Acute respiratory distress syndrome: Clinical sciences

Acute respiratory distress syndrome, or ARDS for short, is a life-threatening condition associated with acute lung injury that results in progressive respiratory dysfunction and hypoxia. ARDS is caused by inflammatory alveolar damage and capillary endothelial injury, which ultimately leads to decreased lung compliance and pulmonary arterial vasoconstriction. Based on arterial oxygenation, patients are grouped into mild, moderate, and severe ARDS.

Now, if you suspect ARDS, you should first perform an ABCDE assessment. Patients with ARDS are usually unstable, so begin acute management immediately! Stabilize the airway, breathing, and circulation, which means you will likely need to intubate the patient and place them on mechanical ventilation. Next, obtain IV access, put your patient on cardiac telemetry, and begin continuous vital sign monitoring, including blood pressure, heart rate, and pulse oximetry.

Once you stabilize the patient, obtain a focused history and physical examination. Your patient will report progressive shortness of breath and may have a history of trauma, sepsis, pneumonia, pancreatitis or drug toxicity. Physical exam often reveals features of hypoxia like cyanosis, tachypnea, accessory respiratory muscle use, and diffuse pulmonary crackles. In addition, pulse oximetry will reveal low oxygen saturation level. At this point, you should suspect ARDS!

Next, order an arterial blood gas, or ABG, and chest x-ray. In addition, you should estimate the fraction of oxygen in the patient’s inspired air, or FiO2. FiO2 is typically 0.21 when breathing room air. For patients on supplemental oxygen, it varies depending on the mode of oxygen delivery, and the amount of oxygen being delivered.

Now, here’s a high-yield fact! To help rule out cardiac dysfunction as the cause of lung infiltrates, look for absence of jugular venous distension on examination, normal troponin or B-type natriuretic peptide levels, a normal ECG, and a normal echocardiogram.

Your next step is to measure the PaO2/FiO2 ratio by dividing the patient’s arterial oxygen level, or PaO2, by the estimated FiO2. For example, if a patient’s ABG shows a PaO2 of 90 millimeters of mercury and they are breathing room air, which has a FiO2 of 0.21, then their PaO2/FiO2 ratio is 429 millimeters of mercury. Normal PaO2/FiO2 ratio is between 400 and 500 millimeters of mercury.

Next, assess whether your patient meets ARDS criteria. These include that the respiratory condition starts within a week of an inciting medical event like; trauma, sepsis, pneumonia, pancreatitis, or drug toxicity; chest x-ray showing bilateral lung opacities  of non-cardiac origin; and a PaO2 to FiO2 ratio of 300 mmHg or less, measured while receiving positive end-expiratory pressure or continuous positive airway pressure of at least 5 cmH2O. If these criteria are not met, consider an alternative diagnosis. On the other hand, if ARDS criteria are met, then diagnose ARDS and assess the PaO2/FiO2 ratio!

Here’s a clinical pearl! Several pulmonary conditions can mimic ARDS, such as cardiogenic pulmonary edema, bilateral pneumonia, and alveolar hemorrhage. So, always include these conditions in your differential diagnosis!