

     • Passive range of motion

     • Active range of motion

     • Water treadmill

     • Massage

Alternative medicine choices would also need to include the addition of eastern medicine with acupressure and acupuncture. In the United States, only qualified veterinarians are allowed to perform acupuncture on the veterinary patient.

Life Style Management

Diet and nutritional supplements should be considered for post-operative patients for adjunctive therapy. There are multiple choices currently available to assist with patients facing orthopedic rehabilitation as well as degenerative joint disease. It is vital that all patients be guided to maintain a normal body weight to relieve bone and joint pain as much as possible. Client counseling is often difficult as they tend to equate feeding as giving love.

     • Owner and pet behavior modification!

     • Annual geriatric screenings

     • Weight reduction/healthy maintenance

     • Addition of nutritional supplements- be proactive and start early in cases of degenerative joint disease

     • Gentle exercise- daily walks for owners and pets