Inhalation Anesthetics Overview and Uptake Factors

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.