6.1 Emergency Anesthesia Protocols & High-Risk Patient Stabilization

Emergency Anesthesia Protocols & High-Risk Patient Stabilization

VTS Critical Concept: Anesthesia does not treat shock—it unmasks and exacerbates cardiovascular collapse. In the emergency and critical care setting, general anesthesia abolishes the patient's endogenous sympathetic tone, compensatory peripheral vasoconstriction, and baroreceptor reflexes. The primary mandate of the veterinary critical care technician is to aggressively resuscitate systemic oxygen delivery (DO2) before anesthetic induction whenever humanly possible.

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1. American Society of Anesthesiologists (ASA) Physical Status Classification

The ASA Physical Status Classification provides a standardized, validated system to communicate pre-existing physiological impairment and stratify perioperative anesthetic risk. In veterinary emergency medicine, patients presenting in shock, acute trauma, or organ failure frequently fall into ASA Class IV or V.

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2. The Pre-Anesthetic Stabilization Hierarchy

Inducing anesthesia in an under-resuscitated, hypovolemic, or acidemic animal dramatically increases the incidence of cardiac arrest during the peri-induction period. Veterinary critical care technicians must systematically work through the Pre-Anesthetic Stabilization Hierarchy prior to premedication:

1. OXYGENATION & VENTILATION  -->  Normalize PaO2 (>80 mmHg) & SpO2 (>95%); relieve pleural space disease
2. INTRAVASCULAR PRELOAD       -->  Resuscitate hypovolemia; restore MAP >65 mmHg & Normal Lactate
3. ELECTROLYTE & ACID-BASE    -->  Correct severe hyperkalemia (K+ >6.0 mEq/L) & profound acidemia (pH <7.15)
4. PACKED CELL VOLUME (PCV)   -->  Ensure PCV >= 25% (or Hgb >= 8 g/dL) for adequate oxygen-carrying capacity
5. TEMPERATURE REGULATION     -->  Rewarm hypothermic patients to >98°F (36.7°C) before induction

Core Physiological Correction Thresholds

• Intravascular Volume: Titrate fractionated isotonic crystalloid aliquots (10-20 mL/kg over 15 min in dogs; 5-10 mL/kg in cats) or synthetic/natural colloids (3-5 mL/kg VetStarch or FFP) until blood lactate drops below 2.0 mmol/L and peripheral pulses normalize.
• Hyperkalemia (K+ > 6.0 mEq/L): Extremely common in feline urethral obstruction (FLUTD), uroabdomen, and hypoadrenocortical crises. Severe hyperkalemia slows cardiac conduction, causing spiked T waves, loss of P waves, widened QRS complexes, and ventricular arrhythmias. Administer 10% Calcium Gluconate (0.5-1.0 mL/kg slow IV over 10-15 min under continuous ECG monitoring) to immediately restore the normal membrane potential threshold and stabilize myocardium. Concurrently administer regular insulin (0.1-0.25 U/kg IV) with 50% dextrose (1-2 g dextrose per unit of insulin) or Terbutaline to shift potassium intracellularly.
• Acidemia (pH < 7.15): Severe metabolic acidosis depresses myocardial contractility, impairs the vascular response to exogenous and endogenous catecholamines, and predisposes to refractory arrhythmias. Resuscitate perfusion before considering cautious sodium bicarbonate therapy.
• Pleural Space Emergencies: Always perform diagnostic/therapeutic thoracocentesis to evacuate air (pneumothorax) or fluid (hemothorax, pyothorax, chylothorax) prior to anesthetic induction. Positive pressure ventilation or recumbency in a patient with unaddressed pleural effusion will cause immediate fatal ventilatory and circulatory collapse.

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3. Premedication & Neuroleptanalgesia in Unstable Patients

Healthy patient premedication protocols (which often utilize phenothiazines like acepromazine or full-dose alpha-2 agonists like dexmedetomidine) are hazardous in hemodynamically compromised critical patients. The gold standard for chemical restraint and preemptive analgesia in the critical patient is Neuroleptanalgesia—the synergistic combination of a potent opioid with a tranquilizer/sedative.

Opioids (Pure mu-Agonists)

Pure mu-opioid receptor agonists provide excellent visceral and somatic analgesia, sedation, and reversibility with minimal negative inotropic or vasodilatory side effects.

• Hydromorphone (0.05-0.1 mg/kg IV/IM): Potent pure mu-agonist; duration 2-4 hours. Causes minimal cardiovascular depression; can cause panting, mild bradycardia (responsive to anticholinergics), and transient hyperthermia in cats.
• Methadone (0.1-0.3 mg/kg IV/IM): Dual-action pure mu-agonist and NMDA receptor antagonist; also inhibits serotonin and norepinephrine reuptake. Unlike morphine, methadone does not induce histamine release when administered intravenously. Highly effective for neuropathic pain and visceral trauma; duration 3-4 hours.
• Fentanyl (2-5 mcg/kg IV bolus): Ultra-short-acting pure mu-agonist with rapid onset (1-2 min) and short duration (20-30 min). Ideal for critically ill patients when administered as an initial bolus followed immediately by a constant rate infusion (CRI).

Benzodiazepines: The Sedative Co-Factor

Benzodiazepines enhance the inhibitory neurotransmitter GABA (GABA-A receptor complex), producing muscle relaxation, anxiolysis, and anticonvulsant activity with virtually zero direct myocardial depression or vascular tone loss.

• Midazolam (0.1-0.3 mg/kg IV/IM): Water-soluble, rapidly absorbed via both IV and IM routes. Does not contain propylene glycol; causes no pain or tissue necrosis on injection. In critically ill, geriatric, or pediatric patients, it produces excellent, reliable sedation when combined with an opioid. Caution: In young, healthy, or non-debilitated animals, benzodiazepines can induce paradoxical excitement, vocalization, and dysphoria.
• Diazepam (0.1-0.3 mg/kg IV ONLY): Lipid-soluble formulation compounded with 40% propylene glycol. Must only be given intravenously (poor/erratic IM absorption). Rapid IV bolusing of propylene glycol can precipitate hypotension, cardiac arrhythmias, and thrombophlebitis.

Sedatives to Avoid in Shock and Cardiovascular Instability

• Acepromazine: A phenothiazine that causes non-competitive blockade of peripheral alpha-1-adrenergic receptors. This induces potent, long-lasting, dose-dependent systemic vasodilation, hypotension, and splenic sequestration of RBCs (dropping PCV by up to 15-20%). Acepromazine is non-reversible and strictly contraindicated in hypovolemia, trauma, shock, and anemia.
• Full-Dose alpha-2-Adrenergic Agonists (Dexmedetomidine 5-20 mcg/kg): Induces intense peripheral vasoconstriction, doubling systemic vascular resistance (SVR), triggering dramatic reflex bradycardia, and slashing cardiac output by 30-50%. While micro-doses (0.5-1.0 mcg/kg) are useful in specific multimodal protocols, full sedative doses are contraindicated in uncorrected shock, dilated cardiomyopathy, and hypovolemia.

4. Induction Agents in Critical Disease: Comparative Pharmacology

The ideal induction agent in critical care provides rapid, smooth induction and enables immediate endotracheal intubation while preserving cardiac output, systemic vascular resistance, and arterial blood pressure.

Comparative Pharmacodynamic Properties

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5. Inhalant Anesthesia Management & MAC-Sparing Strategies

Both Isoflurane and Sevoflurane are halogenated volatile inhalational anesthetics that cause profound, dose-dependent peripheral vasodilation, blunted baroreceptor reflexes, and decreased myocardial contractility. Inhalants are the single most common cause of intraoperative hypotension in veterinary emergency surgery.

Minimum Alveolar Concentration (MAC)

MAC is the alveolar concentration of an inhalant anesthetic at 1 atmosphere of pressure that prevents gross purposeful movement in 50% of patients exposed to a supramaximal noxious stimulus.

• Isoflurane MAC: ~1.30% in dogs; ~1.63% in cats
• Sevoflurane MAC: ~2.36% in dogs; ~2.58% in cats

MAC-Sparing Protocols

Because high inhalant concentrations (> 1.2-1.5 MAC) induce severe hypotension, the goal in critical care is to operate at the lowest possible vaporizer setting (0.5-0.8 MAC) by implementing aggressive multimodal MAC-sparing techniques:

1. Pure mu-Opioids: Intravenous boluses or infusions of fentanyl, hydromorphone, or methadone reduce inhalant MAC by 30% to 50%.
2. Lidocaine CRI (Dogs Only): Reduces isoflurane MAC by 20% to 40% while providing antiarrhythmic, anti-inflammatory, and prokinetic benefits.
3. Ketamine CRI: Subanesthetic ketamine infusions (2-10 mcg/kg/min) reduce inhalant requirements by 25% to 30% and prevent central sensitization.
4. Locoregional Nerve Blocks: Effective local/epidural anesthesia completely prevents nociceptive signals from reaching the central nervous system, reducing inhalant requirements by up to 50-80%.

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6. Rapid Sequence Induction (RSI) & Aspiration Risk Management

Patients presenting with acute abdomen, gastric dilatation-volvulus (GDV), esophageal foreign bodies, traumatic brain injury, or severe obtundation are at extreme risk for gastroesophageal reflux (GER) and fatal pulmonary aspiration of gastric contents during induction.

Core Tenets of Rapid Sequence Induction (RSI)

1. Preoxygenation: Administer 100% oxygen via a tight-fitting face mask for 3-5 minutes before induction. This replaces nitrogen in the functional residual capacity (FRC) with oxygen ("denitrogenation"), expanding the patient's safe apnea time from 1 minute to over 4-5 minutes without desaturation.
2. Calculated Rapid Titration: Draw up the full calculated induction dose (e.g., Etomidate, Ketamine-Diazepam, or Propofol-Fentanyl). Administer a rapid, smooth IV bolus sufficient to rapidly achieve stage III anesthesia and abolish laryngeal reflexes without hesitation.
3. Cricoid Pressure (Sellick Maneuver): An assistant applies gentle dorsal pressure to the cricoid cartilage to occlude the underlying esophagus, preventing passive regurgitation of gastric fluids into the pharynx until the endotracheal tube cuff is inflated.
4. Direct Visualization & Laryngoscopy: Use a lighted laryngoscope (Macintosh or Miller blade) in every case to visualize the arytenoid cartilages and vocal folds directly.
5. Immediate Cuff Inflation: Inflate the endotracheal tube (ETT) cuff immediately with a syringe until a seal is achieved (no leak heard when the reservoir bag is squeezed to 15-20 cmH2O). Verify placement with capnography (EtCO2 waveform).
6. Continuous Suction Availability: Always have an operational, wide-bore suction unit (Yankauer tip) plugged in and immediately reachable at the induction station before administering any drug.