Overview and Clinical Importance
Analgesia is a cornerstone of veterinary anesthesia and patient care. Effective pain management improves patient welfare, accelerates recovery, reduces morbidity, and is an ethical obligation for veterinary professionals. This guide covers the essential analgesic drug classes tested on the BCSE: opioids (full agonists, partial agonists, agonist-antagonists, and pure antagonists), non-steroidal anti-inflammatory drugs (NSAIDs), multimodal analgesia principles, and constant rate infusion (CRI) protocols including MLK.
The Anesthesia domain comprises 20-23 questions on the BCSE. Analgesia is integrated throughout anesthetic protocols, from premedication through recovery, making this topic highly testable.
Opioid Analgesics
Opioids are the cornerstone of effective pain treatment for moderate to severe pain in veterinary medicine. They bind to opioid receptors in the central and peripheral nervous systems, inhibiting the release of excitatory neurotransmitters from afferent fibers in the spinal cord and thereby inhibiting synaptic transmission of painful stimuli.
Opioid Receptor Types
Three primary opioid receptor types mediate analgesia: mu (μ), kappa (?), and delta (?). All are G-protein coupled receptors that, when activated, inhibit adenylate cyclase, open potassium channels (causing hyperpolarization), and close calcium channels, resulting in decreased neurotransmitter release.
MEMORY TIP - "MuSt Know Receptors": Mu = Most analgesia (somatic), Kappa = Kind of visceral, Delta = Does minor work. Remember: μ receptors cause miosis in dogs but mydriasis in cats!
Opioid Classification by Receptor Activity
Full Mu (μ) Agonists
Full agonists produce maximum receptor activation and have the highest analgesic efficacy. They have NO ceiling effect for analgesia (but ceiling for other effects may exist). Common full agonists include morphine, hydromorphone, oxymorphone, fentanyl, and methadone.
Partial Mu (μ) Agonists
Partial agonists bind to the receptor but produce submaximal activation. They exhibit a ceiling effect for analgesia - increasing the dose beyond a certain point does not increase analgesia but does increase side effects. Key example: Buprenorphine.
Buprenorphine has very high receptor binding affinity, making it difficult to reverse with naloxone. Duration is 6-8 hours. It can be given via oral transmucosal (OTM) route in cats with good bioavailability (making it excellent for at-home pain management). Slow onset (30-45 minutes).
Agonist-Antagonists (Mixed Opioids)
These drugs act as agonists at kappa receptors and antagonists (or weak agonists) at mu receptors. They exhibit a ceiling effect for analgesia. Examples include butorphanol and nalbuphine.
Butorphanol: Duration 1-2 hours; better for visceral pain than somatic; excellent antitussive; commonly combined with sedatives (alpha-2 agonists, acepromazine). Limited analgesic efficacy for moderate-severe pain.
MEMORY TIP - "CEILING on the BUS": Buprenorphine, bUtorphanol, and nalbuphine all have ceiling effects! B-U-S drugs have ceilings. Full agonists (morphine, fentanyl) have NO ceiling for analgesia.
Pure Opioid Antagonists
These drugs have no agonist activity and competitively bind to opioid receptors to reverse opioid effects. Key drugs: Naloxone (short duration, 30-45 min), Naltrexone (long duration, oral), and Nalmefene (intermediate duration).
Species Differences in Opioid Response
MEMORY TIP - "Cats are Different!": Cats get MYDRIASIS (dilated pupils) from opioids; Dogs get MIOSIS (constricted pupils). Cats can have DYSPHORIA at high doses. Remember "M for Mydriasis in Meow" and "M for Miosis in (Bow)wow".
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)
NSAIDs exert antipyretic, anti-inflammatory, and analgesic effects. They are the mainstays of relief from mild-to-moderate pain and are essential components of multimodal analgesia protocols. NSAIDs are widely used in veterinary medicine - approximately 98% of dogs and cats receive perioperative NSAIDs for routine procedures like neutering.
Mechanism of Action
NSAIDs act within the arachidonic acid cascade by blocking cyclooxygenase (COX) enzymes, which catalyze the conversion of arachidonic acid to prostaglandins. Prostaglandins mediate inflammation, pain, and fever.
Two main COX isoforms exist:
- COX-1 (Constitutive): Produces prostaglandins involved in "housekeeping" physiological functions including gastroprotection (mucosal blood flow, mucus production), renal blood flow regulation, and platelet aggregation (thromboxane A2 production).
- COX-2 (Inducible): Primarily upregulated at sites of tissue injury and inflammation to produce inflammatory prostaglandins. Also has some constitutive expression in brain, kidney, and reproductive tract.
MEMORY TIP - "COX-1 Keeps the House Clean": COX-1 = Constitutive = "Caretaker" (housekeeping functions: GI protection, renal flow, platelets). COX-2 = Inducible = "Inflammation" (upregulated with tissue injury). Inhibiting the "Caretaker" causes problems (GI ulcers, renal issues, bleeding).
NSAID Classification
NSAID Adverse Effects
NSAID-related adverse effects are most commonly related to the gastrointestinal tract (anorexia, vomiting, diarrhea, decreased appetite). GI effects are usually self-limiting, but ulceration and perforation can occur with inappropriate administration.
Other adverse effects include: decreased platelet aggregation (aspirin is irreversible - avoid preoperatively), renal toxicity (especially in dehydrated/hypotensive patients), and hepatotoxicity (rare, usually idiosyncratic).
NSAID Contraindications
- Hypovolemia, dehydration, or hypotension
- Pre-existing renal disease (use with extreme caution in cats with CKD IRIS stages I-III only if monitored closely)
- GI ulceration or bleeding
- Hepatic disease
- Coagulation disorders
- Concurrent corticosteroid use (greatly increases GI ulceration risk)
MEMORY TIP - "No NSAIDs in DRGH": D = Dehydration/hypotension, R = Renal disease, G = GI ulcers/bleeding, H = Hepatic disease. Also remember: NEVER combine NSAIDs with corticosteroids ("steroids + NSAIDs = GI disaster")!
Multimodal Analgesia
Multimodal analgesia (also called balanced analgesia) involves using two or more analgesic drugs with different mechanisms of action to target different steps in the pain pathway. This approach aims to maximize pain relief while minimizing side effects from any single medication.
Principles of Multimodal Analgesia
- Target Multiple Pain Pathway Sites: Peripheral (NSAIDs, local anesthetics), spinal cord (opioids, alpha-2 agonists, NMDA antagonists), and supraspinal (opioids)
- Synergistic Effects: Combinations may produce additive or synergistic analgesia, allowing lower doses of individual drugs
- Reduced Side Effects: Lower individual drug doses typically mean fewer dose-dependent adverse effects
- Preventive Analgesia: Administering analgesics BEFORE the painful stimulus (preemptive) and continuing through the perioperative period prevents central sensitization and "wind-up"
MEMORY TIP - "ONLAK" for Multimodal: O = Opioids, N = NSAIDs, L = Local anesthetics, A = Alpha-2 agonists, K = Ketamine. These five classes target different parts of the pain pathway. Remember: "Only Nice Llamas Are Kind" to help recall multimodal components!
Constant Rate Infusion (CRI) Protocols
Constant rate infusions (CRIs) provide sustained analgesic drug concentrations, avoiding the "peaks and troughs" of intermittent bolus dosing. CRIs reduce inhalant anesthetic requirements (MAC-sparing effect), provide consistent analgesia, and allow easy dose titration.
MLK (Morphine-Lidocaine-Ketamine) Protocol
MLK is the most commonly used combination analgesic CRI in dogs. It combines three drugs with different mechanisms of action, embodying multimodal principles.
MLK Recipe Example (Dogs)
Standard MLK Formula for 500 mL IV Fluid Bag:
- Morphine: 10 mg (1 mL of 10 mg/mL)
- Lidocaine: 150 mg (7.5 mL of 20 mg/mL)
- Ketamine: 30 mg (0.3 mL of 100 mg/mL)
- IMPORTANT: Remove the volume of drugs (~8.8 mL) from the fluid bag BEFORE adding the MLK mixture to maintain correct concentrations.
Administration: Loading dose of 10 mL/kg, then CRI at 10 mL/kg/hr. Titrate based on patient response.
MEMORY TIP - "MLK = MiLK": Think of MLK like milk - it's a common "staple" CRI in veterinary anesthesia! M = Morphine (opioid), L = Lidocaine (local anesthetic - NOT for cats!), K = Ketamine (NMDA antagonist). Also remember: "MLK gives MAC Reduction" - all three components reduce MAC.