Mitral Valve Disease

Mitral Stenosis

Most cases of mitral stenosis are due to rheumatic fever, and at present, this is seen predominantly in developing countries where there is still a lack of widespread use of antibiotics and access to care. The estimated prevalence of rheumatic disease worldwide is between 15 and 20 million people, whereas there are approximately 200,000 deaths every year from acute or chronic sequelae of rheumatic fever. Rheumatic fever occurs as an immune response to repeated episodes of untreated group A β-hemolytic streptococcal pharyngitis, whereby immune antibodies attack the valve tissue due to the molecular mimicry seen between the streptococcal antigens and the valve. This immune attack leads to leaflet thickening and calcification, as well as commissural fusion of the mitral valve and chordae shortening, all of which contribute to further narrowing of the mitral valve orifice and decreased mobility of the mitral valve leaflets. Approximately 40% of all patients with rheumatic fever have mitral stenosis, with the rest having a mixture of mitral regurgitation (MR) and/or aortic valve disease. Other causes of mitral stenosis involve calcific mitral stenosis and congenital mitral stenosis, both of which are rare.

The normal mitral valve cross-sectional area in diastole is 4 to 6 cm ² . Blood flow is impaired when the valve orifice is narrowed to <2 cm ² , creating a pressure gradient with exertion. A valve area <1 cm ² is considered critical mitral stenosis and results in a significant pressure gradient across the valve at rest with chronically increased LA pressures ( Fig. 46.1 ).

Chronically increased LA pressures associated with mitral stenosis result in LA enlargement and predisposition for atrial fibrillation. Valves affected by mitral stenosis are also vulnerable to recurrent thrombus formation due to blood stasis in the LA and implantation of bacteria in a diseased valve, which leads to infective endocarditis.

The hemodynamic effects of chronic mitral stenosis include pulmonary venous and arterial hypertension; right ventricular (RV) hypertrophy, dilation, and failure; peripheral edema; tricuspid regurgitation; ascites; and hepatic injury with cirrhosis ( Fig. 46.2 ).

Mitral Regurgitation

Numerous etiologies contribute to MR, including mitral valve prolapse, coronary artery disease (ischemic MR), rheumatic heart disease, bacterial or fungal endocarditis, certain collagen vascular diseases, and heart failure (functional MR). In general, it is important to differentiate the etiology of MR between primary and secondary causes. Primary MR refers to abnormal valve function due to degenerative changes seen at the valve level itself, either due to myxomatous mitral valve prolapse, rheumatic leaflet changes, or ischemic heart disease that results in a dysfunctional papillary muscle apparatus. Secondary MR, or functional MR, is a result of progressive enlargement of the LV that leads to mitral annular dilatation and further displacement of the papillary muscle structures. Secondary MR is due to worsened heart failure due to nonischemic cardiomyopathy.

With MR, blood leaks back into the LA in addition to traveling its usual forward route through the aortic valve and into the aorta. If the regurgitant volume is significant, LA remodeling occurs with dilation to accommodate increased volumes without intolerable LA hypertension ( Fig. 46.3 ). Over time, as an increasing fraction of ventricular volume is regurgitant, the forward ventricular output is reduced, and symptoms and other findings of MR become obvious ( Fig. 46.4 ). Patients are generally clinically well if the regurgitant fraction (regurgitant volume/total ejection volume) is <0.4. Patients with regurgitant fractions >0.5 predictably develop LV failure and have high morbidity and mortality. Any evidence of LV failure (LV ejection fraction <60%) is a critical marker of poor prognosis and an indication to proceed to mitral valve surgery.

Infectious endocarditis, spontaneous rupture of chordae tendineae, or ischemic injury of a papillary muscle may cause acute MR. In these cases of abruptly increased regurgitant flow, because there is no adaptation of the LA or pulmonary vasculature to the increased regurgitant volumes, acute pulmonary edema suddenly occurs. Aggressive use of vasodilators is the medical treatment of choice, but this is often a temporary measure because survival often depends on emergency repair or replacement of the valve.

Mitral Stenosis

Patients notice the effects of moderate (valve area = 1–2 cm ² ) mitral stenosis with activity. However, in severe mitral stenosis, dyspnea with minimal exertion and paroxysmal nocturnal dyspnea may occur. In some cases a sudden, dramatic onset of atrial fibrillation produces the first symptoms, occasionally resulting in acute episodes of pulmonary edema. When the development of atrial fibrillation is clinically silent, the initial event may be a stroke or other thromboembolic event. The classic presentation of severe cor pulmonale with ascites and edema is rarely seen today except in medically underserved populations. Mitral valve disease increases the risk for bacterial seeding into the diseased valve tissue, which results in infective endocarditis.

Auscultatory findings in a patient with mitral stenosis include a loud first heart sound, an opening snap after the second heart sound, and a low-pitched diastolic murmur with presystolic accentuation if the patient is in sinus rhythm. The opening snap is the sound generated by sudden full opening of the mitral valve. It reflects the severity of the pressure gradient across the mitral valve, because greater LA pressures generate earlier opening than lesser pressures. Therefore, the shorter the interval from the second heart sound to the opening snap, the greater the pressure gradient, and the more severe the stenosis.

The characteristic diastolic, low-frequency “rumble” or murmur associated with mitral stenosis is best heard at the apex with the patient in the left lateral decubitus position and the bell over the point of maximal ventricular intensity. The rumble occurs throughout diastole, with accentuation in late diastole (presystole) in patients who have preserved normal sinus rhythm. This murmur can be difficult to hear, and is soft and brief when the stenosis is minor. Therefore, heightened awareness of possible mitral stenosis is necessary. If the murmur is inaudible, it can be accentuated by having the patient exercise before auscultation or perform maneuvers such as isometric handgrip. This murmur sequence—loud first sound, opening snap, and diastolic rumble—is specific for mitral stenosis. Murmurs that mimic mitral stenosis include the Austin Flint murmur with aortic regurgitation, mitral diastolic murmurs in patients with large intracardiac shunts, and occasionally murmurs that are caused by an LA myxoma. However, none have all three components of classic mitral stenosis.

ECG changes in mitral stenosis may range from minor ST-segment and T-wave abnormalities to ECG evidence of severe pulmonary hypertension and RV enlargement. The ECG pattern of LA and RV enlargement is a classic indicator. Atrial fibrillation is common.