Pulmonary Embolism

Perhaps the most serious and unpredictable condition affecting the pulmonary circulation is pulmonary embolism. It consists of occlusion of sections of the pulmonary arterial tree by thrombi carried in the bloodstream from the venous part of the circulation or from the right side of the heart. The sites of formation of thrombi vary. Whereas inflammatory diseases of the veins are usually associated with thrombosis (thrombophlebitis), pulmonary embolism occurs much more frequently in veins not affected by inflammation (phlebothrombosis). It is known that in susceptible persons thrombi can form in veins, particularly during periods of inactivity. The veins most commonly affected by "silent" thrombosis are deep veins of the legs (not varicose veins) and veins in the pelvic organs (reproductive organs in women and the prostate in men). Because inactivity is an important cause of such thrombi,

venous thrombosis and pulmonary emboli are particularly likely to develop in people who are bedridden.

Several types of pulmonary embolism can be distinguished. In pulmonary infarct a small single embolus affects a small segment of a lung, which becomes clogged with blood. This relatively benign condition may cause chest pain, spitting up of blood, and some fever; it appears on the chest X ray as a shadow similar to that of pneumonia. Prompt recovery usually occurs in a single infarct without sequelae. The significance of pulmonary infarct is as a predictor of larger, more serious emboli developing from the source of the small embolus.

A large pulmonary embolism or multiple emboli can cause overloading of the pulmonary circulation. When a certain area of the pulmonary arterial tree (usually more than half) is occluded by clots, pressure in the pulmonary artery rises. The sudden onset of pulmonary hypertension often does not give the heart time to adapt and rapidly leads to right ventricular failure. Such an event, occasionally referred to as acute cor pulmonale, can be fatal, but it may reverse itself when pulmonary emboli are no longer forthcoming, either because of cessation of thrombosis or in response to surgical intervention. Patients with acute cor pulmonale are short of breath, may be in shock, and show signs of right ventricular failure.

Massive pulmonary embolism occluding the main pulmonary artery or the two principal branches causes death, usually instantly and without warning. It is the prevention of this dreaded complication that has placed so much emphasis on signs of any disturbance of the venous circulation.

Small, repeated pulmonary emboli may feed the pulmonary circulation over a period of weeks, months, or years, causing gradual elevation of pressure in the pulmonary artery and leading to chronic, irreversible pulmonary hypertension. Fortunately uncommon, this severe, usually fatal form of pulmonary hypertension is almost indistinguishable from primary pulmonary hypertension.

Treatment of pulmonary embolism mainly involves anticoagulant therapy to prevent recurrences of emboli. Acute cases may call for thrombolytic therapy. Interventional therapy includes surgical removal of clots, located by angiography, from the pulmonary artery or its principal branches. Various methods of occluding the inferior vena cava in cases where emboli are thought to arise in the

lower part of the body are occasionally used. Occlusion blocks the pathway for emboli and forces the blood to return to the heart through small collateral veins.