Infarction

Definition:  An infract is an area of ischemic necrosis caused by occlusion of either the arterial supply or venous drainage in a particular tissue. 

 Nearly 99% of all infarcts result from thrombotic or embolic events. Other mechanisms include [almost all of them are arterial in origin]: 

•  Local vasospasm 

•  Expansion of atheroma due to hemorrage in to athermotous plaque.

•  External compression of the vessels. e.g trauma  

•  Entrapment of vessels at hernial sacks etc.   

The development & the size of an infarct are determined by the following factors:

A.  The nature of the vascular supply 

B.  The rate of development of occlusion

C.  Suceptibility of the tissue for hypoxia

D.  Oxygen content of the blood

E.  The severity & duration of ischemia

A.  The nature of vascular supply

The following organs have a dual blood supply. 

 • Lung → pulumonary artery 

→ Bronchial artery 

• Liver → hepatic artery 

→ Portal vein 

•  Hand & forearm

→ Radial arteries 

→ Ulnar arteries. 

The effect of such a dual blood supply is that if there is obsrtuction of one of the arterial supplies, the other one may offset the rapid occurrence of infarction in these organs unlike the renal & splenic circulations which have end arterial supply. 

Infarction caused by venous thrombosis is more likely to occur in organs with single venous outflow channels, such as testis &ovary. 

B: Rate of development occlusion

 Slowly developing occlusions are  less likely to cause infraction since they provide time for the development of collaterals. 

C: Tissue susceptibility to hypoxia:

 The susceptibility of a tissue to hypoxia influences the likelihood of infarction. Neurons undergo irreversible damage when deprived of their blood supply for only 3 to 4 minutes. Myocardial cells die after 20-30 minutes of ischemia. Fibroblasts are more resistant, especially those in the myocardium. 

D: Oxygen content of blood 

Partial obstruction of the flow of blood in an anemic or cyanotic patient may lead to tissue infarction.  

E: The severity & duration of ischemia.

Types of infarcts 

Infarcts are classified depending on: 

 A) the basis of their colour (reflecting the amount of haemorrhage) into:

1. Hemorrhagic (Red) infarcts

2. Anemic (White) infarcts

B) the presence or absence of microbial infection into:

  1. Septic infarcts

  2. Bland infarcts

 1. Red infarcts occur in:

 a)  Venous occlusions as in ovarian torsion 

b)  Loose tissues such as the lung which allow blood to collect in infarct zone. 

c)  Tissues with dual circulations (eg. the lung), permitting flow of blood from unobstructed vessel in to necrotic zone. 

d)  In tissues that were previously congested because of sluggish outflow of blood. 

e)  When blood flow is reestablished to a site of previous arterial occlusion & necrosis.

2. White infarcts occur in:

 a)  Arterial occlusion in organs with a single arterial blood supply.

b)  Solid organs such as the heart, spleen, & kidney, where the solidity of the tissue limits the amount of hemorrhage that can percolate or seep in to the area of ischemic necrosis from the nearby capillaries.

Morphology of infarcts

Gross:  All infarcts are wedge-shaped with the occluded vessel at the apex and the periphery of the organ forming the base of the wedge. The infarction will induce inflammation in the tissue surrounding the area of infarction. Following inflammation, some of the infarcts may show recovery, however, most are ultimately replaced with scars except in the brain. 

Microscopy:

The dominant histologic feature of infarction is ischemic coagulative necrosis. The brain is an exception to this generalization, where liquefactive necrosis is common. 

Clinical examples of infarction:

A. Myocardial infarction

 -  Usually results from occlusive thrombosis supervening on ulcerating atheroma of a major coronary artery.

- Is a white infarct.

-  Can cause sudden death, cardiac failure, etc...

B. Cerebral infarcts 

-  May appear as pale or hemorrhagic 

-   A fatal increase in intracranial pressure may occur due to swelling of large cerebral infarction, as recent infarcts are raised above the surface since hypoxic cells lack the ability to maintain ionic gradients & they absorb water & swell. 

-  Is one type of cerebrovascular accidents (CVA) or stroke which has various clinical manifestations.

C. Lung infarcts 

-  Are typically dark red & conical (wedge-shaped).

-  Can cause chest pain, hemoptysis, etc…

D. Splenic infarcts 

- Conical & sub capsular 

- Initially dark red later turned to be pale.   

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.