Reclassifying Acute Respiratory Distress Syndrome.

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The ratio of PaO2 to FiO2 (P/F) defines ARDS severity and suggests appropriate therapies.


We investigated 1) whether a 150 mmHg P/F threshold within the range of moderate ARDS (100-200 mmHg) would define two, more homogeneous subgroups, 2) which criteria led the clinicians to apply ECMO in severe ARDS. Methods & Measurements: At 150 mmHg P/F threshold, moderate patients were split in mild-moderate (n=50) and moderate-severe (n=55). Severe ARDS patients (FiO2 not available in 3 patients ) were split in higher (n=63) and lower-FiO2 (n=18) at 80% FiO2 threshold.


Compared to mild-moderate ARDS, patients with severe-moderate ARDS had higher peak pressures, PaCO2 and pH. They also had heavier lungs, greater inhomogeneity, more non-inflated tissue, and greater lung recruitability. Within 84 severe ARDS patients (P/F lower than 100 mmHg), 75% belonged to the higher-FiO2 subgroup. They differed from the severe ARDS patients with lower FiO2 only in PaCO2 and lung weight. Forty-one of 46 patients treated with ECMO belonged to the higher-FiO2 group. Within this group, the ECMO patients had higher PaCO2 than the 22 non-ECMO patients. The inhomogeneity ratio, total lung weight and non-inflated tissue were also significantly higher.


Using the 150 mmHg P/F threshold gave a more homogeneous distribution of ARDS patients across the severity subgroups and identified two populations that differed in their anatomical and physiological characteristics. The patients treated with ECMO belonged to the severe ARDS group and almost 90% of them belonged to the higher-FiO2 subgroup.

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