Factors Affecting the Rate of Development of Anesthetic Concentration in the Lung

Gases diffuse from areas of high partial pressure to areas of low partial pressure; thus, the tension of anesthetic in the alveoli provides the driving force to establish brain tension. In fact, the tension of anesthetic in all body tissue will tend to rise toward the lung tension as equilibrium is approached. Consequently, factors that control or modify the rate of accumulation of anesthetic in the lung (e.g., rate of gas delivery, uptake of gas from the lung into the pulmonary circulation) will simultaneously influence the rate at which tension equilibria in other body compartments is established.

Graphs of the alveolar tension plotted against time are used in this chapter to illustrate the changes in lung partial pressure as anesthetic is inhaled. Only a fraction of total lung gases are exchanged during one breathing cycle. Therefore, the volume of gases already in the lung dilutes the first breath of anesthetic (breathing cycle 1 in Fig. 25.3). In subsequent breathing cycles, the alveolar tension will continue to rise toward the inspired level along an exponentially declining curve. The net change of anesthetic tension becomes smaller with each breathing cycle, and the curve of alveolar tension will approach the inspired level more slowly.

The alveolar tension-time curve always declines in an exponential manner, but the position of the curve can be greatly affected by the rate of delivery of anesthetic gases and the rate of their uptake into the pulmonary circulation. For this reason, it is important to consider factors that modify or regulate delivery and uptake.

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