Respiratory mechanics and ventilator waveforms in the patient with acute lung injury

Acute lung injury/acute respiratory distress syndrome is a syndrome of low respiratory compliance. However, longstanding knowledge of applied respiratory mechanics and refined imaging techniques have shown that this is clearly an oversimplified view. Though the average compliance of the respiratory system is reproducibly low, regional mechanics may vastly differ; lung, airway, and chest wall mechanics may be variably affected; finally, these abnormalities may be very dynamic in nature, being influenced by time, posture, and the way positive-pressure ventilation is applied. Modern mechanical ventilators are equipped to display pressure, flow, and volume waveforms that can be used to measure respiratory compliance, airway resistance, and intrinsic positive end-expiratory pressure. These basic measurements, once the domain of applied physiologists only, are now available to aid clinicians to choose the appropriate ventilator settings to promote lung recruitment and avoid injury during lung-protective ventilatory strategies. High-resolution lung imaging and bedside recording of physiologic variables are important tools for clinicians who want to deliver specialized care to improve the outcome of critically ill patients in acute respiratory failure.