Heart Disease
After hypertension, the second most powerful risk factor for Stroke is heart disease, especially a condition known as atrial fibrillation. Atrial fibrillation is irregular beating of the left atrium, or left upper chamber, of the heart. In people with atrial fibrillation, the left atrium beats up to four times faster than the rest of the heart. This leads to an irregular flow of blood and the occasional formation of blood clots that can leave the heart and travel to the brain, causing a stroke.

Atrial fibrillation, which affects as many as 2.2 million Americans, increases an individual's risk of stroke by 4 to 6 percent, and about 15 percent of stroke patients have atrial fibrillation before they experience a stroke. The condition is more prevalent in the upper age groups, which means that the prevalence of atrial fibrillation in the United States will increase proportionately with the growth of the elderly population. Unlike hypertension and other risk factors that have a lesser impact on the ever-rising absolute risk of stroke that comes with advancing age, the influence of atrial fibrillation on total risk for stroke increases powerfully with age. In people over 80 years old, atrial fibrillation is the direct cause of one in four strokes.

Other forms of heart disease that increase stroke risk include malformations of the heart valves or the heart muscle. Some valve diseases, like mitral valve stenosis or mitral annular calcification, can double the risk for stroke, independent of other risk factors.

Heart muscle malformations can also increase the risk for stroke. Patent foramen ovale (PFO) is a passage or a hole (sometimes called a "shunt") in the heart wall separating the two atria, or upper chambers, of the heart. Clots in the blood are usually filtered out by the lungs, but PFO could allow emboli or blood clots to bypass the lungs and go directly through the Arteries to the brain, potentially causing a stroke. Research is currently under way to determine how important PFO is as a cause for stroke. Atrial septal Aneurysm (ASA), a congenital (present from birth) malformation of the heart tissue, is a bulging of the septum or heart wall into one of the atria of the heart. Researchers do not know why this malformation increases the risk for stroke. PFO and ASA frequently occur together and therefore amplify the risk for stroke. Two other heart malformations that seem to increase the risk for stroke for unknown reasons are left atrial enlargement and left ventricular hypertrophy. People with left atrial enlargement have a larger than normal left atrium of the heart; those with left ventricular hypertrophy have a thickening of the wall of the left ventricle.

Another risk factor for stroke is cardiac surgery to correct heart malformations or reverse the effects of heart disease. Strokes occurring in this situation are usually the result of surgically dislodged plaques from the aorta that travel through the bloodstream to the arteries in the neck and head, causing stroke. Cardiac surgery increases a person's risk of stroke by about 1 percent. Other types of surgery can also increase the risk of stroke.