1. William, a 62 year old man, presents to the Emergency Room with fatigue, a productive cough, and a history of emphysema. On examination he has decreased air entry to the lower left lobe with scattered wheezes throughout, his respirations are rapid, he is barrel-chested, his lips and nail beds are cyanotic, and his sputum is green and blood streaked. He states he has a cough and increasing shortness of breath on exertion. He has recently quit smoking last week but has a pack a day habit for 40 years. A chest x-ray reveals left lower lobe pneumonia.

Question: William's blood gases reveal he is hypoxic because of his low PO2. Why would this happen with a diagnosis of pneumonia?

A. Acidosis depresses respirations: While acidosis can affect the respiratory drive, it is not the primary cause of hypoxemia in pneumonia. William’s low PO2 is due to impaired gas exchange rather than depressed ventilatory effort.

B. Oxygen diffusion is impaired by the congestion: In pneumonia, alveoli fill with inflammatory exudate, mucus, and cellular debris. This fluid buildup creates a barrier to oxygen diffusion from the alveoli into the pulmonary capillaries, resulting in hypoxemia and low arterial PO2.

C. Infection reduces the effectiveness of the heart: Cardiac function may be influenced by systemic infection, but William’s hypoxemia is primarily pulmonary in origin. The heart’s pumping ability does not directly explain decreased oxygenation in the alveoli.

D. Inflammatory exudates absorb oxygen from alveolar air: Exudates do not actively consume oxygen. Instead, they physically block gas exchange by filling alveolar spaces. The impaired diffusion, rather than absorption by exudates, leads to low oxygen levels in the blood.