Epidemiology And Etiology

The true incidence of VTE in the general population is unknown because many patients, perhaps more than 50%, have no overt symptoms or go undiagnosed. An estimated 2 million people in the United States develop VTE each year; 600,000 are hospitalized and 60,000 die. The estimated annual direct medical costs of managing the disease are well over $1 billion. The incidence of VTE nearly doubles in each decade of life over the age of 50 and is slightly higher in men. As the population ages, the total number of cases of DVT and PE continues to rise.

'O' The risk of VTE is related to several factors including age, prior history of VTE, major surgery (particularly orthopedic procedures of the lower extremities), trauma, malignancy, pregnancy, estrogen use, and hypercoagulable states (Table 10—1). VTE risk factors can be categorized in one of the three elements ofVirchow's triad: stasis in blood flow, vascular endothelial injury, and inherited or acquired changes in blood constituents that cause hypercoagulation states. These risk factors are additive and some can be easily identified in clinical practice. A prior history of venous thrombosis is perhaps the strongest risk factor for recurrent VTE, presumably because of the destruction of venous valves and obstruction of blood flow caused by the initial event. Rapid blood flow has an inhibitory effect on thrombus formation, but a slow rate of flow reduces the clearance of activated clotting factors in the zone of injury and slows the influx of regulatory substances. Stasis tips the delicate balance of procoagulation and anticoagulation in favor of thrombogenesis. The rate of blood flow in the venous circulation, particularly in the deep veins of the lower extremities, is relatively slow. Valves in the deep veins of the legs, as well as contraction of the calf and thigh muscles, facilitate the flow of blood back to the heart and lungs. Damage to the venous valves and periods of prolonged immobility result in venous stasis. Vessel obstruction, either from a thrombus or external compression, promotes clot propagation. Numerous medical conditions and surgical procedures are associated with reduced venous blood flow and increase the risk of VTE (Table 10-1). Greater than normal blood viscosity, seen in myeloproliferative disorders like polycythemia vera, for example, may also contribute to slowed blood flow and thrombus formation.

Table 10-1 Risk Factors for VTE

Risk Factor

Example

Age Risk doubles with each decade after age 50

Prior history of VTE Strongest known risk factor for DVT and PF

Venous stasis Major medical illness (e.g., congestive heart failure)

Major surgery (e.g., general anesthesia for greater than 30 minutes) Paralysis (e.g., due to stroke or spinal cord injury) Polycythemia vera Obesity Varicose veins

Vascular injury Major orthopedic surqery (e.q., knee and hip replacement) Trauma (especially fractures of the pelvis, hip, or leg) Indwelling venous catheters

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