Which has greater influence on oxygenation and why?

• A. Both FiO2 and PEEP influence oxygenation: FiO2 directly increases oxygen fraction, while PEEP recruits alveoli to raise mean airway pressure.
• B. FiO2 only affects oxygenation.
• C. PEEP only affects oxygenation.
• D. Neither FiO2 nor PEEP influences oxygenation.

Answer: (A)

Oxygenation depends on both the amount of oxygen reaching the lungs and how effectively the lungs transfer that oxygen into the blood. Increasing FiO2 directly raises the fraction of inspired oxygen, so more oxygen is available for diffusion into the bloodstream. PEEP works by keeping alveoli open at end-expiration, recruiting collapsed alveoli, increasing functional surface area for gas exchange, and improving ventilation-perfusion matching. This rise in available alveolar units and better gas exchange efficiency translates into higher arterial oxygen tension. Because oxygenation is shaped by both delivering more oxygen and making more alveoli participate in exchange, both FiO2 and PEEP influence it, with FiO2 having a direct, immediate effect and PEEP providing a substantial, more lung-mechanic improvement when alveolar collapse is present.