Pulmonary embolism symptoms can vary greatly, depending on how much of your lung is involved, the size of the clots, and whether you have underlying lung or heart disease.

Common signs and symptoms include:

• Shortness of breath. This symptom typically appears suddenly and always gets worse with exertion.
• Chest pain. You may feel like you're having a heart attack. The pain may become worse when you breathe deeply (pleurisy), cough, eat, bend or stoop. The pain will get worse with exertion but won't go away when you rest.
• Cough. The cough may produce bloody or blood-streaked sputum.

Other signs and symptoms that can occur with pulmonary embolism include:

• Leg pain or swelling, or both, usually in the calf
• Clammy or discolored skin (cyanosis)
• Fever
• Excessive sweating
• Rapid or irregular heartbeat
• Lightheadedness or dizziness

Pulmonary embolism occurs when a clump of material, most often a blood clot, gets wedged into an artery in your lungs. These blood clots most commonly come from the deep veins of your legs. This condition is known as deep vein thrombosis (DVT).

In most cases, multiple clots are involved but not necessarily all at once. The portions of lung served by each blocked artery are robbed of blood and may die. This is known as pulmonary infarction. This makes it more difficult for your lungs to provide oxygen to the rest of your body.

Occasionally, blockages in the blood vessels are caused by substances other than blood clots, such as:

• Fat from the marrow of a broken long bone
• Collagen or other tissue
• Part of a tumor
• Air bubbles

Although anyone can develop blood clots and subsequent pulmonary embolism, certain factors can increase your risk.

Medical history

You're at higher risk if you or any of your family members have had venous blood clots or pulmonary embolism in the past. This may be due to inherited disorders that affect blood, making it more prone to clot.

In addition, some medical conditions and treatments put you at risk, such as:

• Heart disease. Cardiovascular disease, specifically heart failure, makes clot formation more likely.
• Cancer. Certain cancers — especially pancreatic, ovarian and lung cancers, and many cancers with metastasis — can increase levels of substances that help blood clot, and chemotherapy further increases the risk. Women with a personal or family history of breast cancer who are taking tamoxifen or raloxifene also are at higher risk of blood clots.
• Surgery. Surgery is one of the leading causes of problem blood clots. For this reason, medication to prevent clots may be given before and after major surgery such as joint replacement.

Prolonged immobility

Blood clots are more likely to form during periods of inactivity, such as:

• Bed rest. Being confined to bed for an extended period after surgery, a heart attack, leg fracture, trauma or any serious illness makes you more vulnerable to blood clots. When the lower extremities are horizontal for long periods, the flow of venous blood slows and blood can pool in the legs.
• Long trips. Sitting in a cramped position during lengthy plane or car trips slows blood flow in the legs, which contributes to the formation of clots.

Other risk factors

• Smoking. For reasons that aren't well-understood, tobacco use predisposes some people to blood clot formation, especially when combined with other risk factors.
• Being overweight. Excess weight increases the risk of blood clots — particularly in women who smoke or have high blood pressure.
• Supplemental estrogen. The estrogen in birth control pills and in hormone replacement therapy can increase clotting factors in your blood, especially if you smoke or are overweight.
• Pregnancy. The weight of the baby pressing on veins in the pelvis can slow blood return from the legs. Clots are more likely to form when blood slows or pools.

Pulmonary embolism can be life-threatening. About one-third of people with undiagnosed and untreated pulmonary embolism don't survive. When the condition is diagnosed and treated promptly, however, that number drops dramatically.

Pulmonary embolism can also lead to pulmonary hypertension, a condition in which the blood pressure in your lungs and in the right side of the heart is too high. When you have obstructions in the arteries inside your lungs, your heart must work harder to push blood through those vessels. This increases the blood pressure within these vessels and the right side of the heart, which can weaken your heart.

In rare cases, small emboli occur frequently and develop over time, resulting in chronic pulmonary hypertension, also known as chronic thromboembolic pulmonary hypertension.