Pulmonary agenesis and hypoplasia
Pulmonary agenesis and hypoplasia is usually asymptomatic with mediastinal displacement towards a dense hemithorax. Poliomyelitis can cause atrophy of the overlying pectoral muscles. MacLeod’s syndrome is a late sequel of childhood bronchiolitis with a small lung, small pulmonary arteries and expiratory air trapping on the affected side. If there is an embolus lodged in a major pulmonary artery, the vessels distal to the obstruction will be underperfused with associated loss of lung volume.