SN6 SHORT NOTE 5M ★ High-yield

Define atelectasis. Classify it and describe its causes and consequences.

Pathology · 2nd Professional — Para-Clinical

Definition and Classification

Atelectasis (collapse) is incomplete expansion of the lung, or the collapse of previously inflated lung, producing areas of relatively airless parenchyma.

Type	Mechanism	Mediastinal shift	Causes
Resorption (obstructive)	Complete airway obstruction; the trapped air is absorbed	Towards the affected side	Mucus plug (post-operative, asthma, bronchitis), foreign body, bronchial tumour, enlarged lymph node
Compression (passive, relaxation)	The pleural cavity is filled and the lung is pushed in	Away from the affected side	Pleural effusion, pneumothorax, haemothorax, elevated diaphragm, ascites
Contraction (cicatrisation)	Fibrosis prevents full expansion	Towards the affected side; irreversible	Localised or generalised pulmonary fibrosis
Patchy (microatelectasis)	Loss of surfactant	None	Respiratory distress syndrome of the newborn; ARDS

CLINICAL PEARL

The direction of the mediastinal shift tells you the mechanism. In resorption collapse the lung shrinks and pulls the mediastinum towards it. In compression collapse something is pushing from the pleural space, so the mediastinum is displaced away. One radiograph therefore distinguishes an obstructing tumour from a large effusion.

Neonatal Atelectasis

Primary atelectasis — the lung has never been expanded; seen in stillbirth and in extreme prematurity
Secondary atelectasis — the lung expanded and then collapsed
Respiratory distress syndrome of the newborn (hyaline membrane disease) — surfactant deficiency in the preterm infant; the alveoli collapse at end-expiration, giving stiff lungs, hypoxia and hyaline membranes
Risk factors — prematurity, maternal diabetes, caesarean section without labour, male sex
Prevented by antenatal corticosteroids given to the mother, and treated with exogenous surfactant and respiratory support

Clinical Features and Consequences

Dyspnoea and tachypnoea, with reduced chest movement on the affected side
Dullness to percussion, absent or reduced breath sounds, and displacement of the trachea and apex beat
Hypoxaemia from ventilation–perfusion mismatch and shunting
Predisposes to infection, since retained secretions cannot be cleared — collapse is a common precursor of post-operative pneumonia
Resorption and compression collapse are reversible if the cause is removed promptly; contraction collapse is not

Applied Aspects

Post-operative atelectasis is the commonest cause of fever in the first 48 hours after surgery, from shallow breathing, pain and retained secretions
Prevention — adequate analgesia, early mobilisation, deep breathing exercises, incentive spirometry and physiotherapy
Foreign body inhalation in a child characteristically lodges in the right main bronchus, and may cause obstructive emphysema before it causes collapse
Persistent collapse of a segment or lobe in an adult smoker demands bronchoscopy to exclude carcinoma
Bronchoscopy is both diagnostic and therapeutic for a mucus plug or foreign body

All questions in Respiratory Pathology

Every Pathology chapter →