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PH-RS20

Lung abscess and gangrene

How aspirated mouth bacteria, destructive pneumonias, septic emboli and blocked bronchi turn lung into pus, why the cavity shows an air–fluid level, when necrosis becomes gangrene, and how antibiotics, drainage and surgery act.

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  • 5 steps · 10 questions
  • 3D anatomy, affected parts marked

By the end, you can

  • Explain how aspiration of mouth anaerobes causes an abscess, and why it favours the dependent segments of the right lung.
  • Compare abscesses from necrotising pneumonia (Staphylococcus aureus, Klebsiella), septic emboli and an obstructing tumour.
  • Relate liquefaction, walling off and drainage into a bronchus to foul sputum and the cavity with an air–fluid level.
  • Distinguish necrotising pneumonia, abscess and lung gangrene, and explain how pus spreads to the pleura, the blood and other organs.
  • Tell an abscess apart from cavitating tuberculosis, cavitating carcinoma and an infected bulla, and explain how antibiotics, drainage and surgery act on the mechanism.

What it is

A lung abscess is a localised collection of pus in a cavity carved out of lung tissue by necrotising infection and walled off from the lung around it. Lung gangrene is necrosis of a large area of lung, usually after its vessels thrombose, that putrefies before it can be walled off. Abscesses are primary (in a previously healthy lung, mostly after aspiration) or secondary (behind an obstruction, from septic emboli or by spread).

4 types to tell apart
Aspiration abscess
The commonest: mixed mouth anaerobes and streptococci inhaled when consciousness or swallowing fails; usually single, in a dependent segment, developing over weeks.
Necrotising pneumonia
Destructive organisms such as Staphylococcus aureus or Klebsiella liquefy parts of a pneumonia, leaving many small cavities; it develops quickly, over days.
Haematogenous abscesses from septic emboli
Infected clot from right-sided endocarditis, an infected vein or catheter lodges in the lung: many small abscesses in both lungs, towards the periphery.
Abscess behind an obstruction
A tumour or inhaled foreign body blocks a bronchus, secretions pool beyond it and become infected; it heals poorly until the blockage is removed.

Normal function

Keeping the mouth’s bacteria out of the lung

The mouth is crowded with bacteria, many of them anaerobes living in the crevices between gums and teeth, where oxygen is scarce; plaque and gum disease multiply them. Little of this reaches the lungs. Swallowing closes the larynx, the gag and cough reflexes expel what slips through, and the small amounts inhaled during sleep are trapped in mucus, swept upwards by cilia and engulfed by alveolar macrophages.

Below the , the is wider, shorter and more vertical than the , so inhaled material more often enters the right lung. Each lobe divides into bronchopulmonary segments, each with its own and artery; aspirated fluid runs downhill into whichever segments are lowest at the time.

The lung has two blood supplies: the pulmonary arteries, which carry blood for gas exchange, and the bronchial arteries from the aorta, which feed the airway walls and supporting tissue. In most pneumonias the alveolar walls and their capillaries survive, so the lung heals without losing tissue. An abscess or gangrene forms when the infection destroys them.

Point to it on the model