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ARDS: P/F Ratio Explained

Acute respiratory distress syndrome (ARDS) is defined as an acute process, characterized by widespread inflammation and diffuse alveolar destruction in the lungs, resulting in a moderate to severe loss of lung function. If you remember ARDS got a lot of attention during the pandemic. Those global stories of patients maxing out the ventilator settings and dying.... yep ARDS secondary to a viral pneumonia in most cases. You see the "S" in ARDS stands for syndrome which means that this condition has widespread and systemic effects on the body. Look for bilateral issues that in worst case scenarios can manifest other problems across other organ systems. ARDS isn't a stand-alone disease, but rather a consequence of another underlying problem. The list of pathologies and underlying causes is vast and can differ quite dramatically.

ARDS severity and subsequent mortality can be partially determined by calculating a patient's P/F ratio. Some experts question the utility of the ratio as the only inclusion criteria as ventilator settings can alter the ratio and potentially exclude patients incorrectly. However, the vast majority of health systems still track and trend this ratio over the patient's long + complex hospital course.

Their PaO2 (via ABG) is divided by their inspired O2 (remember room air (RA) would be considered an FiO2 of 21%). You can also estimate what a patients PaO2 should be by multiplying their FiO2 x 5. In patients that are free from disease a normal PaO2 should be around 80-100 mmHg on RA. When this math shakes out, you will find a normal P/F ratio should be > 400. Be sure when dividing that you convert the denominator (FiO2) to a decimal.

The P/F ratio is calculated in most intensive care units for the most critically ill ventilator patients. This simple calculation is essentially determining how much of the supplemental oxygen we are giving is actually reaching the patients' blood/tissues.

Ratios in the 250 range would constitute mild ARDS. Ratios in the 150 range would constitute moderate ARDS. Lastly ratios < 100 would indicate severe ARDS where the mortality rate would climb to nearly 50%.... not very good odds. Below I have included a random example from MD Calc so you can see this play out. (82/.50) = 164. If we increase the inspired O2 and the PaO2 fails to climb the ratio also falls and the ARDS is progressing.


Damage to the alveoli and neighboring capillaries reduces the ability of the lungs to send oxygen into the blood. This happens because the lung injury causes fluid to leak into the spaces between the capillaries and the alveoli. Pressure on the alveoli increases, and eventually fluid gets in there, too. 

This is what gives ARDS its characteristic trait—accumulation of fluid in the lungs, causing the alveoli to collapse. This leads to a series of cascading problems, each further decreasing the lungs’ capacity to move oxygen into the blood and directly impacting the body’s tissues and organs. A viscous cycle that is hard to stop.

What’s more, ARDS also triggers an immune response. The injury causes a release of cytokines—a type of inflammatory protein. Problems arise when some of these proteins and cells leak into nearby blood vessels and, via the circulatory system, are sent throughout the body, causing inflammation in other organs. This inflammation, in combination with low levels of blood oxygen, can lead to such problems as organ failure and sometimes multiple organ failure. 

I hope you enjoyed this deep dive into an important respiratory concept. Be sure to join me next week when we tackle another interesting topic that also has systemic sequalae -- Rhabdomyolysis.

May 13, 2024

Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP

Pass with PASS, LLC.

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