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Ischemia

• Ischemia involves a decrease in blood supply to a part of tissue.
• This is due to the occlusion of its artery.

Types of Ischemia

• Sudden (acute) ischemia can be caused by thrombosis, embolism, arterial spasm, surgical ligature, or twisting of an organ's pedicle.
• Gradual (chronic) ischemia is most commonly caused by atherosclerosis, or pressure on the artery by a tumor or enlarged lymph node.

Effects of Vascular Occlusion

• The effects range from no or minimal effect to death of a tissue or person.
• This depends on the nature of the vascular supply (end artery or dual blood supply), the rate of vascular occlusion, and the vulnerability of tissue to hypoxia.
• Neurons are vulnerable to hypoxia in 3 to 4 minutes.
• Myocardial cells are vulnerable to hypoxia in 20 to 30 minutes.

Infarction Definition

• Infarction is an area of coagulative necrosis (or liquefactive necrosis in the brain).
• This is caused by an inadequate blood supply to the affected area.
• Infarct is an example of ischemic necrosis.

Red (Hemorrhagic) Infarct

• Red infarcts occur in loose tissues such as the lung, in vascular organs with dual blood supply like the lung, liver, and intestine.
• Red infarcts also occur in previously congested tissue (lung and liver), after reperfusion of previously ischemic tissue, or due to occlusion of a vein.

Pale (Anemic) Infarct

• Pale infarcts occur in solid and less vascular organs such as the kidneys, spleen, and heart.
• Infarction of the brain may be pale or red.

Infarction Characteristics

• The size of the infarct area is related to the size of the obstructed artery and the susceptibility of the tissue to ischemia.
• Infarcts are typically wedge-shaped (pyramidal) because the arteries have a fan-like distribution.
• The base of the wedge is directed towards the surface of the organ, and the apex is deep.
• Subcapsular infarcts are raised when recent due to edema and depressed when healed due to fibrosis.
• Infarcts are surrounded by a red zone of hyperemia (inflammation) and are firm (soft in the brain).
• They cause sero-fibrinous inflammation of the overlying serosa.
• Infarcts are an area of coagulative necrosis (liquefactive in the brain) surrounded by a zone of acute inflammation (hyperemia).

Fate of Infarcts

• In small infarcts, necrotic tissue is removed by macrophages.
• Granulation tissue fills the defect, followed by fibrosis.
• Large infarcts are surrounded by a fibrous capsule.
• They undergo dystrophic calcification.
• In the brain (due to high lipid content), they leave a cyst surrounded by glial tissue.

Gangrene Definition

• Gangrene is a type of necrosis that most often affects the lower extremities or bowel.
• It is secondary to vascular occlusion and is associated with saprophytic bacterial infection.

Dry Gangrene

• Dry gangrene begins in the distal part of a limb due to ischemia.
• It spreads slowly upwards until it reaches a point where the blood supply is adequate to keep the tissue viable.
• A line of separation forms between the gangrenous part and the viable part.

Wet Gangrene

• Wet gangrene develops rapidly due to blockage of venous and arterial blood flow simultaneously.
• The affected part is stuffed with blood, which favors the rapid growth of putrefactive bacteria.
• The toxic products formed by bacteria are absorbed, causing profound systemic manifestations of toxemia and death.

Examples of Wet Gangrene

• Diabetic foot: High sugar content in the necrotic tissue favors bacteria growth.
• Bed sores: Occur in bed-ridden patients due to pressure on sites like the sacrum, buttocks, and heels.

Gas Gangrene

• Gas gangrene is a special form of wet gangrene caused by gas-forming clostridia (gram-positive anaerobic bacteria).
• Clostridia gain entry into the tissues through open contaminated wounds, especially in the muscles, or as a complication of operation on the colon.
• Gas gangrene produces various toxins that cause necrosis and edema locally.
• Toxins are also absorbed, producing profound systemic manifestations.

Comparison of Dry and Wet Gangrene

• Dry gangrene typically affects extremities like the lower limb and is caused by gradual arterial obstruction.
• Wet gangrene affects internal organs like the intestine and lung, as well as extremities in crush injuries and diabetes, and is caused by sudden arterial and venous obstruction.
• Dry gangrene progresses slowly with a present line of demarcation, resulting in a black, dry, and mummified part with minimal putrefaction and mild toxemia.
• Wet gangrene progresses rapidly with an absent line of demarcation, resulting in a swollen, edematous part with ulcerated skin, maximal putrefaction, and severe toxemia.