What is preoxygenation and why is it performed?

• A. Breathing 100% oxygen before induction to build oxygen reserves and reduce hypoxemia during apnea.
• B. Breathing room air to acclimate; to avoid oxygen toxicity.
• C. Administering oxygen only after induction; to prevent hyperventilation.
• D. Breathing 50% oxygen with nitrogen to create a reserve; to maintain blood gas.

Answer: (A)

Preoxygenation is the deliberate replacement of the air in the lungs with oxygen before induction of anesthesia, to maximize the oxygen stored in the lungs. When you breathe 100% oxygen, the nitrogen in the lungs is flushed out and the functional residual capacity becomes filled with oxygen. This creates a larger reservoir of oxygen that can be drawn on during the period of apnea that follows induction, buying time before oxygen saturation drops.

That’s why breathing 100% oxygen before induction is the best approach: it directly increases the body's oxygen reserves and reduces the risk of hypoxemia during apnea. Breathing room air wouldn’t build those reserves, administering oxygen after induction misses the protection during the critical apnea period, and using a mix with nitrogen (like 50% oxygen with nitrogen) would not maximize the oxygen reservoir.