Anesthesia Student Survival Guide.pdf - Index of

64 ● AnesthesiA student survivAl Guide
The patient is otherwise healthy and has a normal appearing airway, and
you judge that maintaining the airway by mask will be successful. You agree
to induce anesthesia by inhalation. The patient has an IV and standard
monitors are in place.
Which inhalation agent will you choose?
The ideal agent would have several properties. It would be relatively potent,
so that a high multiple of the minimum alveolar concentration (MAC)
could be delivered by the vaporizer during induction. It would have low
solubility, so that the “tank” needed to be filled before the brain concentration
reaches that needed for anesthesia would be small. Importantly for
inhalation induction in an awake patient, it would be pleasant smelling
and would not irritate the airway. Of the available drugs in clinical practice
today, halothane, nitrous oxide, and sevoflurane are not pungent, and are
therefore potentially suitable for such “mask” induction. Nitrous oxide is
not potent and indeed at 1 atmosphere the MAC exceeds 100%, meaning
it is not possible to fully anesthetize a patient with nitrous oxide alone. It
is, however, insoluble and thus has a rapid uptake into the brain. Halothane
is potent but much more soluble than the other agents, making inhalation
induction slow. Sevoflurane is relatively potent (a commercial vaporizer can
deliver approximately 4 MAC inhaled agent) and is of low solubility, making
it the preferred choice for most anesthesiologists.
Would a combination of more than one inhaled agent offer any advantage?
Theoretically, adding nitrous oxide should help speed the induction with
sevoflurane. This is because of the two-part “second gas effect.” First, the rapid
uptake of nitrous oxide from the alveoli will concentrate sevoflurane there,
effectively increasing the inhaled concentration. Second, this same uptake will
entrain more gas from the trachea (which contains sevoflurane in the case
of inhalation induction), effectively increasing alveolar flow of this
“second” gas. These physiologic effects have been conclusively demonstrated
in research studies. However, in practice, their effect on clinical
induction is minimal. Indeed, randomized trials comparing inhalation
induction with sevoflurane in oxygen vs. in N 2 O plus oxygen have demonstrated
no difference in the rate of induction.