You might want to read the alveolar gas equation and gas basics sections to brush up on some of the concepts in this entry.


Acute respiratory distress syndrome (ARDS) represents inflammation of the airspaces causing respiratory failure. A variety of pulmonary and extrapulmonary insults can lead to ARDS. Treatment involves treating the underlying cause and careful ventilation strategies. Until very recently, a milder form of ARDS – acute lung injury (ALI) – was recognised. We’re moving away from making this distinction towards viewing ARDS as a spectrum from mild to severe (much like we do with asthma).

The history of ALI/ARDS and recent changes in definitions

Until very recently, acute lung injury (ALI) and Acute Respiratory Distress Syndrome (ARDS) were considered different ends of a clinical spectrum of inflammation of the airspaces of the lung causing respiratory failure. The distinction, which you will still find in many texts and online resources for some time, was that at a certain point, the ratio of the partial pressure of O2 in arterial blood (PaO2) the fraction of O2 inhaled (FiO2) distinguished ALI from ARDS. That’s to say that you administer lots of supplemental O2 and still don’t achieve a satisfactory oxygenation of blood. The old ALI/ARDS distinction was:

PaO2/ FiO2 < 40 KPa for ALI

PaO2/ FiO2 < 27 KPa for ARDS

What do these numbers mean? Let’s consider a normal, healthy set of lungs doing their job breathing room air (21% O). The FiO2 is 0.21 (the fraction – not percentage – of inspired O2 in room air) and the PaO2 is probably around 13 on a good day. So, the PaO2/ FiO2 ratio (often abbreviated in jargon as the P/F ratio) will be:

That’s nowhere in the realm of ALI or ARDS (< 40 kPa). So, to meet old distinction for ALI, when breathing room air a patient would need to be distinctly hypoxaemic with a PaO2 of 8.4 or less:

Such a patient would also need to have the relevant history for lung injury. That PaO2/ FiO2 ratio only suggests respiratory failure, it doesn’t necessarily point to what we used to call ALI. Asthma, COPD or any number of other respiratory problems might give you a similar picture.

To meet the old definition of ARDS on room air, a patient would need to have a PaO2 of 5.6 kPa or less, which is essentially the same as normal venous blood:

Of course, the point of this old definition was that patients would never be allowed to reach such low PaO2 values; they’d be receiving supplemental O2. So a patient with the old definition of ARDS might be someone getting an FiO2of 0.4 (i.e. 40% O2 through a mask in which you could be sure about FiO2, such as a venturi mask) with a semi-respectable PaO2 of 10 kPa:

This was the point of the definition: there comes a point when you deliver a lot of O2 into the lungs and it doesn’t appear in the blood. That’s respiratory failure. When the PaO2/ FiO2 ratio is below 27 the outcomes were known to be statistically worse. One of the problems with this ALI/ARDS definition was that it didn’t take into consideration the different kinds of ventilatory support (from none to CPAP to intubation) that patients might be receiving. Invasive and non-invasive ventilation strategies have come a long way since 1994 when this original distinction was made.

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