A healthy 65 -year-old man with a tidal volume ( $\mathrm{VT}$ ) of $0.45 \mathrm{~L}$ has a breathing frequency of 16 breaths/min. His arterial $\mathrm{PCO}_{2}$ is $41 \mathrm{~mm} \mathrm{Hg}$, and the $\mathrm{PCO}_{2}$ of his expired air is $35 \mathrm{~mm} \mathrm{Hg}$. What is his alveolar ventilation?

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Quizplus · Accepted Answer

[ $\mathrm{I}$ A $5 \mathrm{~b}, 6 \mathrm{~b}$ ]. Alveolar ventilation is the difference between tidal volume ( $\mathrm{VT})$ and dead space multiplied by breathing frequency. $\mathrm{V}_{\mathrm{T}}$ and breathing frequency are given, but dead space must be calculated. Dead space is $\mathrm{V}_{\mathrm{T}}$ multiplied by the difference between arterial
$\mathrm{PCO}_{2}$ and expired $\mathrm{PCO}_{2}$ divided by arterial $\mathrm{PCO}_{2}$. Thus: dead space $=0.45 \times(41-35 / 41)=0.066 \mathrm{~L}$. Alveolar ventilation is then calculated as: $(0.45 \mathrm{~L}-0.066 \mathrm{~L}) \times 16$ breaths $/ \mathrm{min}=6.14 \mathrm{~L} / \mathrm{min}$.

A Healthy 65 -Year-Old Man with a Tidal Volume VT\mathrm{VT}VT ) Of

A Healthy 65 -Year-Old Man with a Tidal Volume $\mathrm{VT}$ ) Of