If an ABG reads PaO2 = 58mmHg with a patient breathing 21% FiO2, what is the source of this blood and what would you expect SaO2 to be?

The correct answer indicates that the source of the blood is arterial blood and the expected saturation (SaO2) is around 90%.

In this scenario, a PaO2 reading of 58 mmHg represents hypoxemia, which means there is not enough oxygen in the blood. When the patient is breathing room air with an FiO2 of 21%, the expected SaO2 (oxygen saturation) will decrease significantly.

Typically, a PaO2 of 58 mmHg correlates with an SaO2 of approximately 90%. This is determined using the oxygen-hemoglobin dissociation curve, which shows how the saturation of hemoglobin with oxygen (SaO2) is dependent on the amount of dissolved oxygen (PaO2) in the blood. In this case, a PaO2 of 58 mmHg indicates the oxygen level is critically low, but it still allows for some saturation of hemoglobin, hence the 90%.

This scenario does not reflect mixed venous blood, which would indicate a much lower PaO2 and a corresponding saturation, making it important to identify the blood source accurately. The readings imply the blood sample was drawn from an arterial source because mixed venous blood would have very different gas exchange