Gas Exchange

A patient with consolidated pneumonia may remain profoundly hypoxaemic despite high inspired oxygen, while a patient with pulmonary embolism may initially maintain SpO2 by increasing ventilation. These presentations reflect different failures in gas exchange. The bedside task is to identify whether the main problem is uneven ventilation and perfusion, blood bypassing ventilated alveoli, ventilation without perfusion, inadequate overall alveolar ventilation, or impaired diffusion. That mechanism predicts whether oxygen will help, whether carbon dioxide will rise, which test is useful, and how quickly respiratory support must escalate. Interpret the oxygen trend together with work of breathing, mental status, respiratory rate, and blood-gas results rather than treating a single monitor value as the diagnosis.