Pathophysiology of ARDS and COVID-19 Lung Injury

Swenson, Kai E.; Swenson, Erik R.

Acute respiratory distress syndrome (ARDS) is a rapidly developing non-cardiogenic pulmonary edema caused by pulmonary and systemic infections or sterile tissue injuries that evoke a severe lung-damaging host inflammatory response. The lung loses its normal gas exchange efficiency with disruption of the tight permeability characteristics of the alveolar capillary barrier. Interstitial and subsequent alveolar edema lead to alveolar collapse/de-recruitment, reduced lung compliance and greater pulmonary vascular resistance, often with marked regional heterogeneity in severity. Regional heightened stress applied to surrounding normal or less injured lung from edematous and collapsed regions along with associated barotrauma, volutrauma and atelectrauma with mechanical or extreme spontaneous ventilatory efforts further propagate injury in a vicious cycle called ventilator induced lung injury. Resulting arterial hypoxemia and hypercapnia arise from the creation of regions of shunt and low ventilation-perfusion (V(A)/Q) ratios, in addition to creation of high V(A)/Q and dead space areas by blood flow obstruction with thrombosis and/or high ventilating pressures. The pathophysiology of ARDS is discussed in this article and focuses on changes in the lung parenchyma and vasculature that reduce compliance, increase vascular resistance and compromise maintenance of dry normally compliant alveolar space by active alveolar fluid clearance and passive lymphatic fluid clearance from the lung interstitium. The COVID-19 pandemic that has emerged with the novel coronavirus, SARS-CoV-2, has generated a new form of severe acute lung injury that resembles ARDS in many of its features, but has different characteristics in its earliest stages leading to more profound hypoxemia and loss of dyspnea perception with less radiologically evident lung injury, not described before in ARDS. Understanding the pathophysiological features of ARDS and COVID-19 lung injury is critical to the best use of supportive therapies such as low tidal volume ventilation, supplemental oxygen concentrations, positive end-expiratory pressure (PEEP), permissive hypercapnia, prone positioning and neuromuscular blockade.