SN1 SHORT NOTE 5M ★ High-yield

Define infarction. Classify infarcts and describe the factors determining their effects.

Pathology · 2nd Professional — Para-Clinical

Definition

An infarct is an area of ischaemic necrosis caused by occlusion of the arterial supply or, less often, of the venous drainage of a tissue.

Nearly all infarcts are caused by arterial thrombosis or embolism
Other causes — vasospasm, external compression (tumour, hernia), torsion (testis, ovary, volvulus), traumatic rupture

Classification

Basis	Types
By colour	Red (haemorrhagic) and white (pale, anaemic)
By infection	Bland (sterile) and septic (infected)
By age	Recent and old (organised)

Feature	Red (haemorrhagic) infarct	White (pale) infarct
Occurs in	Tissues with a dual blood supply, loose tissue, or previously congested tissue; and after venous occlusion	Solid organs with an end-arterial supply
Examples	Lung, intestine, testis (torsion), ovary, brain (after reperfusion)	Heart, kidney, spleen
Reason	Blood seeps in from the collateral or obstructed venous circulation	No collateral supply, and the dense tissue limits seepage

CLINICAL PEARL

Whether an infarct is red or white is decided by the blood supply, not by the organ. The lung infarcts red because the bronchial arteries continue to bleed into the dead tissue; the kidney infarcts white because nothing else supplies it. Ask first whether there is a second source of blood — the colour follows.

Morphology

Shape — typically wedge-shaped, with the apex at the occluded vessel and the base at the organ surface
Coagulative necrosis in all organs except the brain, which undergoes liquefactive necrosis and ends as a fluid-filled cyst
Evolution — poorly defined and haemorrhagic at first, later becoming better defined; an inflammatory response appears at the margins
Healing — by granulation tissue and then a fibrous scar, since most infarcted tissues cannot regenerate

Factors Determining the Effect

Factor	Influence
Nature of the blood supply	Dual supply protects — lung, liver, hand and forearm. End arteries do not — kidney, spleen, retina
Rate of occlusion	Slow occlusion allows collaterals to open; sudden occlusion does not
Vulnerability of the tissue to hypoxia	Neurones 3–5 minutes; myocardium 20–30 minutes; fibroblasts hours
Oxygen content of the blood	Anaemia and hypoxaemia make infarction more likely from a partial occlusion

Applied Aspects

Myocardial infarction — the commonest and most important; diagnosed by troponin and ECG long before any morphological change is visible
Cerebral infarction — liquefactive; the salvageable penumbra is the target of thrombolysis within a narrow window
Splenic and renal infarcts are typically embolic, and their presence should prompt a search for a cardiac source such as atrial fibrillation or endocarditis
Mesenteric infarction carries a very high mortality, because the diagnosis is usually made late — pain out of proportion to the physical signs is the classical clue
Septic infarcts from infective endocarditis become abscesses, and may form mycotic aneurysms

All questions in Haemodynamic Disorders

Every Pathology chapter →