Acute Respiratory Distress Syndrome and Lung Protection

Abstract

Hypoxemic respiratory failure, frequently termed acute respiratory distress syndrome (ARDS), is 1 of the classic problems confronting the clinician using mechanical ventilation. Multiple trials over recent decades have identified the manipulation of mean airway pressure as the essential intervention to improve oxygenation. The easiest method to increase mean airway pressure (that the clinician can titrate) is increasing the positive end-expiratory pressure (PEEP). We have also learned that alveolar pressure rises with a consistent tidal volume and increasing PEEP. Key studies reviewed in the last column identify the value of reducing tidal volume and maintaining static alveolar pressure, measured as plateau pressure less than 30 cm H2O. If tidal volume is reduced, patients may develop a degree of hypercarbia because of reduced minute ventilation (defined as respiratory rate tidal volume). In general, reduced minute ventilation will cause the pH to decrease. Most critical care providers would accept a pH as low as 7.20 reflecting reduced minute ventilation and respiratory acidosis. Clinician must strike a balance between adequate ventilation and acid/base protection for the patient and the need to increase mean airway pressure while avoiding excessive airway pressure through the increase in PEEP with a reduction in the tidal volume. The articles discussed in this column describe current definitions for ARDS, review important comanagement strategies, and present physiologic causes for injury to the lungs by mechanical ventilation as reflected in the failure of recent trials involving high-frequency oscillatory ventilation (HFOV).