Factors Governing Increases in Alveolar Anesthetic
Partial Pressure
Ventilation, solubility, and distribution of blood flow have a
combined impact on the increase in the alveolar anesthetic partial pressure (i.e.,
the rise in the FA/FI
ratio). For all agents, FA/FI
initially increases rapidly, regardless of agent solubility[11]
[12]
( Fig.
5-2
) because of the absence of an alveolar-to-venous anesthetic partial
pressure difference (i.e., there is no anesthetic in the lung to create a gradient)
and the absence of uptake in the first moment of induction. The capacity of ventilation
to increase FA/FI
is unopposed. Delivery of anesthetic to the alveoli by ventilation increases the
alveolar-to-venous partial pressure difference. The resulting increase in uptake
increasingly opposes the effect of ventilation to drive the
Figure 5-2
The rise in alveolar (FA)
anesthetic concentration toward the inspired (FI)
concentration is most rapid with the least soluble anesthetics, nitrous oxide and
desflurane, and slowest with the most soluble anesthetic, halothane. All data are
from human studies. (Data from Yasuda N, Lockhart SH, Eger EI II, et al:
Kinetics of desflurane, isoflurane, and halothane in humans. Anesthesiology 74:489–498,
1991, and from Yasuda N, Lockhart SH, Eger EI II, et al: Comparison of kinetics
of sevoflurane and isoflurane in humans. Anesth Analg 72:316–324, 1991.)
alveolar concentration upward. Ultimately, removal by uptake balances the input
by ventilation. The FA/FI
ratio at which the balance is struck depends on the solubility factor in the uptake
equation (see earlier equation describing anesthetic uptake). A higher solubility
produces more uptake for a given alveolar-to-venous partial pressure difference.
Hence, the initially rapid rise in FA/FI
halts at a lower level with a more soluble agent. This process results in the first
"knee" in the curve, which is higher for desflurane than for sevoflurane, higher
for sevoflurane than for isoflurane, and higher for isoflurane than for halothane.
The position of nitrous oxide is discussed later (see "Concentration Effect").
The FA/FI ratio indicates
the percent of anesthetic removed by uptake. An FA/FI
ratio of 0.33 indicates that 67% of the anesthetic is being taken up; an FA/FI
ratio of 0.67 indicates that 33% of the anesthetic is being taken up.
The balance struck between ventilation and uptake does not remain
constant. FA/FI
continues to rise, but at a slower rate than in the first minute. This secondary
rise results from the progressive decrease in uptake by the VRG, a decrease to an
inconsequential amount after 8 minutes. By about 8 minutes, three fourths of the
cardiac output returning to the lungs (i.e., the blood from the VRG) contains nearly
as much anesthetic as it had when it left the lungs. The consequent rise in venous
anesthetic partial pressure decreases the alveolar-to-venous partial pressure difference
and therefore the uptake, allowing ventilation to drive the alveolar concentration
upward to a second knee at roughly 8 minutes.
After 8 minutes, the MG and FG become the principal determinants
of tissue uptake. The slow decrease in the anesthetic partial pressure difference
between arterial
blood and MG and FG produces the gradual ascension of the terminal portion of each
curve in Figure 5-1
, an
ascension determined by the progressive equilibration of the MG and, to a lesser
extent, the FG with the arterial anesthetic partial pressure. If the graphs were
extended for several hours, a third knee would result from equilibration of the MG.
Uptake after that time would principally depend on the partial pressure gradient
between arterial blood and the FG.
