Veterinary Analgesia ==================== þ Behaviour signs of pain - In the presence of predators (e.g., vets) dogs do not show behavioral signs of pain, even if they show them in the absence of predators. (NCSU study presented at NDMS 2006) þ Stress Response to Pain - Increased ACTH and cortisol, catecholamines - Decreased GI motility - Retention of sodium, water - Decreased ventilation, oxygentation - General catabolism, lipolysis - Results in decreased healing þ PRN Pain Medication vs. Continuous - Study stopped by IRB before completion - Study of human NICU patients - Those with PRN pain medication (vs. continuous) had a much higher level of sepsis. þ Central Neuronal Excitability: Windup - Intense noxious stimulus (broken bone > Dorsal horn > Brain > perceived as pain - Innocuous stimulus (petting dog) > Dorsal horn > Brain > perceived as pleasant - Constant bombardment of dorsal horn > Turns on NMDA receptors in dorsal horn > magnifies output from dorsal horn to brain > much higher pain perception - Allodynia: innocuous stimulus may elicit pain as a result (petting dog is painful): may become chronic - (Sounds like human Reflex Sympathetic Dystrophy) þ Opiods in dogs and cats and other animals - unless head injury or anesthetized, large doses of narcotics don't cause hypoventilation; may cause bradycardia - treating pain in advance with narcotics a good idea in dogs, etc.; wide safety margin compared to humans - remifentanyl: no liver metabolism or kidney excretion, used for head trauma, can turn off for neuro exam and is reliably gone 10 minutes later. - mu agonists most effective - Demerol decreases cardiac contractility in animals, short duration, toxic metaoblites in dogs and likely cats, not a good drug. - Opiods are antiemetics centrally, but when acting peripherally causes vomiting. - 80% incidence of vomiting from morphine in dogs, preventable with pretreatment with fentanyl. Not found with methadone. - Morphine causes histamine release in dogs, causes hypotension. - Dose-response curves for opiods: potency relates only to dose, not to how strong or pain control it provides (for mu agonists). - Butorphanol has different dose-response, won't treat pain as well; once you give a bigger dose, it can actually increase pain, and should not be in VMAT cache. - Morphine is cheap. - Morphine doses in dogs: 30 mg for 30 kg dog is OK. Really! 0.2-1.0 mg/kg SQ!, IM; 0.1-0.2 mg/kg/hr - Never give morphine to cats; have to metabolize to M6G to get analgesia, morphine itself causes dysphoria; cats don't make M6G hardly at all. - Hydromorphone (Dilaudid): half as potent as oxymorphone. No histamine release. Less vomiting. Works well in cats. Horse doses uncertain. - Morphine makes well horses (cats and dogs too) hyperactive and get an ileus: doesn't happen when they're in pain. - Oxymorphone (Numorphan, Opana): twice as potent as hydromorphone (Dilaudid) - Fentanyl patches don't work well in animals, come loose and get subtherapeutic levels. May get diverted for abuse. Onset about 24 hours for dogs, a bit less for cats. Hairdryer increases absorption. - Buprenorphine (Buprenex): mu agonist/antagonists. Lasts 6-12 hours in cats and dogs; better for cats than dogs; not as strong as pure narcotics; in cats (not dogs) can give intrabuccal. Binds receptor, can't get off. Takes 1200x the normal dose of Narcan to reverse, or need to give stronger mu agonists. Will reverse Morphine as has high affinity for receptors. - Butorphanol (Stadol) Kappa agonist.: analgesia only lasts about 45 minutes in dogs, though sedation can last for hours. Lasts 4 hours in cats. Expensive. No place in vet medicine. - Nalbuphine (Nubain) kappa agonist: Nonscheduled. Lasts 1-2 hours. Better than butorphanol. Cheap. $.60/mL. Usable for many species. 0.5 mg/kg IV, 1 mh/kg IM for dogs. Half the dog dose for cats. - Morphine OK for lactating dogs þ Peripheral Opiod Administration þ Local/systemic alpha-2 agonists - Used for sedation Visceral analgesia, sedation; synergystic with opiods Significant cardiac effects Increase parasympathetic, decrease sympathetic Hypertension from vasoconstriction, then hypotension from cardiodepressor effects. Pulmonary effects, especially at high altitude and in cats and sheep: pulmonary edema. No effect on ventilation, but decreases oxygenation. Reversible with atipamezole Zylaxine Medetomadine Romifadeine þ TCAs þ NMDA blockers (receptor anagonists) - Ketamine: NMDA receptor antagonist, prevents windup, no direct analgesia, no longer used for kats þ Lidocaine/Bupivicaine - 2 mg/kg max for cats and dogs þ NSAIDs - Etodolac (Lodine), Carprofen, Deracoxib, Meloxicam (Mobic), Tepoxalin, Firocoxib commonly used - Human NSAIDS like ASAP, Piroxicam, Pheylbutazone, ketoprofen, ibuprofen BAD for dogs - acetaminophen can be mixed with NSAIDs in dogs; used to be thought bad in dogs but those were 40 mg/kg longterm, but 10-15 mg/kg for less than 5 days it's OK. Tylenol is COX-3 inhibitor (in CNS) - NO acetaminophen for cats!! - cats and dogs have no problems with CAD. - cats and dogs get ulcers from NSAIDs - cats don't glucuronidate very well, no repeat doses of ASA (half a baby aspirin, or a dose or Remidyl or meloxicam 0.3 mg/kg IM, will last for 3 days) þ Antiepileptics þ Benzodiazepines - may alter perception of pain þ How to Assess Pain in Dogs and Cats - Generally show no signs of pain with mild to moderate pain - Vocalization: varies by breed, species (some northern breeds whine all the time): ureliable - lack of vocalization doesn't mean no pain - non-weightbearing or "toe-touching" with only partial weight-bearing - response to palpation - need to distinguish from dysphoria (palpation of painful area helps) - posture and facial expression: tail between legs, arch back, twist body with abdominal pain, don't move or constantly moving, agressive from the pain, claw and bit selves, - cats purr when happy or when hurt or afraid, not reliable - many pain scoring systems, Glasgow one for dogs is best validated - solitary cats become social and vice versa - cats stop grooming selves - cats mutilate selves when in pain - horses restles, poor eating, lowered head, head-pressing, tremors, sweats, increased HR, RR, groan, grind teeth, walk in circles, roll and thrash, - DO anthropomorphize! þ Drugs for Dogs - tramadol one tablet per 30p lbs. Dosage not very important, can't overdose. - NSAIDS - narcotics - acetaminophen þ Drugs for Cats - Tramadol half a pill twice a day, no side effects other than sedation; but very better; seems to work well in marshmallows (who would know? accident at CSU vet clinic)