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Thrombosis

• Formation of a solid mass (compact mass) composed of circulating blood elements inside a vessel or heart cavity (CVS) during life.

Causes & Pathogenesis of Thrombosis

• The causes of Thrombosis are explained by Virchow's Triad

Normal vs Abnormal Endothelium

• Normally, Endothelium secretes prostacyclin which inhibits thrombosis.
• Damaged Endothelium does not secrete prostacyclin.
• Causes of damage include: inflammatory conditions, degenerative diseases and toxins.
• Inflammatory conditions include phlebitis, arteritis, and endocarditis
• Degenerative diseases include atheroma
• Toxins such as tobacco smoke

Normal vs Slow Blood Flow

• In normal blood stream, blood cells occupy the central part, and plasma is in the peripheral part.
• In slow blood flow, platelets cross plasmatic zone and come in contact with the vascular endothelium.
• An incompetent venous valve prevents proper outflow, causing stasis.
• Slow blood flow can be caused by:
• Heart failure
• Varicose veins
• Postoperative period
• Long drives or flights
• Pregnancy

Changes of Composition of Blood (Hypercoagulability)

• Excessive increase in the number of blood elements.
• Increase in blood factors that help blood clotting.

Mechanism of Thrombus Formation

• Platelets adhere to the damaged endothelium and release thromboxane A2.
• Formation of lines of Zahn.
• Deposition of blood corpuscles.

Morphology of a Thrombus

• Thrombi are attached to the surface of the vessel or heart chamber in which they arise.
• Lines of Zahn consist of pink (platelets & fibrin) and red (RBCs).
• Formed only during life, thrombi are used to distinguish from postmortem clots.
• Made of fibrin, a thrombi tends to easily lead to embolization.
• Thrombi take the shape of the vessels.

Types of Thrombi according to Site

• Venous thrombosis (phlebothrombosis): deep vein thrombosis in the lower limb.
• Arterial thrombi occur in coronary & femoral arteries.
• Heart thrombi: More common on the left side, and have many types:
• Auricular thrombi: develops in the left atrium with mitral stenosis
• Vegetations: occurs over the cusps of valves in Rheumatic fever
• Mural thrombi: occurs over an infarct in the left ventricle

Presence or Absence of Organism (Infection)

• Septic thrombi contains pyogenic bacteria.
• Aseptic thrombi contains no microorganisms.

Extent of Occlusion

• Mural thrombi partially occlude the lumen.
• Occlusive thrombi obstruct the lumen completely.

Fate & Complications

• Septic Thrombus: Fragmented by proteolytic enzymes forming septic emboli which become pyaemic abscesses.
• Aseptic Thrombus:
• If small in size: Dissolution occurs.
• If large in size:
• Propagation: Thrombi accumulate additional platelets and Fibrin.
• Embolization: Aseptic emboli cause ischaemia.
• Organization & Recanalization occurs.

Embolism Definition

• Circulation of insoluble material (solid, liquid, or gaseous) in the blood and its sudden impaction in a narrow vessel.
• The material is called an embolus.

Types of Emboli

• Thrombo-embolism.
• Fat embolism.
• Amniotic fluid embolism.
• Tumor emboli.
• Air embolism.
• Parasitic emboli.

Thrombo-embolism Details

• A detached thrombus may originate from:
• Systemic veins and reaches the right side of the heart, then lungs, causing pulmonary embolism.
• Cardiac thrombi which usually originate in the left side of the heart, and carried by systemic arterial circulation to impact in any organ (kidney, brain...).
• Portal vein or its branches and passes to the liver (portal embolism).

Fat Embolism

• Common in sites containing fat, such as bone fractures & abdomen due to acute pancreatitis.
• Fat globules enter through the ruptured veins.

Tumor Emboli

• Malignant cells pass as emboli in the circulation and give metastases in the organs.

Parasitic Emboli

• Bilharzial ova and worms

Amniotic Fluid Embolism

• Rare, but occurs during delivery, causing fatal pulmonary embolism.

Air Embolism

• Sudden admission of 100 cc air in the blood stream causes sudden death.
• The air may block the right ventricle or pulmonary arteries causing acute heart failure.
• Causes:
• Injury of large neck veins causing air to be sucked into the vessels then heart.
• Criminal abortion (air passes into uterine veins).
• Caisson's disease: A type of air embolism occurs when deep divers work under high atmospheric pressure, where their nitrogen gas is dissolved in the tissues and blood. Sudden decompression i.e. sudden ascent produces nitrogen bubbles which act as gas emboli.

Effect of Embolism

• Emboli lodge in vessels too small to permit further passage, causing partial or complete vascular occlusion which results in ischemic necrosis (infarction) of the tissue supplied by this vessel.
• Depends on:
• The size of the embolus.
• Nature of the embolus: septic (pyemic abscess) or aseptic (infarction).
• State of the collateral circulation in the affected organ.

Pulmonary Embolism Sources of Embolus

• Mainly arises from recent thrombi of calf veins in lower limbs (most common).
• Commonly above the knee.
• Depends upon:
• Size of the embolus (large, medium or small).
• State of the lung (pulmonary congestion or not).

Pulmonary Embolism Effects

• Large embolus: Occludes the pulmonary trunk or one of its main branches producing sudden death (no time for infarction).
• Medium-sized emboli:
• If the lung is healthy, no effect will occur as the lung has double blood supply (pulmonary and bronchial arteries).
• If the lung is suffering from chronic venous congestion, lung infarct occurs.
• Recurrent small-sized emboli: Produce pulmonary hypertension due to lung fibrosis and right-sided heart failure (cor-pulmonale).