Division of Pulmonary, Allergy, Critical Care and Sleep Medicine Division of the Department of Medicine, University of Minnesota, Minneapolis, MN, U.S.A.
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The pressure of arterial oxygen to fractional inspired oxygen concentration (PaO2/FIO2) ratio is a commonly used indicator of lung function in critically ill patients. For many years, physicians have relied on it to define and characterise the severity of the acute respiratory distress syndrome (ARDS), and this ratio is still a central element of the new ARDS definition (Berlin definition). In addition, clinicians utilise this ratio to track change in lung conditions, to set positive end expiratory pressure, to assess the response to different ventilatory strategies and/or to make decisions regarding the requirement for advanced supportive treatment modalities (e.g., paralysis, prone position, extracorporeal membrane oxygenation). Despite having the merit of simplicity and availability, the PaO2/FIO2 is more complex to interpret than being acknowledged and can at times be misleading. This risk is particularly present if one does not understand or consider the key determinants of the PaO2/FIO2 ratio in each individual patient and why this ratio may change over time. We review here the main determinants of PaO2/FIO2 ratio and discuss how the application of a few physiological key concepts can be used to optimise the management of patients with hypoxic respiratory failure.
We need a more individualised approach of hypoxic respiratory failure and ARDS. It is questionable that the new Berlin ARDS definition was the most required change to our approach of ARDS. One could argue that our patients could be better off, if we had moved away from trying to find commonality between very different conditions as the old and new ARDS definitions do. The ‘one size fits all’ approach tried for many years has not led to substantial progress. It may be high time for a different strategy and during the mean time, it may also be wise to use physiology as a compass to avoid the obvious mistakes associated with a cookbook approach.