Anesthetic Induction and Maintenance

Introduction to Anesthesia

Stages of Anesthesia

The progression of anesthesia from consciousness to unconsciousness is classified under different stages, with the Gwedel classification being most widely utilized.

  • Stage 1: Analgesia (Pain relief) and Amnesia (Loss of memory)

  • Stage 2: Delirium (Excitement)

  • Stage 3: Surgical anesthesia (Optimal stage for surgery)

  • Stage 4: Overdose or dangerous depth, nearing death

Methods of Induction

  1. Intravenous (IV) Induction: The primary route of induction which is reliable if the catheter is in place.

  2. Intramuscular (IM) Induction: Suitable for animals less tolerant of IV, especially in wildlife capture situations.

  3. Inhalation Induction: Less used in companion animals due to stress but applicable in safe, restrained environments (e.g., birds).

  4. Preoxygenation: Critical step to prevent desaturation, using either flow-by or breathing masks to deliver 100% oxygen for five minutes before induction.

Induction Agent Selection Criteria

The selection of an induction agent should consider:

  • Route of Administration: Some agents like propofol require IV, while others like alfaxalone can be used IM.

  • Onset Time: Quick-acting agents are preferred in emergencies.

  • Species and Legal Regulations: Especially relevant in food-producing animals or horses.

  • Safety Considerations: Sometimes heavier doses may be necessary to maintain control and ensure safety for both the animal and the personnel.

  • Environmental Factors: Influences choice based on procedure type and location.

Common Induction Agents

1. Inhalant Agents

  • Sevoflurane

  • Isoflurane

  • Desflurane: Not commonly used due to greenhouse gas effects.

2. Injectable Agents

  • Propofol

    • Dosing: 4-6 mg/kg without premedication, 2-4 mg/kg with premedication

    • Known for its GABAergic effects and extrahepatic metabolism.

  • Alfaxalone

    • Dosing: 2-5 mg/kg IV, usually not recommended for short procedures due to recovery issues

    • Primarily metabolized by the liver.

  • Ketamine

    • Routes: IV, IM, and subcutaneous with various benefits including analgesia when combined with benzodiazepines.

  • Thiopental

    • Ultra-short acting with a quick onset but not preferred generally due to induction speed and risk of seizures.

  • Less Common Agents:

    • Telazol, Zolazepam, MS222 (used in fish) and others.

Pharmacodynamics and Pharmacokinetics

  • Pharmacodynamics: Effects of drugs on the body, focusing on altering the central nervous system, cardiovascular system, and respiratory system.

  • Pharmacokinetics: How the body processes drugs, including absorption, distribution, metabolism, and excretion.

Central Nervous System Effects

  • Most agents diminish CNS activity, with considerations for intracranial pressure (ICP).

  • Cumulative depression of cerebral metabolic requirements generally aids in managing patients with intracranial pathologies.

Respiratory Effects

  • Induction agents typically result in hypoventilation or apnea, elevating risks of hypoxemia.

  • Acoustic Breathing Patterns: Certain agents like ketamine may induce irregular breathing but usually resolve.

  • Etomidate: Known for minimal respiratory effects, although it's tricky for administration.

Cardiovascular Effects

  • Cardiovascular stability is crucial, with most induction agents leading to decreased mean arterial pressure and heart rate.

  • Equations:

    • Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)

    • Mean Arterial Pressure (MAP) = CO x Systemic Vascular Resistance (SVR)

  • Alfaxalone and Ketamine: Notably preserve cardiovascular stability better than others.

Co-Induction Considerations

  • Co-induction practices can improve outcomes by using a combination of agents, usually a benzodiazepine or short-acting opioids to lessen doses of primary induction agents and mitigate cardiovascular impacts.

  • The timing of administration is crucial to avoid poor recovery quality due to residual effects, particularly with Ketamine.

Practical Applications and Considerations

  • Selection Based on Health and Disease: Medication strategies should be adjusted based on the health status and psychiatric conditions; lower doses may be necessary in compromised patients.

  • One Health Relevance: Understanding pharmacology ensures safer anesthesia practices and minimizes reliance on volatile agents to reduce environmental impact.