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
