Chapter 25: General Anesthetics

ORGANIZATION OF CLASS

The state of general anesthesia is a drug-induced absence of perception of all sensations. Depths of anesthesia appropriate for surgical procedures can be achieved with a wide variety of drugs. General anesthetics are administered primarily by inhalation and intravenous (IV) injection. These routes of administration allow control of the dosage and time course of action.

For these drugs, understanding the principles of uptake, distribution, and elimination are the major focus, particularly for the inhaled anesthetics. The mechanism of action of most of the anesthetics is unknown. You should be able to recognize the names of the general anesthetics and know a few specific facts about these drugs (Figure 25–1).

UPTAKE AND DISTRIBUTION OF INHALATIONAL ANESTHETICS

The tension of a gas in a mixture is proportional to its concentration. Therefore, the terms tension and concentration are often used interchangeably. The term partial pressure is also used interchangeably with tension.

The level of general anesthesia is dependent on the concentration of anesthetic in the brain.

The blood–gas partition coefficient represents the ratio of anesthetic concentration in blood to the concentration in the gas phase. The blood–gas coefficient is high for very soluble agents and low for relatively insoluble anesthetics such as nitrous oxide.

The potential reservoir for relatively soluble gases is large and will be filled more slowly. Therefore, for soluble gases the rate at which the tension (partial pressure) in the arterial blood approaches the inspired partial pressure is slow. Also, the rate at which the brain partial pressure approaches the arterial partial pressure is slow. The opposite is true for more insoluble anesthetics.

Onset of anesthesia is also related to pulmonary ventilation, rate of pulmonary blood flow, tissue blood flow, and ...