Atelectasis, Pneumonia, and Aspiration

Atelectasis and pneumonia are commonly encountered in the inpatient setting, particularly in the intensive care units (ICUs) where patients are intubated and seriously ill, and often subject to variety of co-morbidities. The two entities are often confused as they have overlapping imaging appearances and may coexist. Atelectasis represents incomplete expansion of the lung parenchyma, with associated loss of volume –whereas pneumonia is an infection of the parenchyma and not typically associated with volume loss. Recognition of the characteristic imaging findings of these diseases allows a confident diagnosis to be made in many cases, and a helpful differential diagnosis to be offered in others. Clues to lobar or total lung atelectasis include volume loss, mediastinal shift, fissural and hilar displacement, and a homogeneous opacity with straight borders obscuring adjacent structures (e.g., hemidiaphragm or heart border). Pneumonia may manifest with many different patterns – often nonspecific for a given organism; however, the pattern may help to narrow the differential diagnostic considerations. Aspiration most commonly manifests with dependent centrilobular nodules and/or consolidation. Chest radiographs may change rapidly in patients affected by aspiration or the more severe forms of atelectasis (i.e. lobar or whole lung) and the patient’s symptoms typically change just as rapidly, frequently developing hypoxemia and respiratory distress. While atelectasis in the inpatient setting is a common finding, outpatients who present with lobar atelectasis should be suspected to have an obstructing tumor until proven otherwise (except for patients with CF or asthma).