• One Health
  • Pain Management
  • Oncology
  • Anesthesia
  • Geriatric & Palliative Medicine
  • Ophthalmology
  • Anatomic Pathology
  • Poultry Medicine
  • Infectious Diseases
  • Dermatology
  • Theriogenology
  • Nutrition
  • Animal Welfare
  • Radiology
  • Internal Medicine
  • Small Ruminant
  • Cardiology
  • Dentistry
  • Feline Medicine
  • Soft Tissue Surgery
  • Urology/Nephrology
  • Avian & Exotic
  • Preventive Medicine
  • Anesthesiology & Pain Management
  • Integrative & Holistic Medicine
  • Food Animals
  • Behavior
  • Zoo Medicine
  • Toxicology
  • Orthopedics
  • Emergency & Critical Care
  • Equine Medicine
  • Pharmacology
  • Pediatrics
  • Respiratory Medicine
  • Shelter Medicine
  • Parasitology
  • Clinical Pathology
  • Virtual Care
  • Rehabilitation
  • Epidemiology
  • Fish Medicine
  • Diabetes
  • Livestock
  • Endocrinology

Pharmacologic management of pain in the equine patient (Proceedings)

Article

What is pain? "An unpleasant sensory and emotional experience associated with actual or potential tissue damage" (International Association for the Study of Pain)

What is pain?

• "an unpleasant sensory and emotional experience associated with actual or potential tissue damage" (International Association for the Study of Pain)

How is pain evaluated in the equine patient?

• Very Difficult!!!!

• Based mostly on subjective evaluation

• May not be evident until severe

o Example of signs of pain: depression, agitation, anorexia, tachycardia, tachypnea, lameness, colic

Pathway of pain

• First order neurons

o Two types of pain receptors (nociceptors):

• Type Aδ myelinated nerve fibers = first, sharp pain

• Type C non-myelinated nerve fibers = slow, burning pain

• Second order neurons

o Dorsal horn of grey matter

o "Gated"

• Excitatory

• Inhibitory

• Spinothalamic tract

o Major ascending pathway for pain conduction

o Contralateral side

• Ventral white matter

• Third order neurons

o In thalamus

o Project to cortex

What is pain? – There are 2 types:

• Physiologic pain

o Stems from noxious stimuli

o Perception is proportional to intensity of stimuli

o "Protective Pain"

o Stimulates a response

• Pathologic pain

o From tissue damage (e.g. inflammation)

o Perception is greater than stimulus

Goals of pain management

• Reduce the morbidity and mortality in patient.

Methods of pain management

• Drugs – IV, IM, oral

• Epidurals

• Continuous rate infusions

• Transdermal

• Novel approaches

NSAIDS

• Inhibit cyclooxygenases (COX)

• Decrease prostaglandin and thromboxane A2

• Effective as analgesics when inflammatory response is present

• Reduce hypersensitivity to pain (local effects)

• May have some central effects (speculated)

• Phenylbutazone

o 2.2 to 4.4 mg/kg BID

o Cumulative affects by 48 hours post-administration

o Oral absorption highly variable especially in fed animal (2-12 hours for peak), but good total absorption

o Peak concentration 3-4 hours (fasted)

o T ½ = 3 - 7 hours (dose dependent)

o Strong COX-1

• Flunixin meglumine

o 1.1 mg/kg

o Effective for visceral pain

o Rapid onset of action

o Duration of action 6 to 12 hours

o Strong COX-1

• Firocoxib

o Equiox

o Coxib

o Highly selective COX-2 inhibitor

o As safe as phenylbutazone at recommended doses (Doucet MY et al. JAVMA Jan 1 232:1; 91-97, 2008)

o Safer at chronic doses?

o Comparable efficacy to phenylbutazone for horses with osteoarthritis (Doucet MY et al. JAVMA Jan 1 232:1; 91-97, 2008)

• Ketoprofen:

o 2.2 mg/kg SID

o Effective in visceral and musculoskeletal models

o COX1 and COX 2

• Carprofen:

o 0.7 mg/kg BID

o Strong COX-2

o Use in foals due to less GI toxicity

• Meloxicam:

o 0.6 mg/kg IV??? or PO SID ??

o IV – rapid clearance from plasma (two studies)

o Very effective NSAID

o Strong COX-2

o Use in foals?

• Aspirin:

o 17 mg/kg PO EOD

o Anti-thrombotic

• Eltenac:

o 0.5-1.0 mg/kg IV SID

o 1 hour onset, 24 hours analgesia

o No sig GI effects

Alpha 2 agonists

• Provide analgesia by binding to α2 receptors in CNS

• Most potent visceral analgesics

• Sedatives

• Xylazine:

o 1.1 mg/kg

o Duration of action = 20 minutes

• Detomidine:

o 20 – 40 µg/kg

o Duration of action = 45 - 90 minutes

Opioids

• Mechanism of action:

o Agonists and mixed agonists and antagonists that suppress nociceptive cells.

o Inhibition of pain transmission in the dorsal horn of the spinal cord and brain

o Activate µ, k, δ receptors

o µ receptor selective drugs have most effective analgesia

o Cause excitement

• Butorphanol:

o 0.1 – 0.4 mg/kg IV or IM

o 3 minutes until onset after IV administration

o Peak 15 to 30 minutes

o Provides 60 to 90 minutes of analgesia IV and up to 4 hours IM.

o Good for visceral analgesia, especially with alpha 2 agonists

o µ and k receptor actions

• Buprenophine:

o 0.005 mg/kg IM

o 6 to 12 hours of analgesia

o High affinity for µ receptors

o Mixed agonist/antagonist

• Morphine:

o 0.05 – 0.6 mg/kg IM

o Causes significant agitation/excitement

• Fentanyl:

o 100 µg patches

o Blood levels and effectiveness not published in the horse

o 48 hours of analgesia

Epidural

• Indications for epidural

o Procedures involving rectum, vagina, perineum, urethra and bladder

• Obstetric manipulations

• Analgesia of in hind-limbs

• Intraoperative

• Contraindications

o Infection at puncture site

o +/- Sepsis

o Uncorrected hypovolemia

o Anticoagulation therapy

o Anatomic abnormalities

• Anatomy

o Epidural space is within the spinal canal outside the visceral layer of the dura matter.

o Not subarachnoid space (site of CSF collection).

• Where do I go?

o Lumbosacral

• Subarachnoid is easier than epidural

• Fastest and best controlled flank anesthesia

• Need special equipment and aseptic technique

o Caudal

• Simple to do, no special equipment

• Can preserve locomotor function of hindlimb

o Lumbosacral subarachnoid

• Same site as CSF collection

• 1 to 2 cm caudal to a line drawn from the cranial edge of tuber sacrale and dorsal midline

• Need a 17.5 cm 17 g spinal needle

o Caudal epidural

• Palpate Co1-Co2 as the first midline depression caudal to the sacrum

• First movable coccygeal articulation when the tail is raised and lowered

• Technique

• 18 gauge 1.5 in needle

• Enter the center of the space perpendicular to the skin

• May feel popping as interarcuate ligament is penetrated

• "Hanging drop"

• Loss of resistance

• Injects easily, air bubble does not compress

o Epidural catheters

• Indicated for continuous epidural analgesia

• Pelvic fractures, hind limb fractures, septic joints, etc.

• Best for repeat dosing

• 17 gauge Huber point directional needle with stylet

• Catheters can cause localized inflammation and fibrosis

• Complications minimal

• Report of 43 catheterizations (Martin CA et al JAVMA 2003 May 15 1394-98)

o Technical problems with catheter predominate

• Kinking, leakage, obstruction, dislodgement, patient problems, non-response to treatment

• What to use in epidural administration

o Local anesthetics – for desensitization

• Where does this block?

• Desensitized skin in a standing horse will depend on amount used

• Lidocaine, Mepivicaine

o Alpha-2 Agonists

• Less motor effects other than profound sedation

• "patchy" analgesia so best when combined

• Can be reversed

o Opioids

• Morphine, meperidine, butorphanol, methadone, hydromorphone

• Most effective and long-acting analgesics

• No excitatory effects when given epidurally

o Dissociatives

• Ketamine

• Only good for short-term analgesia

• Mechanism – NMDA antagonist

o Drug combinations

• Can act synergistically and prolong analgesia

• May provide better analgesia than one agent alone

• Minimize side effects of individual agents

• Complications of epidural administration

o Hypoventilation

o Bradycardia

o Pruritis

o Upward fixation of the patella

o Sepsis

o Recumbency

Continuous rate infusions

• Provide consistent analgesia

• Via fluid infusion pump or drip rate calculations

• Butorphanol

o Loading Dose 17.8 µg/kg

o Rate = 13 µg/kg/hr

o In celiotomy patients: (Sellon DC et al J Vet Intern Med. 2004 Jul-Aug;18(4):555-63)

• Improved behavior scores

• Decreased weight-loss

• Decreased plasma cortisol levels

• Increase time to first defecation

• Lidocaine

o Sodium-channel blocker

o Mechanism:

• Decreases sympathetic tone

• Provides somatic analgesia

• Anti-inflammatory

o Loading dose 1.3 mg/kg

o Rate = 0.05 mg/kg/min

o Used for pain management and decreasing ileus in humans for decades

o Shown to reduce MAC in anesthetized horses

o Side effects include muscle faciculations, ataxia, collapse

• Ketamine (Fielding CM, et al. Am J Vet Res. 2006 Sep;67(9):1484-90)

o 0.4 – 0.8 mg/kg/hr

o No excitation at this dose

o No analgesic effect

• Has analgesic effects in other species

o Complications

• Mild bradycardia and hypotension

Transdermal

• Fentanyl (Orsini et al. J Vet Pharmacol Ther. 2006 Dec;29(6):539-46)

o 2 to 4 - 10 mg patches for adult horses (Orsini et al. J Vet Pharmacol Ther. 2006 Dec;29(6):539-46; Maxwell et al Eq Vet J 2003; 35(5):484-490)

o Need > 1 ng/ml for analgesia

o Absorption is variable among individual horses (Orsini et al. J Vet Pharmacol Ther. 2006 Dec;29(6):539-46

• Only ⅓ of horses reached analgesic levels

• Site of application dependent (Mills et al. Res Vet Sci; 2007; 82:252-256.

• Less absorption from leg vs. groin or thorax

o Lidocaine

• Not absorbed across equine skin (Bidwell et al.Vet Anes and Analg, 2007; 34:443-446)

Things to consider

• Implantable drug delivery systems (pumps)

• Intravenous regional anesthesia

o Currently used in humans

o Can be continuous infusion

Related Videos
© 2024 MJH Life Sciences

All rights reserved.