During cardiac arrest, what percentage of oxygen should be administered?

• A. 100% oxygen
• B. 50% oxygen
• C. 21% oxygen
• D. 90% oxygen

Answer: (A)

Maximizing oxygen delivery during CPR is the key idea. When the heart isn’t circulating, tissues quickly become hypoxic, so giving the highest possible arterial oxygen content helps support brain and heart viability while chest compressions are in progress. That’s why 100% oxygen is recommended during cardiac arrest. Using a bag‑valve mask or an advanced airway with 100% O2 ensures the patient receives the maximum oxygen available, improving the chance of oxygen reaching vital organs.

Lower percentages—like ambient air (about 21%), or even 50% or 90%—don’t guarantee the same level of oxygen delivery and can leave tissues more hypoxic during resuscitation. After circulation is restored, clinicians often titrate oxygen to avoid hyperoxia, but during the arrest itself the priority is to deliver the most oxygen possible.