In ACS assessment with no dyspnea and no signs of heart failure, what noninvasive measurement helps guide oxygen therapy?

• A. Arterial blood gas with PaCO2
• B. Chest radiograph
• C. Noninvasively monitored oxyhemoglobin saturation
• D. Cardiac MRI

Answer: (C)

The key idea is using real-time, noninvasive monitoring of how well blood is carrying oxygen to guide oxygen therapy. Pulse oximetry gives the oxyhemoglobin saturation (SpO2) continuously, so you can titrate supplemental oxygen to keep SpO2 in a safe range. If SpO2 is adequate, you avoid unnecessary oxygen; if it’s low, you supplement to raise it.

Arterial blood gas with PaCO2 would provide detailed oxygen and ventilation data but requires an arterial blood sample and is more invasive, not needed to guide routine oxygen in a stable patient. A chest radiograph or a cardiac MRI doesn’t tell you the patient’s current oxygenation status in real time, so they aren’t used to guide oxygen therapy.