INSPIRED-TO-ALVEOLAR ANESTHETIC RELATIONSHIP
Of the steps between delivered and brain anesthetic partial pressures,
none is more pivotal than that between the inspired and alveolar gases. The alveolar
partial pressure governs the partial pressures of anesthetic in all body tissues;
all ultimately approach and equal the alveolar partial pressure. By use of high
inflow rates (e.g., a non-rebreathing system), the anesthetist can precisely control
the inspired partial pressure of anesthetic.
Effect of Ventilation
Two factors determine the rate at which the alveolar concentration
of anesthetic (FA) rises toward the concentration
of inspired anesthetic (FI): the inspired concentration
(see "Concentration Effect") and alveolar ventilation. The unopposed effect of ventilation
on induction rapidly increases the alveolar concentration (i.e., FA/FI
ratio quickly approaches 1). This occurs with preoxygenation, which normally produces
a 95% or greater washin of oxygen in 2 minutes or less when a non-rebreathing (or
high-inflow-rate) system is used ( Fig.
5-1
).
Figure 5-1
The washin of oxygen was determined in two patients breathing
from a mask. A 63% change is obtained in about 0.5 minute, an 86% change in about
1 minute, and a 95% to 98% change in 2 minutes. (From Eger EI II, unpublished
data, 1992.)
However, the rapid washin of oxygen exceeds that of potent inhaled
anesthetics. The solubility of anesthetics far surpasses that of oxygen (or nitrogen),
causing uptake of a substantial anesthetic by the pulmonary blood flow. This uptake
opposes the effect of ventilation to increase the alveolar anesthetic concentration.
A balance between the delivery of anesthetic by ventilation and its removal by uptake
determines the FA/FI
ratio. The relationship is a simple one. For example, if uptake removes one third
of the inspired anesthetic molecules, FA/FI
equals 0.67; if uptake removes two thirds of the inspired molecules, FA/FI
equals 0.33.
