Acute respiratory failure, part 1: Establishing the diagnosis

The 3 most important pathophysiologic mechanisms of acute respiratory failure are hypoventilation, ventilation-perfusion (V̇/Q̇) mismatch, and shunt. V̇/Q̇ mismatch is the most common cause of clinically important hypoxemia and hypercapnia; underlying causes include pulmonary edema, pulmonary vascular disease, and emphysema. Certain signs and symptoms may suggest the underlying cause-for example, cough, sputum production, and fever suggest pneumonia, while acute pleuritic chest pain may indicate pulmonary embolism or pneumothorax. Decreased breath sounds may be present in patients with chronic obstructive pulmonary disease. Key components of the workup include arterial blood gas analysis and chest radiography. The most commonly used measurements for assessing respiratory muscle strength at the bedside are maximum inspiratory and expiratory pressures, forced vital capacity, and maximum voluntary ventilation.