Metabolism of Anesthetic
Although percutaneous loss of anesthetic may not appreciably affect
anesthetic uptake, biodegradation can, particularly with agents that undergo extensive
biodegradation. Berman and colleagues[25]
found
that phenobarbital pretreatment in rats decreased the arterial level of methoxyflurane.
In humans, Carpenter and coworkers[26]
found that
one half of the halothane and three fourths of the methoxyflurane taken up was biodegraded.
Such biodegradation explains why the alveolar concentration of halothane decays
more rapidly on recovery from anesthesia than the alveolar concentration of isoflurane,
an anesthetic significantly less soluble in blood.[27]
[28]
In contrast to halothane or methoxyflurane,
isoflurane and desflurane resist biodegradation.[29]
[30]
For all potent inhaled anesthetics, anesthetizing
concentrations appear to saturate the enzymes responsible for anesthetic metabolism.
[31]
Because higher anesthetic partial pressures
exist during anesthesia, saturation of enzymes may limit the impact of metabolism
more during the washin than the washout of anesthetic. The combined effect of these
factors remains to be determined, but it appears that metabolism is not a significant
determinant of FA/FI
during anesthesia with isoflurane or desflurane.[11]
[12]
A small influence on the kinetics of sevoflurane
may exist, explaining why there is a more substantial difference between the FA/FI
for desflurane compared with sevoflurane during induction and maintenance of anesthesia
than for the parallel washout curves (FA/FA0
)
during recovery.[32]
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