About Valvular Heart Disease
Atrial Fibrillation (AF)
Atrial fibrillation (AF) is the most common major heart rhythm disorder. In the United States, alone, AF affects more than two million people. AF compromises the heart’s ability to pump blood efficiently but is not directly life-threatening.
Though AF can occur in the absence of other apparent heart disease, this arrhythmia is particularly common in patients with mitral stenosis and/or mitral regurgitation. In these patients, development of AF can lead quickly to debilitating symptoms. If it continues, it may result in a shortened life expectancy. Medical evaluation and treatment should be undertaken rapidly. In some cases, the development of AF is an indication for mitral valve surgery.
AF is far less common in patients with aortic valve disease but, when it does occur, it can also lead to debilitating symptoms.
Normal Heart Rhythm
Each heartbeat is stimulated by an electrical impulse, which normally starts in the sinoatrial or sinus node — the heart’s equivalent of a battery — located in the right atrium. Impulses originating in the sinus node spread through the right atrium, travel to the left atrium and, then, spread to the ventricles.
When a healthy person is at rest, the initial electrical impulse is generated regularly from the sinus node at a rate of 60 to 90 times per minute. This is called “normal sinus rhythm.” The electrical activity leads to mechanical activity of the atria and ventricles, which are stimulated to contract in a coordinated and well-organized manner, resulting in efficient ejection of blood from the heart.
Atrial fibrillation (AF) occurs when impulses arise simultaneously or almost simultaneously from several different areas in the atria. The result is disorganized and even chaotic electrical activity throughout the atria. It leads to similarly disorganized mechanical activity of the atria and irregular transmission of electrical signals to the ventricles.
The disorganized contraction of the various portions of the atria prevents these chambers from ejecting blood efficiently. In fact, they do not actively pump the blood at all.
Instead, they fibrillate, or quiver, acting as passive conduits through which blood flows to the ventricles from the large veins — inferior vena cava and superior vena cava (into the right atrium) and pulmonary veins (into the left atrium). The absence of active contraction of the atria can result in incomplete filling of the ventricles.
Irregular transmission of electrical signals from the atria to the ventricles also results in an irregular ventricular rhythm. This generates an unsteady arterial pulse rate with some beats following very rapidly after one another and others following only after relatively long pauses. When two ventricular contractions occur in very close proximity to one another, the time between them can be too short to allow a normal amount of blood to fill the ventricles. This limits the amount of blood that can be pumped out of the ventricles during the next beat.
Absence of organized atrial contraction causes areas of very slow blood flow — static blood pools along the atrial walls, which promote formation of blood clots. Fragments of the clots — emboli — tend to break off as other blood flows past the stagnant areas.
Complications of AF
When emboli break off, they leave the heart and lodge in arteries downstream, blocking them and depriving body tissues of blood and oxygen. Severe health consequences can result from blocked arteries in any part of the body. Emboli can lodge in the arteries of the:
In patients with valve disease, the rapid, irregular heartbeat of AF — and the associated changes in the heart’s filling and emptying — can markedly impair the heart’s efficiency as a blood pump. This can result in heart failure.
Even in the absence of such major complications, quality of life can be impaired by the palpitations, fatigue and dizziness sometimes associated with AF.
When AF is established, anticoagulant drugs are given to prevent normal blood clotting and thus to avoid emboli, the most feared complication of AF. Use of these drugs requires regular monitoring, through blood tests, to ensure that blood clotting is being appropriately modified.
Unfortunately, this therapy results in an abnormal risk of excessive bleeding — hemorrhage — which can be life-threatening, especially after an unexpected trauma (such as fall or auto accident). Even with careful monitoring, about 0.6 percent of patients annually have bleeding that requires transfusion or more complex treatments.
Drug therapy can control AF in some patients but never can cure the rhythm disorder. Patients with valve diseases — particularly those with mitral stenosis or regurgitation — may favorably respond to drug therapy in the short-term, but generally prove resistant in the long-term. In addition, all drugs have potentially undesirable side effects.
The Maze Procedure
A surgical treatment for AF — the Maze procedure — was developed more than 10 years ago. Developed primarily for patients without valve disease, it has been increasingly applied during valve surgery for patients with AF and valve disease. The procedure short-circuits the chaotic signals that cause AF, allowing the sinus node to reassert its direction of electrical signaling. Restoration of normal rhythm can preclude the need for drug therapy.
How it is performed
A radio frequency probe (a device that emits sound waves at a frequency that can injure or destroy small and very highly selected tissue regions) or a cryoprobe (a device that freezes and thus injures or destroys similarly limited areas of tissue) is used to injure a ring of tissue:
Catheter-based treatment similar to the Maze procedure now has been developed and sometimes is used in patients who do not have other heart disease requiring surgery.
A catheter equipped with a radio frequency emitter is placed across the interatrial septum after a small hole is made for it by another catheter. The radio frequency catheter is then manipulated to allow injury of some or all of the areas that can be reached surgically.
In general, catheter-based procedures are not as complete as those performed at surgery and, therefore, success rates have not been as high. However, even when not completely successful, these procedures often increase the effectiveness of drugs, allowing patients to live without the limitations of AF.
The catheter approach is most effective for patients who have intermittent, rather than persistent, AF. The surgical approach is highly successful for both intermittent and persistent AF.