The subclavian vein provides the primary pathway for venous blood to flow from the arm to the heart. When the subclavian vein is partially or completely blocked, and the patient has symptoms of venous blood clot in the arm, venous thoracic outlet syndrome is present.
Blood flow in the body follows a well-described pattern. Blood passes through the lungs and absorbs oxygen. This oxygenated blood is pumped to the body by the heart under high pressure. This high-pressure oxygenated blood flows through arteries, which have a thick muscular wall, to every part of the body.
Each artery branches unto smaller and smaller arteries until they reach a capillary bed. Capillaries are tiny branching blood vessels with walls that are one cell layer thick. Tissues extract oxygen from the blood in the capillary bed and release metabolic end-products such as carbon dioxide and lactic acid. The capillaries then take this deoxygenated blood and send it to small veins, which join other small veins to form larger veins, which eventually empty into the two largest veins of the body, the inferior vena cava (from the abdomen and lower body), and the superior vena cava (from the head, neck, and upper extremities). The inferior vena cava and superior vena cava return blood to the heart under low pressure. Because venous blood is under low pressure, vein walls are thin and have no muscles.
Once the venous blood returns to the heart, the heart pumps it through the lungs. In the lungs, the blood releases carbon dioxide and absorbs fresh oxygen. This oxygenated blood returns to the heart, which pumps the oxygenated blood under high pressure to the body through arteries, and the cycle repeats.
In order to understand venous TOS, let us look more specifically at blood flow through the arms. Oxygenated blood is pumped by the heart into each arm through a single, large, muscular subclavian artery. When this blood reaches a capillary bed, the tissues in that area extract oxygen from the blood and deposit metabolic end-products into the blood. This deoxygenated venous blood returns to the heart through a single, large subclavian vein (although a few smaller veins may participate). This process is known as ‘venous drainage.’
Under normal circumstances, low-pressure venous blood passes easily out of the arm on its trip back to the heart. But when a person develops a venous blood clot in the arm, or if the subclavian vein is extrinsically compressed, symptoms of venous thoracic outlet syndrome may occur. This patient can now receive the diagnosis of venous TOS.
Both Paget and von Schroetter described cases of venous TOS associated caused by a venous blood clot in the arm. McLaughlin and McCleery described mechanical extrinsic compression of the subclavian vein in the arm. Both sets of patients demonstrated similar symptoms of venous TOS, albeit due to different mechanisms.