Pharmacology Notes

Inhalation Anesthetics
Certain drugs that are gases or volatile liquids at room temperature are administered by inhalation in combination with air or oxygen. Th e only gas used routinely for anesthesia is nitrous oxide, commonly called laughing gas. It is usually administered in combination with oxygen. Nitrous oxide provides analgesia equivalent to 10 mg of morphine sulfate but may cause occasional episodes of nausea and vomiting. Volatile liquids are converted into a vapor and inhaled to produce their anesthetic effects. Commonly administered volatile agents are halothane, enfl urane, and isofl urane.

Mechanism of Action
Inhaled general anesthetics are all very lipophilic (able to dissolve much more easily in lipids than in water). When the lipophilic anesthetic enters the lipid membrane, the whole membrane is slightly distorted and closes the sodium channels, causing a marginal blockage, which prevents neural conduction.

Indications
Volatile anesthetics are rarely used as the sole agents for both induction and maintenance of anesthesia. Most commonly, they are combined with intravenous agents in regimens of so-called balanced anesthesia. Of the inhaled anesthetics, nitrous oxide, desfl urane, and sevofl urane are the most commonly used in the United States.

Adverse Effects
Nitrous oxide at higher doses causes anxiety, excitement, and aggressiveness. It also produces nausea, vomiting, and difficulty in breathing. Volatile anesthetics may cause headache, shivering, muscle pain, mental or mood changes, sore throat, and nightmares.

Contraindications and Precautions
Inhaled general anesthetics are contraindicated in patients with known hypersensitivity to these agents. They are also contraindicated in patients who have received monoamine oxidase inhibitors (MAOIs) within the previous 14 days. Th ey should not be used by those who are intolerant to benzodiazepines, or have myasthenia gravis, acute narrowangle glaucoma, acute alcohol intoxication, status asthmaticus, and acute intermittent porphyria. Inhaled general anesthetics should be used cautiously during pregnancy,
and in children younger than 12.

Drug Interactions
Inhaled general anesthetic drugs may interact with levodopa and increase the level of dopamine in the CNS. Skeletal muscle weakness, respiratory depression, or apnea may occur if halothane is administered with polymyxins, lincomycin, or aminoglycosides.