Pao2 / Fio2 Ratio || #aetcm || #emergencymedicine||

Added: 2026-05-18

[00:00:05]Welcome to AT CM, the emergency medicine channel. In today's clinical update, an essential parameter for assessing oxygenation, the PAO2 to FAO2 ratio, commonly known as the PF ratio.

[00:00:19]The PF ratio is a measure of how effectively oxygen moves from the lungs into the bloodstream.

[00:00:26]It is calculated as PAO2 divided by FAO2, where PAO2 represents arterial oxygen pressure obtained from arterial blood gas analysis, and FAO2 represents the fraction of inspired oxygen.

[00:00:41]Normal PF ratio values range between 300 and 500.

[00:00:48]A value between 200 and 300 indicates mild impairment in oxygenation.

[00:00:55]A value between 100 and 200 indicates moderate impairment. A value less than 100 indicates severe impairment.

[00:01:05]Clinically, the PF ratio provides a more accurate assessment of oxygenation than oxygen saturation alone, especially in patients receiving supplemental oxygen.

[00:01:17]It is widely used in conditions such as acute respiratory distress syndrome, pneumonia, and critically ill patients requiring respiratory support.

[00:01:29]Let's look at two clinical scenarios.

[00:01:32]In the first scenario, a patient has an oxygen saturation of 98% but is receiving oxygen with an FAO2 of 0.5.

[00:01:43]Arterial blood gas shows a PAO2 of 80 mm of mercury.

[00:01:48]So, the PF ratio is 160. Despite a normal oxygen saturation, this indicates moderate impairment in oxygenation. This means the lungs are not efficiently transferring oxygen and the patient may require closer monitoring and escalation of support.

[00:02:09]In the second scenario, a patient has an oxygen saturation of 90% on room air with an FiO2 of 0.21.

[00:02:20]Arterial blood gas shows a PaO2 of 75 mm of mercury. The PF ratio is approximately 357 357. Despite a lower oxygen saturation, this suggests near normal oxygenation efficiency.

[00:02:37]This patient may not need aggressive respiratory intervention.

[00:02:42]So, the clinical message is oxygen saturation reflects how much oxygen is bound to hemoglobin, but the PF ratio reflects how well oxygen is actually moving from the lungs into the blood.

[00:02:56]And the key point is a normal SpO2 does not always mean normal oxygenation and a low SpO2 does not always indicate severe lung dysfunction.

[00:03:09]Thank you for watching.