Correct!
3. Hypoventilation

There are multiple causes of hypoxemia (Table 1).

Table 1. Causes of hypoxemia.

• Ventilation-perfusion mismatch (vascular anomaly or obstruction, alveolar filling anomalies)
• Right to left shunting (congenital heart, pulmonary arteriovenous malformations, lobar lung collapse)
• Hypoventilation (drugs, neuromuscular, myopathic, obesity, chronic lung disease)
• Decreased diffusion (interstitial lung disease, pulmonary hypertension)
• Low FiO2 (higher altitudes)
• Low mixed venous O2 (shock)

Of these ventilation-perfusion mismatch, where blood flows to poorly ventilated areas in the lung, is the most common. Some debate the existence of decreased diffusion at least in interstitial lung disease. Hypoventilation is incorrect because the alveolar-arterial gradient (A-a gradient) is not normal as indicated by hypoxemia with a normal of low pCO2.

The patient was placed on 100% oxygen and the ABGs were repeated. There was minimal change.

Based on this observation, which of the following is the most likely cause of the patient's hypoxemia? (Click on the correct answer to proceed to the third of six panels)

1. Low FiO2
2. Right to left shunting
3. Shock
4. Ventilation-perfusion mismatch
5. None of the above

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