A 12-year-old boy has a severe asthmatic attack with wheezing. He experiences rapid breathing and becomes cyanotic. His arterial $\mathrm{PO}_{2}$ is $60 \mathrm{~mm} \mathrm{Hg}$ and his $\mathrm{PCO}_{2}$ is $30 \mathrm{~mm} \mathrm{Hg}$.
-Which of the following statements about this patient is most likely to be true?

Question

Quizplus · Accepted Answer

[VIII B 2 a]. The patient's arterial $\mathrm{PCO}_{2}$ is lower than the normal value of $40 \mathrm{~mm} \mathrm{Hg}$ because hypoxemia has stimulated peripheral chemoreceptors to increase his breathing rate; hyperventilation causes the patient to blow off extra $\mathrm{CO}_{2}$ and results in respiratory alkalosis. In an obstructive disease, such as asthma, both forced expiratory volume $\left(\mathrm{FEV}_{1}\right)$ and forced vital capacity ( $\left.\mathrm{FVC}\right)$ are decreased, with the larger decrease occurring in $\mathrm{FEV}_{1}$. Therefore, the $\mathrm{FEV}_{1} / \mathrm{FVC}$ ratio is decreased. Poor ventilation of the affected areas decreases the ventilation/perfusion (V/Q) ratio and causes hypoxemia. The patient's residual volume (RV) is increased because he is breathing at a higher lung volume to offset the increased resistance of his airways.

A 12-Year-Old Boy Has a Severe Asthmatic Attack with Wheezing PO2\mathrm{PO}_{2}PO2​

A 12-Year-Old Boy Has a Severe Asthmatic Attack with Wheezing $\mathrm{PO}_{2}$