The left atrium is exposed to both volume and pressure increases. However, in chronic MR, left atrial pressure increases are not dramatic because of compliance changes in the left atrium as a function of gradual chamber dilation. Progressive left atrial enlargement eventually leads to atrial fibrillation, which occurs in about 50% of patients who present for surgical correction of MR. When left atrial compliance thresholds are reached, left atrial pressure and pulmonary arterial pressure become elevated. Eventually, if chronically exposed to elevated PA pressure, the right ventricle progressively enlarges and right ventricular dysfunction develops.
The long-term sequelae of MR are related to chronic pressure and volume effects on the left atrium and left ventricle. The left ventricle is exposed to a chronic, isolated volume-overload state. Eccentric hypertrophy of the left ventricle develops, causing chamber enlargement without significant increases in wall thickness. Forward cardiac output is preserved because of eccentric hypertrophy and the low impedance of the left atrium—a physiologic equivalent of afterload reduction.16 The larger stroke volume ejected by the left ventricle is composed of normal venous return into the left atrium plus the regurgitant volume from the prior cardiac cycle. With time, however, compensatory eccentric hypertrophy fails to preserve left ventricular systolic function, and gradual systolic failure ensues, as noted on pressure-volume loops (Figure 4). A reduction in left ventricular Figure 4: Pressure-volume loop in mitral insufficiency (regurgitation).ejection fraction below 60% or an increase in end-systolic dimension exceeding 40 mmHg indicates the need for surgical repair or replacement of the mitral valve.23
With acute onset of MR (e.g., due to myocardial infarction and rupture of papillary muscles), there has been no time for left atrial compensatory changes to occur. Therefore, there is a sudden increase in left atrial pressure and pulmonary capillary wedge pressure. Patients with acute severe MR are usually in cardiogenic shock and do not present for noncardiac surgery. Pharmacologic support of the left ventricle, often accompanied by mechanical support with intra-aortic balloon pump (IABP) counterpulsation, may be necessary to prepare the patient for emergency cardiac surgery