Concentration Effect
The previous analysis ignores the impact of the concentration
effect on FA/FI.
The inspired anesthetic concentration influences the alveolar concentration that
may be attained and the rate at which that concentration may be attained.[13]
[14]
Increasing the inspired concentration accelerates
the rise in FA/FI.
At an inspired concentration of 100%, the rate of rise is most rapid, dictated solely
by the rate at which ventilation washes gas into the lung. At 100% inspired concentration,
uptake no longer limits the rise in FA/FI.
The cause of this extreme effect is readily perceived. At 100% inspired concentration,
the uptake of anesthetic creates a void that draws gas down the trachea. The additional
inspiration replaces the gas taken up. Because the concentration of the replacement
gas is 100%, uptake cannot modify the alveolar concentration. This explains why
FA/FI of nitrous
oxide in Figure 5-2
rises
more rapidly than that for desflurane despite the identity of their blood-gas partition
coefficients.
The concentration effect results from two factors: a concentrating
effect and an augmentation of inspired ventilation.[15]
Both are illustrated in Figure 5-3
.
The first rectangle (see Fig. 5-3A
)
represents a lung containing
Figure 5-3
A, The rectangle represents
a lung filled with 80% nitrous oxide plus 1% of a second gas. B,
Uptake of one half of the nitrous oxide does not halve the concentration of nitrous
oxide, and the reduction in volume thereby increases the concentration of the second
gas. C, Restoration of the lung volume by addition
of gas at the same concentration as that contained in A
increases the nitrous oxide concentration and adds to the amount of the second gas
present in the lung. (Adapted from Stoelting RK, Eger EI II: An additional
explanation for the second gas effect: A concentrating effect. Anesthesiology 30:273–277,
1969.)
80 volumes (80%) nitrous oxide. Uptake of one half of the nitrous oxide yields a
residual 40 volumes of nitrous oxide in a total of 60 volumes, a concentration of
67% (see Fig. 5-3B
). Uptake
of one half of the nitrous oxide does not halve the concentration, because the remaining
gases are concentrated in a smaller volume. If the void created by uptake is filled
by drawing more gas into the lungs (i.e., augmentation of inspired ventilation),
the final concentration equals 72% (see Fig.
5-3C
).
This explanation has been criticized as overly simplistic because
it ignores the realities of some aspects of ventilation.[16]
For example, if ventilation is controlled with a volume-limited respirator, augmentation
of inspired ventilation is limited to the period of the expiratory pause. If ventilation
is spontaneous, this limitation is minimized. In any event, the anesthesiologist
should be aware that although Figure
5-3
describes the basic factors governing the concentration (and second
gas effects as discussed later), the actual situation is more complex.
The impact of the concentration effect on FA/FI
may be thought of as identical to the impact of a change in solubility[17]
;
as the inspired concentration increases, the effective solubility decreases. At
50% inspired nitrous oxide, the FA/FI
rises as rapidly as the FA/FI
of an anesthetic that has one half the solubility of nitrous oxide and is given at
1% inspired concentration, and 75% inspired nitrous oxide acts as does an anesthetic
given at 1% that has one fourth the solubility of nitrous oxide.
