A PaO2 of 45 mmHg indicates which ventilatory concern?

• A. Hyperoxemia
• B. Hypoxemia
• C. Normal oxygenation
• D. Severe hypercapnia

Answer: (B)

Understanding arterial oxygen tension helps determine how well oxygen is being transferred into the blood. Normal PaO2 in arterial blood is about 80–100 mmHg, and clinicians consider hypoxemia when PaO2 falls below the level needed to sustain adequate tissue oxygenation (often <60 mmHg is used as a threshold for clinically significant hypoxemia).

A PaO2 of 45 mmHg is well below that threshold, indicating hypoxemia. This means the blood is not adequately oxygenated despite ventilation, signaling the need to improve oxygenation or ventilation (for example, by increasing FiO2, addressing V/Q mismatch, diffusion limits, or shunt). It is not hyperoxemia (which would be too high PaO2) and not normal oxygenation, and it does not describe hypercapnia, which relates to elevated PaCO2, not PaO2. In practice, this level would prompt escalation of support to raise arterial oxygenation toward safer targets.