Infarction is defined as , it is a process
of coagulative necrosis as a result of ischaemia resulting from sudden arrest
of blood supply due to occlusion of physiological or anatomical end arteries .
The effect of the vascular obstrunction may be judged by the texture of the
tissue , spread of onset of obstruction , nature of the vessels involved and
state of collaterals .
Both the artery and vein may be involved and they develop ischaemia .
But in most cases , the artery will be involved .
The sites of infarction are common in lungs , bowels etc .
Causes of Infarction ……
The causes of infarction are –
i)
Occulsion of an artery or vein
by an embolus or thrombus may cause infarction .
ii)
Atherosclerosis change .
iii)
An aseptic embolus may produce
infarct resulting a simple necrosis of area .
iv)
Septic embolus will produce
septic infarction .
Types of Infarct –
The types are …..
- Red or haemorrhagic infarct – Initially all infarcts are red or
haemorrhagic . formation of red infarct depends on (i) Blood supply of the
organ with established collateral circulation and (ii) Compactness of the
tissue of the organ . The haemorrhagic infarct is usually seen in the lung
s and intestine where the tissues are loose and capillaries are poorly supported
. Here blood can be easily aspirated and infarct remains end .
- Pale infarct – If the dead tissue coagulates quickly , the red
blood cells will break down as a result the blood will not come inside the
tissue and infarcts become pale . this type of infarct is found in kidney
, spleen , heart .
Mechanism of infarction –
- At first , there will be sudden occlusion of the artery by an
embolus or otherwise , there will b e stoppage of the arterial flow to the
area supplied by the vessels .
- Due to the stoppage of the blood flow , the blood pressure in
the area distal to the block of the arterial sides become almost nil .
Oxygen tension in the area will diminished . Inividual cells show
degenerative changes .
- As the arterial pressure falls in the affected area , the
venous pressure gets the upper hand and a venous reflux sets in venules ,
capillaries and arteriols of the affected area as a result there will be
intense congestion of te infarct . The oxygen tension will be gradually
diminished and ultimately leads to necrosis of the tissue .
- The collateral circulation will try to compensate and attempt
to pour in blood into the area as a result there will be haemorrhagic
border around the infarct . The diallated capillaries will also form a
haemorrhagic border . If the zone of infarct is wider , the affected part
becomes flooded with blood and looks like a haemorrhagic infarct .
- Formatin of red infarct and pale infarct depends on the
collateral circulation . Pale infarct is usually seen in kidney and heart
, both pale and red infarct are seen in spleen and red infarct is seen in
lung and liver as lung and liver are getting double sources of blood
supply .
- The lesions are often wedge shaped or fan shaped corresponding
to the area of distribution of the occulded artery . The part will be firm
in feel . The type of necrosis of the affected area will be coagullative
type exception in brain where liquefactive necrosis sets in .
- Vascular permeability in the affected area is increased .
Transfusion will take place . Area will swell up due to oedema . Firm con
sitency of the tissue exerts pressure over congested vessels and cogged
blood will be expelled out .
- Macroscopically , the infarct is usually triangular in shape ,
base is directed to the periphery and apex is indicated to the [point of
vascular obstruction . The affected area will be surrounded by a pink zone
of hyperaemia . The infarct may be either pale or red depending on the
collateral circulation . The cut surface is dry , granular and friable .
- Microscopically , three distinct zones can easily be identified
and these zones are …..
i)
Zones of infarction – The
central area of infarct shows structureless tissue taking pink stain . Remnants
of nuclei may be seen in the periphery of infarct .
ii)
Zone of hyperaemia – A zone of
congestion and infiltration of polymorph may be sen at the margin of infarct .
In old standing case , a zone of fibrous tissue proliferation may be seen in
between healthy and infarct tissue .
iii)
Zone of normal tissue – A zzone
of normal tissue will be found beyond the zone of hyperaemia .
