TYK138

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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