Blood clots in the lungs can cause a number of symptoms that can appear in patients suddenly or develop slowly over time. This condition, also known as pulmonary thromboembolism or pulmonary emboli (PE), most frequently causes problems with breathing. It can also cause chest pain, irregular beating of the heart, and even sudden death in some cases. Often the blood clots found in the lungs actually originate in the veins of the legs; as a result, many affected patients also have redness and swelling in one of their legs.

One of the most common symptoms of blood clots in the lungs is difficulty with breathing. Many people have shortness of breath, and feel like they can't take in enough air. Often this sense of shortness of breath results in an increase in their rate of breathing, a condition also known as tachypnea.

Other symptoms of blood clots in the lungs can involve other parts of the body. Often patients with this condition have tachycardia, or an increased heart rate. Some people may develop a low-grade fever in response to the blood clot. Additionally, some patients can have a sharp, stabbing chest pain that is more severe when they take a deep breath in.

Occasionally a large blood clot to the lung, called a saddle embolus, can block blood flow to the lungs. This can cause severe symptoms such as chest pain, weakness, and irregularities in the beating of the heart. The stress that this state causes can prevent the heart from pumping blood throughout the body, and therefore can be fatal.

Another of the common symptoms of blood clots in the lungs is leg swelling. Often the blood clot develops in the veins of the legs due to a condition called deep vein thrombosis (DVT). The blood clot, or thrombus, breaks off and travels to the lungs. The symptoms of a DVT can include leg redness, leg swelling, and leg pain. Typically, these symptoms only affect one leg at a time.

Symptoms of blood clots in the lungs can be less severe in patients who chronically have small clots in their lungs, as compared to people who acutely have a large blood clot in their lungs. Instead of experiencing the sudden onset of shortness of breath and chest pain, those with small clots can have a progressive onset of increased breathing difficulties. At first they might only feel short of breath when exercising, but with the progression of the disease they may have trouble breathing even at rest.