An embolus is a “travelling clot” and pulmonary embolism (PE) is a complication of deep venous thrombosis (DVT) clot formation in a vein elsewhere in the body. The clot or clots break off (now called an embolus) and travel through the veins and the right side of the heart, and blocks one or several lung arteries, preventing blood flow to parts of the lung. This results in less oxygen reaching the rest of the body. There are many known risk factors for this condition. Diagnosis may be missed or delayed due to non-specific symptoms, but special tests can be done and specific treatment started. With correct diagnosis and early treatment, the outcome is favourable, and recurrences can be prevented.
Pulmonary thromboembolism is the sudden occlusion of a lung artery due to a blood clot that was dislodged from somewhere else in the body, usually the legs. This leads to obstruction of the blood supply to the lung tissue, Clots can be of different sizes and more than one clot can break off over time. The larger the clot, the greater the risk of sudden death. If not treated aggressively, it can lead to respiratory and/or circulatory collapse and death.
The results and clinical picture of the PE will depend on how large the embolus is: a large embolus will block a major (large) lung artery, whilst a small embolus will reach a smaller, peripheral lung artery before becoming stuck and blocking it off. Blocking a small artery affects only a small part of the lung and may either be asymptomatic or cause any of the symptoms listed below. Blocking a large central artery affects more – sometimes the whole of – one or even both lungs, and will thus produce the sudden, dramatic symptoms described below. Obstructing lung arteries raises the pressure against which the right ventricle of the heart has to pump; in severe cases, this can cause sudden heart failure.
What causes PE?
The causes of pulmonary embolism are basically the same as the causes of venous thrombosis. The causes can be divided into three categories:
- Damage to the venous endothelium (lining of the vein) due to indwelling catheters, injection of irritating substances into the vein, sepsis, etc.
- Increased coagulability (e.g. blood disorders, cancers, oral contraceptives, etc.)
- Venous stasis (postoperative, after child birth, varicose veins, etc.)
More than 90 percent of pulmonary emboli primarily originate from a deep vein thrombosis (DVT) of the leg, but they can also arise from the abdomen and rarely the arms. The higher up the leg the clot forms, the more likely it is to break off and embolise to the lungs. Less than 20 percent of lower leg (calf) DVTs will extend upwards into the iliofemoral (thigh) veins, where about 50 percent break off and travel to the lungs.
The risk factors for developing thrombosis and thus PE can be divided into major (where the risk is greater than 5-20 times normal) and minor (where the risk is 2-4 times normal).
Major risk factors include the following:
- Trauma (including fractures, especially of the femur, hip and pelvis).
- Surgery within the last three months (especially of the hip, knee and abdomen or pelvis).
- Malignancy/cancer (especially of lung, pancreas or colon).
- Prolonged immobilisation/inactivity (in bedridden patients, e.g. strokes, spinal cord injury, severe burns, etc.)
- Pregnancy & Caesarean section (especially post-delivery).
- Varicose veins.
- Previous DVT or PE-existing respiratory disease.
Minor risk factors include:
- Oestrogen-containing contraceptive pills and hormone replacement therapy (HRT).
- General medical illness (e.g. heart disease, lung disease, hypertension, TB, kidney disease causing nephrotic syndrome).
- Obesity (BMI > 29kg/m²).
- Travel (long distance sedentary, especially by air: “Tourist Class Syndrome”).
- Advanced age.
- Heavy cigarette smoking (>25/day).
- Indwelling venous lines.
Some patients develop PE without obvious risk factors and these may have unsuspected abnormalities which favour the development of DVT and PE, e.g. increased levels of clotting factors in their blood.