Overview and Clinical Importance
Anesthesia induction agents are essential drugs used to transition patients from a conscious state to an anesthetized state suitable for endotracheal intubation and surgical procedures. Understanding the pharmacology, mechanisms, dosing, and species-specific considerations of these agents is critical for safe veterinary anesthesia practice.
The BCSE tests your knowledge of induction agents across multiple contexts including: mechanism of action, appropriate dosing, cardiovascular and respiratory effects, species differences, contraindications, and clinical decision-making for patient selection.
Propofol
Overview
Propofol (2,6-diisopropylphenol) is the most widely used injectable induction agent in veterinary medicine. It is a phenolic compound formulated as a white, lipid-based emulsion containing soybean oil, glycerol, and egg lecithin. The drug provides rapid, smooth induction with excellent recovery characteristics.
Mechanism of Action
Propofol is a potent positive allosteric modulator (PAM) of GABA-A receptors in the central nervous system. By enhancing GABA-mediated chloride conductance, it hyperpolarizes neurons and produces CNS depression, leading to unconsciousness and muscle relaxation.
MEMORY AID: PROP-ofol = PRO-motes GABA
Propofol PROMOTES GABA activity at GABA-A receptors. Think: 'PRO-GABA' for propofol's mechanism.
Pharmacokinetics
- Onset: 30-60 seconds after IV administration
- Duration: 5-10 minutes (single bolus)
- Metabolism: Rapid hepatic and extrahepatic conjugation
- Recovery: Redistribution-dependent; full standing recovery typically within 20 minutes in dogs
- No active metabolites; suitable for repeated dosing or CRI
Dosing Guidelines
Cardiovascular and Respiratory Effects
Cardiovascular Effects: Propofol causes dose-dependent cardiovascular depression including decreased systemic vascular resistance, mild myocardial depression, and hypotension. Heart rate may remain unchanged or decrease slightly.
Respiratory Effects: Dose-dependent respiratory depression is common. Apnea lasting 30-60 seconds frequently occurs, especially with rapid administration. Always be prepared to provide ventilatory support.
MEMORY AID: Propofol = 'Pro-Hypotension'
Remember: Propofol causes vasodilation and decreased SVR, leading to hypotension. The 'Pro' in propofol can remind you it's 'Pro-Hypotensive.'
Clinical Pearls
- Excellent for patients with hepatic dysfunction (short duration due to redistribution)
- Safe for cesarean sections (rapid neonatal clearance)
- Suitable for patients with seizure disorders (anticonvulsant properties)
- Sighthounds (Greyhounds) may have prolonged recovery
- Avoid in patients allergic to soy, eggs, or peanuts (emulsion components)
Alfaxalone
Overview
Alfaxalone (3-alpha-hydroxy-5-alpha-pregnane-11,20-dione) is a synthetic neuroactive steroid that has gained popularity as an induction agent. It is formulated in 2-hydroxypropyl-beta-cyclodextrin (HPCD), a water-soluble carrier that does not cause histamine release. The drug produces reliable anesthesia with good muscle relaxation.
Mechanism of Action
Like propofol, alfaxalone acts as a positive allosteric modulator of GABA-A receptors. As a neurosteroid, it binds to specific steroid-binding sites on the GABA-A receptor, enhancing chloride ion conductance and producing CNS depression, unconsciousness, and muscle relaxation.
MEMORY AID: ALFAxalone = A Lovely Fast Alternative
Alfaxalone provides A Lovely Fast Alternative to propofol with similar GABA-ergic mechanism but different formulation. Both are GABA PAMs.
Pharmacokinetics
- Onset: Less than 60 seconds after IV administration
- Duration: Dose-dependent (6-26 minutes depending on dose)
- Metabolism: Rapid hepatic metabolism
- Can be administered IM in cats (unlike propofol)
- Recovery may be slightly longer than propofol
Dosing Guidelines
Cardiovascular and Respiratory Effects
Cardiovascular Effects: Similar to propofol, alfaxalone causes dose-dependent cardiovascular depression. Studies show decreased arterial blood pressure and systemic vascular resistance, but cardiac output may be maintained or slightly increased due to enhanced contractility and stroke volume compared to etomidate.
Respiratory Effects: Dose-dependent respiratory depression occurs. Post-induction apnea is common (similar to propofol). Administer slowly to minimize respiratory complications.
Clinical Pearls
- Smooth induction and recovery comparable to propofol
- Safe for cesarean sections with good neonatal outcomes
- No histamine release (unlike original Saffan formulation)
- May see paddling, twitching, or myoclonus during induction or recovery
- Combining with benzodiazepines or ketamine can reduce dose requirements
Ketamine (Dissociative Anesthetic)
Overview
Ketamine is a phencyclidine derivative classified as a dissociative anesthetic. It produces a unique state of 'dissociative anesthesia' characterized by profound analgesia, catalepsy, and apparent separation of the patient from the environment while maintaining protective reflexes. Ketamine is typically combined with a benzodiazepine or alpha-2 agonist to provide muscle relaxation.
Mechanism of Action
Ketamine is a non-competitive antagonist of NMDA (N-methyl-D-aspartate) receptors. By blocking glutamate-mediated excitatory neurotransmission, it disrupts thalamocortical pathways and produces the dissociative state. This mechanism differs fundamentally from GABA-ergic agents like propofol and alfaxalone.
MEMORY AID: K-NMDA = Ketamine Blocks NMDA
Ketamine KNOCKS out NMDA receptors. The 'K' in Ketamine = KNOCKS out NMDA. This is the OPPOSITE of GABA agents which ENHANCE inhibition.
Pharmacokinetics
- Onset: 1-2 minutes IV; 5-10 minutes IM
- Duration: 20-40 minutes (species and dose dependent)
- Metabolism: Hepatic N-demethylation to norketamine (active metabolite)
- Recovery: Longer than propofol/alfaxalone; may see emergence delirium
- Cats: Slower metabolism leads to prolonged effects
Dosing Guidelines
Cardiovascular and Respiratory Effects
Cardiovascular Effects: SYMPATHOMIMETIC - Ketamine increases heart rate, blood pressure, and cardiac output through central sympathetic stimulation. This makes it valuable for compromised or hypovolemic patients. However, direct myocardial depression may occur in catecholamine-depleted patients.
Respiratory Effects: Minimal respiratory depression at clinical doses. Maintains airway reflexes (laryngeal and pharyngeal). Bronchodilation makes it useful for asthmatic patients.
MEMORY AID: Ketamine = 'Keep-the-Heart-Going'
Ketamine KEEPS cardiovascular parameters UP (sympathomimetic). Unlike propofol's hypotension, ketamine supports blood pressure and heart rate.
Unique Characteristics
- Eyes remain open with intact palpebral reflexes (assess depth differently)
- Increased salivation - consider anticholinergic premedication
- Muscle rigidity when used alone - ALWAYS combine with muscle relaxant
- Increases intracranial and intraocular pressure
- DEA Schedule III controlled substance
Etomidate
Overview
Etomidate is an imidazole derivative used primarily for anesthetic induction in high-risk cardiovascular patients. It provides the most cardiovascular-stable induction of all available agents, making it ideal for patients with compromised cardiac function.
Mechanism of Action
Etomidate is a positive allosteric modulator of GABA-A receptors (like propofol and alfaxalone). It enhances inhibitory neurotransmission to produce hypnosis. However, it provides minimal analgesia.
MEMORY AID: Etomidate = 'E' for Emergency Cardiac Cases
Use Etomidate for Emergency cardiovascular patients. It's the 'E'mergency choice when you need stable hemodynamics.
Pharmacokinetics
- Onset: Less than 60 seconds IV
- Duration: 5-15 minutes (single dose)
- Metabolism: Hepatic ester hydrolysis
- Recovery: Rapid redistribution-dependent
Dosing Guidelines
Cardiovascular Effects - The Key Advantage
Etomidate minimally suppresses myocardial contractility and has little effect on systemic vascular resistance. This results in stable heart rate and blood pressure during induction, making it the agent of choice for patients with cardiovascular compromise.
Adverse Effects
- Adrenocortical suppression (most clinically significant)
- Pain on injection (propylene glycol vehicle)
- Myoclonus and muscle twitching
- Nausea and vomiting (more common in humans)
- Hemolysis if stored improperly
MEMORY AID: Etomidate Side Effects: 'MAPS'
M = Myoclonus, A = Adrenal suppression, P = Pain on injection, S = Stable cardiovascular (the benefit, not side effect). Remember MAPS to navigate etomidate's profile.
Tiletamine-Zolazepam (Telazol)
Overview
Tiletamine-zolazepam is a fixed 1:1 combination of tiletamine (a dissociative anesthetic related to ketamine) and zolazepam (a benzodiazepine). Marketed as Telazol, it provides rapid-acting anesthesia with muscle relaxation. The combination eliminates the need to mix separate drugs.
Mechanism of Action
Tiletamine: NMDA receptor antagonist (like ketamine). Produces dissociative anesthesia, analgesia, and catalepsy.
Zolazepam: GABA-A receptor positive allosteric modulator. Provides anxiolysis, muscle relaxation, and reduces the muscle rigidity associated with dissociative agents.
MEMORY AID: Telazol = 'Two-in-One'
Telazol is a Two-in-One: Tiletamine (NMDA antagonist like ketamine) + Zolazepam (benzo like diazepam). Think 'T + Z = Complete package.'
Pharmacokinetics
- Onset: Less than 30-60 seconds IV; 3-5 minutes IM
- Duration: 30-60 minutes (variable)
- Critical point: Tiletamine has longer half-life than zolazepam
- Species variation: Dogs metabolize zolazepam faster than tiletamine (rough recovery); Cats metabolize tiletamine faster (smoother recovery)
Dosing Guidelines
Reconstitution and Stability
Telazol is supplied as a lyophilized powder (250 mg tiletamine + 250 mg zolazepam per vial). Reconstitute with 5 mL sterile water to yield 100 mg/mL total concentration (50 mg/mL each component). Reconstituted solution is stable for 7 days at room temperature or 56 days refrigerated.
Contraindications
- Pancreatic disease (labeled contraindication)
- Severe cardiac or pulmonary dysfunction
- Renal disease
- Cesarean sections (crosses placenta, depresses neonates)
- Hyperthyroidism, pheochromocytoma
- Head trauma, intracranial masses (increases ICP)
Comprehensive Comparison of Induction Agents
Clinical Selection Guide
Memory Aid Summary
MEMORY AID: The 'GABA Gang' vs 'NMDA Blockers'
GABA Gang (enhance inhibition): Propofol, Alfaxalone, Etomidate - all PAMs at GABA-A receptors. NMDA Blockers (block excitation): Ketamine, Tiletamine - antagonize glutamate.
MEMORY AID: 'PAKE' for Mechanism Categories
P = Propofol (GABA), A = Alfaxalone (GABA), K = Ketamine (NMDA), E = Etomidate (GABA). Three are GABA, one is NMDA. Telazol combines both!
MEMORY AID: Cardiovascular Mnemonic: 'PROPofol drops PRESSURE'
Propofol and Alfaxalone both drop blood pressure. Ketamine KEEPS it up. Etomidate keeps it EVEN. Remember: P drops, K keeps, E evens.
MEMORY AID: Recovery Speed: 'Fast PROP, Slow K'
Propofol = fastest recovery. Ketamine/Telazol = slowest. Alfaxalone and Etomidate in between.
MEMORY AID: IM Route Available: 'TAKA'
T = Telazol, A = Alfaxalone, K = Ketamine can all be given IM. Propofol and Etomidate are IV only.
MEMORY AID: Analgesic Properties: 'Only K and T'
Only Ketamine and Tiletamine (Telazol) provide analgesia. The GABA agents (P, A, E) do NOT provide pain relief.