Which of the following best describes the relationship between cardiac output
and speed of induction in infants?
a) Infants have a decreased cardiac output compared to adults, and thus an
increased induction speed.
b) Infants have an increased cardiac output compared to adults; this explains the
observed increase in induction speed.
c) Infants have similar cardiac output (per kilogram); this is unrelated to
induction speed.
d) Infants have faster induction speeds despite having increased cardiac output.
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Answer
Answer d.
Normally the speed of an inhalational induction is inversely related to
cardiac output (CO); that is, it is increased with a low cardiac output and slowed
down by a higher CO. This is counterintuitive, as one would think that with a low
CO the pick-up of volatile anesthetic would be slower. However, low CO means
that the blood returning to the heart is coming back with more vapor in it (which
raises the FA/FI ratio). In infants, the Vessel Rich Group receives a much higher
percentage of CO (18% vs. 8%); thus, the blood returning to the heart rapidly
equilibrates and comes back at higher concentrations, raising the FA/FI similarly
to the mechanism described above for low CO.
Notes
- This question originally printed in the Pediatric Anesthesiology Review Topics kindle book series, and appears courtesy of Naerthwyn Press, LLC.
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