Heart failure can be ongoing (chronic), or your condition may start suddenly (acute).

Heart failure signs and symptoms may include:

• Shortness of breath (dyspnea) when you exert yourself or when you lie down
• Fatigue and weakness
• Swelling (edema) in your legs, ankles and feet
• Rapid or irregular heartbeat
• Reduced ability to exercise
• Persistent cough or wheezing with white or pink blood-tinged phlegm
• Increased need to urinate at night
• Swelling of your abdomen (ascites)
• Very rapid weight gain from fluid retention
• Lack of appetite and nausea
• Difficulty concentrating or decreased alertness
• Sudden, severe shortness of breath and coughing up pink, foamy mucus
• Chest pain if your heart failure is caused by a heart attack

Heart failure often develops after other conditions have damaged or weakened your heart. However, the heart doesn't need to be weakened to cause heart failure. It can also occur if the heart becomes too stiff.

In heart failure, the main pumping chambers of your heart (the ventricles) may become stiff and not fill properly between beats. In some cases of heart failure, your heart muscle may become damaged and weakened, and the ventricles stretch (dilate) to the point that the heart can't pump blood efficiently throughout your body.

Over time, the heart can no longer keep up with the normal demands placed on it to pump blood to the rest of your body.

An ejection fraction is an important measurement of how well your heart is pumping and is used to help classify heart failure and guide treatment. In a healthy heart, the ejection fraction is 50 percent or higher — meaning that more than half of the blood that fills the ventricle is pumped out with each beat.

But heart failure can occur even with a normal ejection fraction. This happens if the heart muscle becomes stiff from conditions such as high blood pressure.

Heart failure can involve the left side (left ventricle), right side (right ventricle) or both sides of your heart. Generally, heart failure begins with the left side, specifically the left ventricle — your heart's main pumping chamber.

Any of the following conditions can damage or weaken your heart and can cause heart failure. Some of these can be present without your knowing it:

• Coronary artery disease and heart attack. Coronary artery disease is the most common form of heart disease and the most common cause of heart failure. The disease results from the buildup of fatty deposits (plaque) in your arteries, which reduce blood flow and can lead to heart attack.
• High blood pressure (hypertension). If your blood pressure is high, your heart has to work harder than it should to circulate blood throughout your body. Over time, this extra exertion can make your heart muscle too stiff or too weak to effectively pump blood.
• Faulty heart valves. The valves of your heart keep blood flowing in the proper direction through the heart. A damaged valve — due to a heart defect, coronary artery disease or heart infection — forces your heart to work harder, which can weaken it over time.
• Damage to the heart muscle (cardiomyopathy). Heart muscle damage (cardiomyopathy) can have many causes, including several diseases, infections, alcohol abuse and the toxic effect of drugs, such as cocaine or some drugs used for chemotherapy. Genetic factors also can play a role.
• Myocarditis. Myocarditis is an inflammation of the heart muscle. It's most commonly caused by a virus and can lead to left-sided heart failure.
• Heart defects you're born with (congenital heart defects). If your heart and its chambers or valves haven't formed correctly, the healthy parts of your heart have to work harder to pump blood through your heart, which, in turn, may lead to heart failure.
• Abnormal heart rhythms (heart arrhythmias). Abnormal heart rhythms may cause your heart to beat too fast, creating extra work for your heart. A slow heartbeat also may lead to heart failure.
• Other diseases. Chronic diseases — such as diabetes, HIV, hyperthyroidism, hypothyroidism, or a buildup of iron (hemochromatosis) or protein (amyloidosis) — also may contribute to heart failure.

Causes of acute heart failure include viruses that attack the heart muscle, severe infections, allergic reactions, blood clots in the lungs, the use of certain medications or any illness that affects the whole body.

A single risk factor may be enough to cause heart failure, but a combination of factors also increases your risk.

Risk factors include:

• High blood pressure. Your heart works harder than it has to if your blood pressure is high.
• Coronary artery disease. Narrowed arteries may limit your heart's supply of oxygen-rich blood, resulting in weakened heart muscle.
• Heart attack. A heart attack is a form of coronary disease that occurs suddenly. Damage to your heart muscle from a heart attack may mean your heart can no longer pump as well as it should.
• Diabetes. Having diabetes increases your risk of high blood pressure and coronary artery disease.
• Some diabetes medications. The diabetes drugs rosiglitazone (Avandia) and pioglitazone (Actos) have been found to increase the risk of heart failure in some people. Don't stop taking these medications on your own, though. If you're taking them, discuss with your doctor whether you need to make any changes.
• Certain medications. Some medications may lead to heart failure or heart problems. Medications that may increase the risk of heart problems include nonsteroidal anti-inflammatory drugs (NSAIDs); certain anesthesia medications; some anti-arrhythmic medications; certain medications used to treat high blood pressure, cancer, blood conditions, neurological conditions, psychiatric conditions, lung conditions, urological conditions, inflammatory conditions and infections; and other prescription and over-the-counter medications.

Don't stop taking any medications on your own. If you have questions about medications you're taking, discuss with your doctor whether he or she recommends any changes.

• Sleep apnea. The inability to breathe properly while you sleep at night results in low blood oxygen levels and increased risk of abnormal heart rhythms. Both of these problems can weaken the heart.
• Congenital heart defects. Some people who develop heart failure were born with structural heart defects.
• Valvular heart disease. People with valvular heart disease have a higher risk of heart failure.
• Viruses. A viral infection may have damaged your heart muscle.
• Alcohol use. Drinking too much alcohol can weaken heart muscle and lead to heart failure.
• Tobacco use. Using tobacco can increase your risk of heart failure.
• Obesity. People who are obese have a higher risk of developing heart failure.
• Irregular heartbeats. These abnormal rhythms, especially if they are very frequent and fast, can weaken the heart muscle and cause heart failure.

If you have heart failure, your outlook depends on the cause and the severity, your overall health, and other factors such as your age. Complications can include:

• Kidney damage or failure. Heart failure can reduce the blood flow to your kidneys, which can eventually cause kidney failure if left untreated. Kidney damage from heart failure can require dialysis for treatment.
• Heart valve problems. The valves of your heart, which keep blood flowing in the proper direction through your heart, may not function properly if your heart is enlarged or if the pressure in your heart is very high due to heart failure.
• Heart rhythm problems. Heart rhythm problems (arrhythmias) can be a potential complication of heart failure.
• Liver damage. Heart failure can lead to a buildup of fluid that puts too much pressure on the liver. This fluid backup can lead to scarring, which makes it more difficult for your liver to function properly.

Some people's symptoms and heart function will improve with proper treatment. However, heart failure can be life-threatening. People with heart failure may have severe symptoms, and some may require heart transplantation or support with a ventricular assist device.