Acute Respiratory Distress Syndrome (ARDS) is a severe, life-threatening lung injury that develops rapidly. It is characterized by widespread inflammation, causing the tiny air sacs (alveoli) to fill with fluid. This fluid accumulation prevents oxygen from properly entering the bloodstream, leading to dangerously low blood oxygen levels. Because ARDS impairs oxygen transfer, it requires immediate medical intervention, often in an intensive care unit.
Initial Clinical Assessment and Risk Factors
The diagnostic process for ARDS begins with a rapid clinical assessment to identify the underlying cause and the patient’s symptoms. ARDS develops within one week of a known clinical insult, which may be a direct lung injury or a systemic illness. Common triggers, or risk factors, include severe infections like sepsis or pneumonia, massive trauma, aspiration of stomach contents, and pancreatitis.
Physicians look for characteristic signs that raise immediate suspicion of lung failure. These include severe shortness of breath (dyspnea) and a rapid, shallow breathing pattern (tachypnea), even when the patient is receiving supplemental oxygen. The patient’s history confirms the respiratory distress is acute, meaning it developed quickly, typically within hours to a few days of the inciting event. This initial suspicion triggers the next steps in the diagnostic workup.
Confirming Lung Damage Through Imaging and Gas Exchange
Confirmation of ARDS requires objective evidence of physical lung damage and impaired oxygenation. Chest imaging, typically an X-ray or Computed Tomography (CT) scan, is necessary to visualize the lung tissue. The imaging must show “bilateral opacities,” which are white areas across both lungs, indicating that the air sacs are filled with fluid and collapsed tissue. These findings must not be fully explained by other issues, such as a collapsed lung lobe or fluid surrounding the lung (pleural effusions).
The failure of gas exchange is measured using an arterial blood gas (ABG) test. This test determines the partial pressure of arterial oxygen (PaO2), which is compared to the fraction of inspired oxygen (FiO2) the patient is receiving. This PaO2/FiO2 ratio, or P/F ratio, measures the lung’s efficiency at transferring oxygen. A low P/F ratio confirms a severe physiological impairment.
Distinguishing ARDS from Heart Failure
Ruling out heart failure is essential, as it can also cause fluid in the lungs (cardiogenic pulmonary edema). ARDS is defined as non-cardiogenic edema, meaning the fluid leakage results from inflammation and capillary damage, not high pressure from a failing heart. This distinction is important because the treatments for the two conditions are completely different.
An Echocardiogram (Echo) is often performed to assess the heart’s pumping ability, particularly the left ventricle function. If the heart is working normally, it suggests the lung fluid is due to injury (ARDS) rather than cardiac backup. The Berlin Definition requires that the respiratory failure cannot be fully explained by heart failure or fluid overload, replacing the need for the historically used invasive test, Pulmonary Capillary Wedge Pressure (PCWP).
Applying the Diagnostic Criteria and Classifying Severity
The accumulated evidence is synthesized using the Berlin Definition, which establishes the formal criteria for ARDS. This framework requires four components: acute onset within one week of the insult, bilateral opacities on imaging, exclusion of cardiac failure, and confirmation of impaired oxygenation. When the P/F ratio is measured, a minimum positive end-expiratory pressure (PEEP) of 5 cmH2O is required to ensure a standardized assessment.
The P/F ratio is the final determinant for classifying the severity of the syndrome. Mild ARDS is defined by a P/F ratio between 201 and 300 mmHg. Moderate ARDS falls within the range of 101 to 200 mmHg. Severe ARDS is given when the P/F ratio is 100 mmHg or less. This classification guides treatment intensity and is directly associated with the patient’s prognosis.