What is indicated by an oxygen saturation of 90% on 100% oxygen (FiO2 1.0)?

An oxygen saturation of 90% while on 100% oxygen (FiO2 1.0) indicates that the patient is experiencing severe hypoxemia. Under normal circumstances, when a patient is receiving 100% oxygen, one would expect their oxygen saturation levels to be much higher, typically close to or at 100%. The fact that the saturation is only 90% suggests that there is a significant impairment in the body's ability to use oxygen, which is characteristic of severe hypoxemia.

In individuals with normal respiratory function, gas exchange is efficient, and oxygen saturation levels would be expected to reflect that efficiency, typically remaining well above 90% even when on lower levels of supplemental oxygen. Therefore, a reading of 90% in this context points to a deeper issue, often related to conditions such as acute respiratory distress or significant lung pathology, which adversely affects the patient's oxygenation despite the high concentration of oxygen being delivered.

Effective oxygenation would imply that the lungs are properly delivering oxygen to the bloodstream, resulting in higher saturation levels. Similarly, normal respiratory function would indicate that the oxygen transport system is functioning properly without significant distress. Minor respiratory distress would not typically lead to such low oxygen saturation levels when 100% oxygen is being administered