Heart failure is when the heart cannot pump blood well. Heart failure is different from cardiac arrest, because the heart is still working. Heart failure can be sudden ("acute"), for example after a heart attack, or can come on slowly.
Someone with heart failure can have shortness of breath, difficulty lying flat, waking up breathless at night, swelling of the legs, and needing to urinate often during the night. There are many reasons why someone can get heart failure, but most often heart attacks, high blood pressure or problems with the heart valves.
A doctor makes a diagnosis of heart failure by asking about the symptoms above, examining the heart, blood vessels, lungs, liver (for swelling) and legs (for swelling or edema). Other tests to prove the diagnosis are X-rays of the lungs, an echocardiogram (an ultrasound test of the heart) and blood tests.
Heart failure can only be cured with a heart transplant, which is not done often, but most people with heart failure need to take diuretic medicines and other medication (ACE inhibitor, statin). Some people with heart failure are treated with an artificial pacemaker that makes the heart work better.