The Hidden Measurement Problem Behind COVID-19 Hypoxia   During the height of the COVID-19 pandemic, clinicians encountered a puzzling trend: individuals with dangerously low arterial oxygen levels often appeared clinically stable and comfortable. While “silent” or “happy” hypoxia drew headlines, an equally critical issue— occult hypoxemia —posed a more subtle risk.  Occult hypoxemia  occurs when pulse oximetry readings overestimate true arterial oxygen saturation (SaO₂), mas