Pathophysiology and diagnosis of severe pneumonia and acute respiratory distress syndrome

Severe pneumonia requires therapeutic interventions that can be provided at a high-acuity level of care, such as in the intensive care unit (ICU), because these patients have a high risk of death. Several severity scores have been developed to predict mortality and hospitalization or ICU admission of patients with pneumonia. The most common organism in patients with severe community-acquired pneumonia is Streptococcus pneumoniae, while some viral pneumonia types, especially those due to influenza virus, can be severe with hypercytokinemia Serum indicators, such as procalcitonin and presepsin, are useful for making a differential diagnosis and predicting the prognosis of severe pneumonia and sepsis. Acute respiratory distress syndrome (ARDS) develops most often in patients who are being treated for various underlying diseases including severe pneumonia and sepsis. Physiologically, ARDS is characterized by increased permeability pulmonary edema and severe hypoxemia. Intra-alveolar accumulation of neutrophils and other inflammatory cells is associated with altered endothelial and epithelial barrier function. A new consensus definition of ARDS, the Berlin definition, has been published recently. This definition includes 4 criteria; timing, chest imaging, origin of edema, and hypoxemia under a minimum level of positive end-expiratory pressure. According to the Berlin definition, ARDS can be stratified into 3 stages according to oxygenation severity at ARDS onset but whether this ARDS stage is associated with patient mortality remains a controversial issue.