The left atrium is the chamber of the heart that receives oxygen-rich blood from the lungs. Blood moves from the left atrium through the mitral valve into the main pumping chamber of the heart -- the left ventricle -- and then out to the body. Any condition that increases pressure in this portion of the cardiac system can lead to enlargement of the left atrium. Left atrial enlargement can be mild, moderate or severe depending on the severity of the underlying condition. Conditions that lead to left atrial enlargement include hypertension, heart valve problems, heart failure and atrial fibrillation.
Hypertension, or high blood pressure, causes the heart to work harder as it pumps blood to the body. Since the heart is made of elastic muscle tissue, it gradually enlarges when it has to pump against elevated pressures. The main pumping chamber of the heart, the left ventricle, typically enlarges first. As the pressure rises in the left ventricle, the left atrium gradually enlarges as well. When left atrial enlargement occurs due to elevated blood pressure, it is associated with an increased risk of heart attack and stroke.
Valvular Heart Disease
Disorders of certain heart valves can also lead to left atrial enlargement. The mitral valve separates the left atrium from the left ventricle. If the mitral valve becomes narrow -- a condition known as mitral stenosis -- blood accumulates in the left atrium and causes enlargement. Another condition called mitral regurgitation can also cause excess blood in the left atrium leading to enlargement. With this condition, a leaky mitral valve allows blood to escape back into the left atrium when the left ventricle contracts.
The aortic valve allows blood to flow from the left ventricle into a large artery called the aorta and out to the rest of the body. Aortic valve narrowing, known as aortic stenosis, forces the heart to pump against a higher pressure to eject blood into the aorta. As with high blood pressure, pumping against increased resistance eventually leads to left ventricular and left atrial enlargement.
Heart failure develops when the heart is unable to pump blood effectively to the rest of the body. Uncontrolled hypertension, coronary artery disease, and drug and alcohol abuse can all lead to heart failure. As the heart becomes progressively weaker, blood tends to pool in the heart and lungs resulting in left ventricular and left atrial enlargement. If heart failure remains untreated, the excess blood volume can eventually affect the right side of the heart causing dilation of the right ventricle and right atrium as well.
Atrial fibrillation is an abnormal heart rhythm that causes the heart to beat irregularly. Under normal circumstances, the atria and the ventricles beat together in a coordinated fashion, but atrial fibrillation disrupts this process. Disorders such as mitral stenosis and heart failure that cause left atrial enlargement often lead to atrial fibrillation. But atrial fibrillation can develop in the absence of left atrial enlargement. With persistent atrial fibrillation, however, the atria eventually enlarge. Untreated atrial enlargement and atrial fibrillation promote the formation of blood clots, which can travel to the brain and cause a stroke. For this reason, most people with atrial fibrillation and left atrial enlargement are treated with blood thinners to reduce the risk of stroke.
Less Common Causes
Other medical conditions are less frequent causes of left atrial enlargement. For example, a ventricular septal defect -- a hole in the wall that separates the right and left ventricles -- can alter blood flow through the heart such that there is overfilling and enlargement of the left atrium. Other congenital heart defects that lead to increased pressure in the left atrium can also cause enlargement. A noncancerous heart tumor called a cardiac myoma is another uncommon cause of left atrial enlargement, as roughly 75 percent of these tumors occur in the left atrium.