Inhalation Anesthetics Overview

Study Notes

Inhalation anesthetics are primarily used for maintenance of anesthesia, allowing rapid alteration of depth by adjusting inhaled concentration.
The goal is to achieve a constant partial pressure of anesthetic in the brain (Pbr) by maintaining equilibrium with alveolar pressure (Palv).

Inspired concentration (Fi): Higher inspired concentrations lead to increased alveolar concentrations and faster induction of anesthesia.
Alveolar ventilation: The rate and depth of ventilation impact the volume of fresh gases entering the alveoli and subsequently increase alveolar concentration (FA).
Time constant: Reflects the time required for gas flow to equal lung capacity; defined as FRC/V alveolar.
Functional Residual Capacity (FRC): Higher FRC results in slower gas wash-in due to pre-existing air volume.
FA/FI Ratio: A significant determinant of induction speed. Factors influencing this ratio include:
- Decreased blood anesthetic solubility increases FA/FI.
- Reduced cardiac output increases FA/FI.
- Increased minute ventilation elevates FA/FI.

Commonly known as laughing gas, a non-flammable, colorless gas with a sweet odor.
Poorly soluble in blood, facilitating rapid induction and recovery.
Typically combined with oxygen at concentrations of 30% to 70% for general anesthesia.
Safety profile: Minimal depression of respiratory function and cardiovascular system.
Side effects may include hallucinations, nausea, vomiting, and potential megaloblastic anemia with prolonged exposure.

A colorless, highly volatile liquid with a pungent odor; flammable and explosive.
MAC (Minimum Alveolar Concentration) is 2-3%.
Advantages: Significant CNS depression and excellent muscle relaxation.
Disadvantages: Flammable, can irritate mucous membranes, lead to breath-holding, and cause nausea and vomiting.

Colorless, non-flammable volatile liquid with a pleasant odor, but light-sensitive.
Produces rapid induction and quick recovery with low water solubility.
Affect cardiovascular system: Relaxes myocardium, decreases oxygen demand, induces vasodilation, and may cause hypotension with overdose.
Respiratory system: Non-irritating, dilates bronchioles, decreases secretions but can depress reflexes.
Risks: Potential for liver damage (Halothane Hepatitis) with repeated use.
MAC is 0.75, used for induction and maintenance in general anesthesia.

Cardiovascular: Myocardial relaxation, decreased cardiac output and heart rate, potential for severe hypotension with overdose.
Respiratory: Non-irritating, bronchodilation, reduced secretions, potential depression of airway reflexes.
Liver: Risk of hepatotoxicity with repeated halothane administration.

Modern inhalation agents aim for rapid induction, effective muscle relaxation, and minimal respiratory stimulation, balancing efficacy with safety profiles to mitigate post-operative nausea, vomiting, and other side effects.