Early hypoxemia detection with minimal FiO₂: Putting routine hyperoxygenation in anesthesia practice to the test

High inspired oxygen fractions (FiO₂) are routinely administered during mechanical ventilation in anesthesia in order to prevent hypoxemia. However, liberal oxygen use often exceeds physiological requirements, increasing the risk of oxygen toxicity and absorption atelectasis. Moreover, by sustaining normal-appearing oxygen saturation, high FiO₂ may obscure the presence of pulmonary congestion, edema, acute respiratory distress syndrome (ARDS), inadvertent endobronchial intubation, pneumothorax, and other conditions associated with impaired gas exchange. This approach may postpone the detection and treatment of underlying pathology caused by ventilation-perfusion (V/Q) mismatch or pure pulmonary shunt.

This narrative review revisits the core principles of Nunn's isoshunt diagrams as a physiological framework for understanding the relationships among shunt fraction, arterial oxygenation (PaO₂), and inspired oxygen fraction (FiO₂). It examines these diagrams' mathematical foundation, historical evolution, and subsequent refinements, emphasizing their diagnostic utility, particularly at lower FiO₂ levels. By integrating classical physiology with contemporary monitoring and clinical evidence we support a paradigm change away from regular hyperoxygenation toward an accurate, patient specific FiO₂ titration.

Maintaining the FiO₂ low, usually around 0.3, not only reduces the damage caused by hyperoxia but also reveals mild abnormalities in gas exchange that are concealed under high FiO₂ settings. By detecting abnormal gas exchange early, this personalized, physiologically informed approach to oxygen therapy in anesthesia challenges conventional approach and supports a more sophisticated, evidence-based practice, ultimately improving patient safety.