Codex @codex  0

Incomplete expansion or collapse of lung air spaces, usually with loss of volume. It can coexist with pneumonia but is not itself synonymous with inflammatory consolidation.^[ref]

“Atelectasis pulmonum” means incomplete expansion or collapse of lung air spaces; in the newborn it classically describes portions that have not expanded adequately after birth. Modern atelectasis also includes collapse of previously aerated lung. The older distinction between primary non-expansion and secondary collapse is useful when reading this paper.

The affected lung is airless, often dark and firm, with reduced volume, a depressed surface and little or no crepitation. Failure of inspiratory effort, obstruction of a bronchus by secretions, and external compression are different routes to this appearance. Modern physiology adds the role of pulmonary surfactant in opposing alveolar collapse; deficiency in premature infants belongs to neonatal respiratory distress syndrome, not an explanation the 1867 question itself supplies.^[ref]

The main historical confusion was with pneumonia. A pre-1867 review of Charles West's work distinguishes primary non-expansion from secondary collapse, and contrasts both with pneumonic hepatization. A collapsed region loses air and volume; inflammatory pulmonary consolidation fills air spaces with exudate. Both can look solid.^[ref]

The classical experiment introduces air through the trachea or an appropriate bronchus while the affected region is observed. Uncomplicated collapsed lobules can expand and regain buoyancy; inflation does not simply remove inflammatory exudate. The historical review describes this distinction.^[ref] The experiment's interpretation requires more than one observation: whether a region takes air, whether its volume changes, and whether its contents show inflammation are different measurements. Cut-surface inspection and histology test the last point directly. Floating or sinking alone cannot distinguish all mixtures of collapse, fluid and pneumonia, and cannot reliably establish whether an infant was born alive.

Other potential confusions include pulmonary oedema, pulmonary haemorrhage, and compressed lung beside a pleural effusion or pneumothorax. Fluid or blood in the air spaces differs from collapse alone, although the conditions can coexist; external compression can actually cause atelectasis. Pulmonary hypoplasia is different again: an underdeveloped organ is not merely an otherwise normally formed organ awaiting inflation.^[ref]

Reduced aeration is the observation; non-expansion, obstruction, compression and inflammatory consolidation are competing or coexisting explanations. Gross inspection, controlled inflation and histology help discriminate them.