KEY POINTS

1. If unopposed, ventilation produces a rapid change (in and out) of gases in the lungs.
2. When the delivered concentration of anesthetic is small (<10%), uptake opposes the effect of ventilation to quickly change the alveolar concentration of anesthetic, and the relationship between the inspired (F_I ) and the alveolar (F_A ) anesthetic concentrations (i.e., the FA/FI ratio) is simple. If uptake removes 80% of the anesthetic delivered by ventilation, the FA/FI ratio is 0.2.
3. Uptake from the lungs is the product of three factors: blood solubility, cardiac output, and the partial pressure driving anesthetic from one phase (e.g., lung) into a second phase (e.g., venous blood).
4. Blood solubility (i.e., blood-gas partition coefficient) differs greatly among anesthetics, and solubility is a primary factor that determines the FA/FI ratio and clinically relevant issues such as the rates of induction of and recovery from anesthesia: lower solubility, faster recovery.
5. Three tissue groups form depots for anesthetic within the body: the vessel-rich group (VRG), the muscle group (MG), and the fat group (FG). Having the greatest perfusion and limited solubility, the VRG equilibrates rapidly (about 8 minutes). Having a lower perfusion, the MG equilibrates more slowly. Having an enormous capacity (i.e., solubility), the FG equilibrates slowest. The differences in perfusion and equilibration determine the shape of the FA/FI ratio.
6. The inspired inhaled anesthetic concentration also determines the rate of rise of the alveolar concentration toward the concentration inspired (i.e., FA/FI ratio). The greater the inspired concentration, the more rapid is the rise in the FA/FI ratio (i.e., the less the impact of uptake). This is called the concentration effect.
7. The concentration effect results from two factors: a concentrating of residual gases and an increase in inspired ventilation. These two factors equally affect other gases given concurrently (i.e., second gas effect).
8. Anesthetic metabolism and intertissue diffusion of anesthetics also influence uptake. Metabolism is minimal for desflurance and isoflurane, modest for sevoflurane, and substantial for halothane.
9. Changes in physiologic variables (e.g., ventilation, circulation, distribution of circulation, ventilation-perfusion abnormalities) influence the FA/FI ratio, and the magnitude of this influence is usually governed by solubility.
10. Nitrous oxide presents unique kinetic problems. It increases the volume or pressure within gas-containing spaces in the body.
11. Low fresh gas inflow rates have increasingly become the norm because of the economy associated with their use. They also provide advantages of decreased pollution, heat conservation, and increased humidification. They can, however, increase rebreathing of potentially toxic degradation products.
12. Through rebreathing, anesthetic circuits can exaggerate the impact of differences in solubility on the rate of rise (i.e., FA/FI ratio) and required delivered concentration of inhaled anesthetics. Least affected are the least soluble anesthetics.