Acute Respiratory Distress Syndrome - Diagnosis

Diagnosis

An arterial blood gas analysis and chest X-ray allow formal diagnosis by the below mentioned criteria. Although severe hypoxemia is generally included, the appropriate threshold defining abnormal PaO₂ has never been systematically studied. Note though, that a severe oxygenation defect is not synonymous with ventilatory support. Any PaO₂ below 100 (generally saturation less than 100%) on a supplemental oxygen fraction of 50% meets criteria for ARDS. This can easily be achieved by high flow oxygen supplementation without ventilatory support.

Any cardiogenic cause of pulmonary edema should be excluded. This can be done by placing a pulmonary artery catheter for measuring the pulmonary artery wedge pressure. However, this is not necessary and is now rarely done as abundant evidence has emerged demonstrating that the use of pulmonary artery catheters does not lead to improved patient outcomes in critical illness including ARDS.

Plain chest X-rays are sufficient to document bilateral alveolar infiltrates in the majority of cases. While CT scanning leads to more accurate images of the pulmonary parenchyma in ARDS, it has little utility in the clinical management of patients with ARDS, and remains largely a research tool.

Four main criteria for ARDS:

1. Acute onset
2. Chest X-Ray: Bilateral diffuse infiltrates of the lungs
3. No cardiovascular lesion
4. No evidence of left atrial hypertension: PaO₂/FiO₂ ratio equal to or less than 200 mmHg.

The criteria for diagnosis of Acute Lung Injury (ALI) are similar except that PaO2/FiO2 ratio is ≤300.

To assess the severity of ARDS, the Murray scoring system is used, which takes into account the chest X-ray, the PaO₂/FiO₂ ratio, the positive end-expiratory pressure, and lung compliance.