Pain Management

Philosophy of treating pain 

• Assessing pain is subjective

• species differences in pain expression

• historical approach to animal pain

• anthropomorphism is OK!

• pain is inadequately treated in both human and veterinary medicine

• IF IN DOUBT, TREAT FOR PAIN!

• It may also be a source of income for practitioners! 

 

Physiology

Types of pain

• Somatic pain

• Visceral Pain

• Neuropathic Pain

 

Peripheral nociceptors

• nociceptor = a receptor for pain

• in peripheral tissue, detect potentially damaging stimuli

• activated or sensitized by endogenous substances released in inflammation (prostaglandins, bradykinin, serotonin, histamine, etc…)

• transmit information to spinal cord via A (sharp pain, rapid transmission) or C fibers (dull, burning pain, slow transmission)

 

Central nervous system

• Nociceptor fibers terminate 1° in the dorsal horn of the spinal cord

• Synapse at this level, relay information via several spinal tracts to the level of the thalamus

• Synapses occur at the level of the thalamus, information then relayed diffusely to the cortex

• Pain perception occurs when the information reaches the cortex

• A large number of different neurotransmitters are involved, including endogenous opioids, substance P, norepinephrine, and many other neuropeptides

• The information can be modulated at each step in the transmission process

 

Concepts of pain management

• Preemptive analgesia: is a concept based on experiments, which suggest that analgesic intervention before a noxious stimulation reduces or eliminates subsequent pain. If small doses of anesthetic (local or opioid) are administered before the stimulus, hyperexcitability may be prevented.

• Balanced analgesia: using more than one analgesic (with less dose and therefore less side effects) agent to prevent pain.

 

Perioperative Pain Management

Nursing care

• Good, basic nursing care should be the first step in pain management

• Clean, dry, appropriate temperature environment

• Adequate rest, minimal external stimuli

• Minimize environmental stress

 

When to treat pain

• Surgical procedures:

  • Orthopedic procedures, multiple fractures

  • Thoracotomy, sternotomy

  • Ocular procedures

  • Total ear ablation

  • Major mastectomies

  • Major abdominal procedures

  • Trauma 

 

• Medical conditions

  • Spinal disc disease

  • Meningitis

  • Some cancers

  • Pancreatitis

• Breed/species characteristics-stoic vs excited animals

• Physical signs/physiological effects of pain:

  • Guarding/splinting affected area

  • Licking, chewing, scratching affected area

  • Restlessness, inability to sleep

  • Vocalization, salivation

  • Withdrawal, apathy

  • Depressed appetite

  • Anxious expression

  • Tachypnea

  • Ventilatory impairment, atelectasis

  • Tachycardia

  • Arrhythmias (PACs, PVCs)

  • Hyperglycemia

  • Hypercortisolemia

 

Currently Available Analgesic Drugs

Four classes: opioids, local anesthetics, NSAIDs, and alpha-2 agents.

Opioids: 

• Analgesic and euphoric effects recognized for over 5,000 years

• Morphine still the "gold standard"

• Mechanism of action:

  • Bind to spinal and cerebral opioid receptors: µ, k , d , s , 

    • µ: supraspinal analgesia, respiratory depression, bradycardia, sedation

    • : spinal analgesia, sedation

    • : modulate µ receptor activity

    • : excitation, respiratory stimulation, tachycardia

 

• Side effects of opioids:

  • Dose dependent respiratory depression

  • Increased intracranial pressure

  • Bradyarrhythmias

  • Other vagal effects

  • Emesis, nausea

  • Sedation/excitation

• Abuse potential

• Agonist vs. partial agonist vs. antagonist

 

Morphine

• µ receptor agonist

• Profound analgesia, sedation (most species)

• Dose dependent respiratory depression

• Bradyarrhythmias

• Vomiting, defecation common initially following premedication

• Constipation with chronic use

• Histamine release (IV)

• Minimal, if any, direct depressant effects on myocardium

• » 2-4 hour duration of action

• Cheap

• Class II drug

 

Oxymorphone

• µ agonist

• Very similar to morphine

• 10x as potent as morphine

• Less tendency to vomiting and better sedation than morphine

• Does not cause histamine release

• Slightly shorter duration of action than morphine

• More expensive than morphine

• Class II drug

 

Meperidine

• µ agonist

• Less sedation than morphine

• Vomiting, defecation not as common as morphine

• Does cause histamine release

• Only opioid with direct myocardial depressant effect

• Tends to increase heart rate due to "mild atropine-like effects"

• Short duration of action, » 1-2 hours

• Class II drug

 

Fentanyl

• µ agonist

• Was primarily used in combination with droperidol (Innovar-Vet)

• Now most commonly used either as a constant rate infusion or as a transdermal patch 

• 75-125x as potent as morphine

• Does not release histamine

• Minimal cardiovascular effects, except bradyarrhythmias

• Respiratory depressant

• Short duration of action, » 45-60 minutes

• Class II drug

 

Butorphanol

• partial agonist, µ antagonist

• Initially used as an antitussive in vet. med.

• 5-10x as potent as morphine

• Minimal sedative effects when used alone

• Minimal cardiopulmonary effects

• Respiratory depression reaches a "plateau"

• » 3-4 hour duration of action

• Class 4 drug

 

Nalbuphine

• partial agonist, µ antagonist

• Similar to butorphanol

• » 1-2 hour duration of action

• We use it primarily to reverse the respiratory depression and sedation caused by µ agonists postoperatively

• Not a classified drug

 

Pentazocine

• partial agonist, µ antagonist, has some activity at higher doses

• First partial agonist used extensively

• Been largely replaced by newer drugs

• » 1-2 hour duration of action

• Class IV drug

 

Buprenorphine

• a µ partial agonist, antagonist

• High affinity for the µ receptor site; results in a long duration of action, » 6-8 hours

• Minimal cardiopulmonary effects

• Rare reports of profound respiratory depression in humans that is difficult to reverse

• Class V drug

• Relatively new in veterinary medicine

 

Routes of administration

• Generally administer IV or IM

• Usually undergo extensive first pass hepatic metabolism if given orally

• Epidural: a relatively new route of administration is epidurally; use lower dose, much longer duration of effect (» 12 - 24 hours!), and much lower incidence of side effects; this technique has become very popular in both human & veterinary medicine.

• Transdermal: newest technique that is finding use in veterinary medicine. Apply transdermal fentanyl patches (come in several sizes, similar in concept to nicotine patches) to clipped portion of skin, cover with bandage. Over several hours, plasma fentanyl concentration rises, plateaus, and then maintains effective concentration for 3-4 days. Useful for long term therapy, especially for elective orthopedic procedures.

• CRI (constant rate infusions) - relatively new methodology in our clinic; administer either morphine or fentanyl by constant rate infusions, both intraoperatively and postoperatively 

 

Neuroleptanalgesia

• Useful when increased chemical restraint or sedation, in addition to analgesia, is required

• A large number of combinations possible

 

Nonsteroidal antiinflammatory drugs (NSAID's)

• Large class of drugs

• Includes aspirin, flunixin meglumine, carprofen, etodolac

• Act by inhibiting the production of the products of inflammation, eg prostaglandins

• Produce analgesia by reducing the sensitization of nociceptors

• Generally effective against mild to moderate pain

• Usually share similar side effects, including possible nephrotoxicity and gastrointestinal irritation or ulceration

 

Alpha-2 and dissociatives

• Xylazine, medetomidine, and detomidine are sedative-analgesic agents.

• Useful for providing sedation, allowing the patient to rest

• Ketamine and tiletamine also contains both sometic and viscera analgesia

• Recent reports indicate the very low doses of ketamine given by CRI may be used to provide analgesia 

• 2-adrenergic agonists: adverse cardiopulmonary effects limit their usefulness in small animal practice; more useful in large animals, especially in equine colics

 

Local anesthetics

• Can be used in pain management

• Act by blocking transmission of pain information through the peripheral nerves

• Variety of techniques can be used, eg intercostal nerve block following lateral thoracotomy

• Major disadvantage is potential for blockade of motor function

 

Transcutaneous electrical nerve stimulation (TENS) and acupuncture

• TENS appears to function similarly to acupuncture

• Provide pain relief by stimulating endogenous pain suppression system (eg release of endogenous opioid peptides)

• It does work, and has a place in pain management. However, it is not widely accepted yet. 

 

Species characteristics:

Canine

• Opioids: work well, widely documented in the literature

• NSAID's: work well, widely used, some toxicity problems with some drugs

• Local anesthesia: techniques established, may be an option

• Epidural opioids ± local anesthetics

• Transdermal fentanyl

 

Feline

• Opioids: produce excitement at high doses ("morphine mania"); however, these drugs can work well if use appropriate dose; if excitement occurs, a tranquilizer, such as acepromazine, will calm the animal

• NSAID's (acetaminophen, aspirin): toxicity is a major problem

• Local anesthesia: more sensitive than dogs to get overdose

• Epidural opioids ± local anesthetics

• Transdermal fentanyl

 

Equine

• Opioids: may cause excitement in the pain free horse, can be prevented with concurrent administration of a tranquilizer

• 2-adrenergic agonists: well tolerated and accepted

• NSAID's: well tolerated and accepted

• Local anesthesia, neurectomies: well tolerated and accepted

• Epidural 2-adrenergic agonists - perineal analgesia

 

Food animal species

• ????: little information on these species

• Opioids: partial agonists appear to work well

• NSAID's: seemly work well

• Local anesthesia: is a vital option

 

Avian

• Butorphanol has been found to be a useful analgesic for birds.