Which of the following blood gas abnormalities is initially most suggestive of pulmonary edema?
A. Anoxia
B. Hypercapnia
C. Hyperoxygenation
D. Hypocapnia
Correct Answer: D. Hypocapnia
In an attempt to compensate for increased work of breathing due to hyperventilation, carbon dioxide decreases, causing hypocapnia. If the condition persists, CO2 retention occurs and hypercapnia results. Hypoxemia and hypocapnia occur in stages 1 and 2 of pulmonary edema because of a ventilation/perfusion (V/Q) mismatch. In stage 3 of pulmonary edema, right-to-left intrapulmonary shunt develops secondary to alveolar flooding and further contributes to hypoxemia.
Option A: Oxygen diffusion is impaired between the alveolus and the pulmonary capillaries. Causes are usually interstitial edema, interstitial inflammation or fibrosis. Clinical examples include pulmonary edema and interstitial lung disease.
Option B: In more severe cases, hypercapnia and respiratory acidosis are usually observed. The decision regarding intubation and the use of mechanical ventilation is frequently based on many clinical parameters, including oxygenation, ventilation, and mental status.
Option C: The alveolar epithelial and alveolar capillary endothelial cells are vulnerable targets for O2-free-radical-induced injury caused by hyperoxia. In acute lung injury (ALI) caused by hyperoxia, hyperpermeability of the pulmonary microvasculature causes flooding of the alveolus with plasma extravasations leading to pulmonary edema and abnormalities in the coagulation and fibrinolysis pathways promoting fibrin deposition.
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