Animal medicine chapter 8

Pain Definition

• Pain is complex, involving sensory and emotional experiences.
• Pain serves as a protective mechanism against tissue injury.
• Pain can influence behavior, causing avoidance and changes in species-specific activities.

Pain Physiology

• Nociception refers to the nervous system's detection of potential or actual tissue injury.
• Physiologic pain is adaptive, associated with minor tissue injury and referred to as "ouch" pain.
• Pathologic pain arises from tissue injury, categorized as acute or chronic.

Pathologic Pain Classification

• Mechanism: Inflammatory, neuropathic, cancer, idiopathic.
• Origin: Visceral (internal organs) or somatic (superficial or deep tissues).
• Severity: None, mild, moderate, severe.

Nociception: Pain Pathway

• Transduction: Transformation of stimuli into electrical signals.
• Transmission: Sensory impulses travel to the spinal cord.
• Modulation: Impulses are amplified or suppressed.
• Perception: Impulses reach the brain for processing and recognition.
• Different receptors are involved at each step of the pain pathway, allowing targeted drug action.
• Multimodal therapy targets multiple receptors and mechanisms for optimal pain management.

Consequences of Unmanaged Pain

• Catabolism and wasting.
• Immune system suppression.
• Inflammation and delayed wound healing.
• Increased anesthetic risk and higher anesthetic doses.
• Patient suffering.

Primary Hyperalgesia

• Peripheral hypersensitivity due to tissue damage and nerve stimulation.
• The area near the injury becomes painful to non-noxious stimuli.

Secondary Hyperalgesia

• Central nervous system hypersensitivity or "windup."
• Hypersensitivity extends beyond the injury site.
• Continuous spinal cord neuron stimulation leads to hypersensitivity.
• Neurons become hyperexcitable and sensitive.

Physiologic Changes Due to Pain

• Hypertension.
• Tachycardia, tacharrhythmia.
• Peripheral vasoconstriction (pale mucous membranes).
• Tachypnea.
• Shallow breathing.
• Exaggerated abdominal component in breathing.
• Panting in dogs.
• Open-mouthed breathing in cats.
• Mydriasis (pupils dilated).

Signs of Pain in Animals

• Pain triggers a stress response, leading to "fight-or-flight" physiological changes.
• Behavioral responses vary based on species, age, breed, temperament, and pain duration/severity.
• Animals may vocalize, change gait, avoid lying down, or exhibit unusual facial expressions.

Preemptive Analgesia

• Administration of pain medication before pain onset.
• This can reduce overall analgesic requirements and duration of treatment.
• Helps prevent "windup" (central hypersensitivity).

Pain Assessment Tools

• Simple descriptive scales: Qualitative description of pain levels.
• Visual analogue scales: Pointing to a line to indicate pain intensity.
• Numeric rating scales: Assigning numbers to pain levels.
• Validated scales: Standardized measures with proven accuracy.
• Feline Grimace Scale: Visual assessment of facial expressions for pain detection.

Assessing Response to Therapy

• Frequent monitoring is crucial to identify pain reduction.
• Assess changes in behavior (appetite, grooming, posture, interaction).
• Monitor pain scores to gauge treatment effectiveness.

Analgesic Drug Administration Routes

• Intravenous (IV).
• Intramuscular (IM).
• Subcutaneous (SQ).
• Oral.
• Epidural.
• Intraarticular.
• Transdermal patch.

Opioid Analgesics

• Opioids vary in potency, duration, and potential side effects.
• Commonly used for moderate-to-severe pain.
• Can be used as premedications, post-surgical analgesia, or sedation.
• Side effects: gastrointestinal (GI) changes (nausea, vomiting, ileus, constipation), respiratory depression, sedation, cardiovascular effects.

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

• Inhibit prostaglandin synthesis, reducing pain and inflammation.
• Adverse effects: GI ulcers, renal toxicity, platelet dysfunction.
• NSAIDs selectively targeting COX-2 have fewer side effects.

Local Anesthetics

• Used to prevent or manage postoperative pain.
• Advantages: complete local anesthesia, low toxicity, rapid onset.
• Disadvantages: short duration of action, potential toxicity with repeated use.

Other Analgesic Agents

• Alpha2-Adrenoceptor Agonists: Limited use in small animals due to side effects (sedation, bradycardia, hypotension).
• Ketamine: Facilitates analgesia but doesn't provide pain relief alone.
• Gabapentin: Treats neuropathic pain and hypersensitivity.
• Amantadine: NMDA receptor antagonist, useful for neuropathic pain.
• Corticosteroids: Powerful anti-inflammatory, but long-term use has adverse effects (immunosuppression, hyperadrenocorticism).
• Tramadol: Non-opioid with mu receptor activity, used for postoperative pain, but should not be given with other serotonin/norepinephrine reuptake inhibitors.
• Tranquilizers: Not analgesics but can help reduce anxiety and potentiate opioids.

Multimodal Therapy

• Using multiple analgesic agents to target different pain pathways.
• Combination therapy can increase effectiveness and reduce individual drug doses.

Home Analgesia

• Fentanyl patches (chronic pain).
• NSAIDs (long-term pain management).
• Oral morphine (sustained release).
• Tramadol (mild-to-moderate pain).
• Butorphanol (mild-to-moderate pain).

Nursing Care

• Providing comfort aids pain management.
• Keep environment clean, dry, and calm.
• Ensure proper bedding and positioning.
• Reduce anxiety with familiar objects.
• Administrate ophthalmic ointment to prevent dry eyes in unconscious patients.
• Reassurance through touch and communication.

Nonpharmacologic Therapies

• Acupuncture.
• Transcutaneous electric nerve stimulation (TENS).
• Massage therapy.
• Heat/Cold therapies.
• Physiotherapy.
• Laser/magnetic therapy.
• Homeopathic/herbal remedies.

The Nature of Pain

• Pain is a complex experience, involving both sensory and emotional components.
• It serves as a warning system, prompting protective actions and avoidance behaviors.
• Pain can modify an animal's natural behaviors.

Physiology of Pain

• Nociception is the nervous system's detection of potential or actual tissue damage.
• Physiologic pain (adaptive) is a short-term response to minor injury, considered "ouch" pain.
• Pathologic pain occurs after tissue injury and can be acute or chronic.

Types of Pain

• Inflammatory pain is caused by inflammation.
• Neuropathic pain arises from nerve damage.
• Cancer pain is associated with cancerous growths.
• Idiopathic pain has an unknown cause.
• Visceral pain originates from internal organs.
• Somatic pain originates from the body's structures, including skin, muscles, bones, and joints.

The Pain Pathway

• Transduction involves the conversion of stimuli into electrical signals (action potentials).
• Transmission refers to the conduction of sensory impulses to the spinal cord.
• Modulation involves the amplification or suppression of these impulses.
• Perception is the processing and recognition of impulses in the brain.

Consequences of Untreated Pain

• Untreated pain leads to catabolism, immune system suppression, delayed wound healing, increased anesthetic risks, and prolonged suffering.

Primary Hyperalgesia

• Also known as peripheral hypersensitivity, resulting from tissue damage and nerve stimulation.
• The area near the injury becomes sensitive to even light touch.

Secondary Hyperalgesia

• Central nervous system hypersensitivity, also called "windup".
• Occurs due to constant stimulation of spinal cord neurons, making them hyperexcitable and sensitive.
• The area of hypersensitivity can be further away from the original injury.

Physiologic Changes Due to Pain

• Pain triggers a stress response, leading to:
  • Hypertension
  • Tachycardia
  • Peripheral vasoconstriction
  • Tachypnea
  • Mydriasis (dilated pupils)
  • Shallow breathing
  • Exaggerated abdominal breathing
  • Panting in dogs
  • Open-mouth breathing in cats

Pain Assessment

• Simple descriptive scales: Use words to describe pain levels.
• Visual analogue scales: Use a line to indicate pain intensity.
• Numeric rating scales: Use numbers to rate pain levels.
• CSU Canine Acute Pain Scale and CSU Feline Acute Pain Scale: Standardized pain scales specific to dogs and cats.
• Feline Grimace Scale: Evaluates facial expressions for pain.
• Chronic Pain Assessment: Assesses long-term pain.

Preemptive Analgesia

• Administering pain medication before pain occurs.
• Can be part of premedication before anesthesia.
• Reduces overall analgesic requirements and prevents windup.

Behavioral Responses to Pain

• Vary depending on species, age, breed, temperament, and the nature of the pain.
• Young animals, cattle, and large dog breeds are often more stoic.
• Cats tend to hide, dogs seek comfort, and herd animals often isolate themselves.

Physical Evidence of Pain

• Gait changes and decreased activity.
• Reluctance to lie down or constant position shifting.
• Vocalization.
• Altered facial expressions and attitude.

Routes of Analgesic Administration

• Intravenous (IV)
• Intramuscular (IM)
• Subcutaneous (SQ)
• Oral (PO)
• Transdermal
• Epidural
• Intraarticular

Opioid Analgesics

• Uses: Moderate to severe pain management
• Adverse Effects: Sedation, respiratory depression, gastrointestinal effects, bradycardia (slow heart rate), mydriasis (dilated pupils), physical addiction in humans.

Opioid Agents

• Morphine: Pure agonist for mu and kappa receptors, may cause excitement in cats and horses.
• Hydromorphone: More potent than morphine, but similar duration.
• Methadone: Synthetic opioid with similar characteristics to hydromorphone, less likely to cause vomiting in cats and dogs.
• Fentanyl: Highly potent, rapid onset with short duration (IV route). Available as a transdermal patch, providing prolonged analgesia.
• Buprenorphine: Partial mu agonist, delayed onset, long duration. Useful against morphine and fentanyl effects.
• Butorphanol: Mixed agonist-antagonist, used for preanesthesia, sedation, and visceral pain.
• Nalbuphine: Kappa agonist, mu antagonist, weaker analgesic, useful for opioid reversal.

Nonsteroidal Anti-inflammatory Drugs (NSAIDs)

• Mechanism of Action: Inhibit prostaglandin synthesis by blocking COX enzymes.
• Uses: Reducing pain and inflammation.
• Adverse Effects: Gastrointestinal ulcers, renal toxicity, liver damage, prolonged bleeding time due to platelet dysfunction.

Local Anesthetics

• Uses: Prevention and treatment of pain (e.g., post-surgical).
• Advantages: Complete anesthesia of the target area, rapid onset, low toxicity.
• Disadvantages: Short duration of action, potential for central nervous system and cardiac toxicity with repeated use.

Alpha2-Adrenoceptor Agonists

• Uses: Sedation, muscle relaxation, analgesia (primarily in horses).
• Adverse Effects: Respiratory depression, vomiting, bradycardia, hypotension, heart block.

Ketamine

• Uses: Adjunct to more potent analgesics , not analgesic by itself.
• Mechanism of Action: Blocks NMDA receptors at spinal cord level.

Gabapentin

• Uses: Neuropathic pain and hypersensitivity, chronic pain unresponsive to NSAIDs.
• Adverse Effects: Drowsiness, toxicity if liquid form contains xylitol.

Amantadine

• Uses: Neuropathic pain, chronic musculoskeletal pain
• Adverse Effects: Agitation, gastrointestinal upset.

Corticosteroids

• Uses: Strong antiinflammatory properties.
• Adverse Effects: Ulcerogenic, immunosuppression with long-term use, hyperadrenocorticism.

Tramadol

• Uses: Non-opioid drug with mu receptor activity, useful for post-surgical pain.
• Adverse Effects: Don't use concurrently with norepinephrine or serotonin reuptake inhibitors.

Tranquilizers

• Not analgesics, but may be used to enhance the effects of opioids in anxious patients.

Multimodal Therapy

• The use of multiple analgesic medications to target different pathways of pain perception.
• Often more effective than single-agent therapy.

Examples of Multimodal Therapy

• Acetaminophen and Codeine: For moderate to severe pain in dogs.
• Fentanyl and Meloxicam: For cats, combining the immediate effects of fentanyl with long-term NSAID action.
• Morphine and an Injectable NSAID (meloxicam or carprofen): For post-surgical analgesia.
• MLK (Morphine, Lidocaine, Ketamine): Used during surgery for pain control.

Home Analgesia

• Fentanyl Patch: Sequential use for chronic pain.
• NSAIDs: For long-term chronic pain management.
• Oral Morphine: Sustained-release tablets for chronic pain.
• Tylenol with Codeine (dogs) and Butorphanol (dogs and cats): For mild to moderate pain.
• Tramadol: For post-surgical and chronic pain.

Nursing Care to Relieve Discomfort

• Maintain cleanliness, dry environment.
• Provide comfortable bedding and a quiet environment.
• Ensure access to urination and defecation.
• Position patients comfortably, possibly requiring rotations.
• Provide familiar toys or blankets for comfort.
• Apply ophthalmic ointment to unconscious patients to prevent corneal drying.
• Offer reassurance through touch and verbal communication.

Nonpharmacologic Therapies

• Used in conjunction with or as an alternative to medication.
• Examples: Acupuncture, transcutaneous electric nerve stimulation (TENS), massage therapy, cold or heat application, physiotherapy, laser/magnetic therapy, homeopathic remedies, herbal remedies.