In mitral regurgitation, blood leaks backward through the mitral valve into the upper left chamber of the heart (the left atrium) every time the heart contracts. Normally, when the left ventricle squeezes, all the blood should push forward through the aortic valve and out to the body. A leaky mitral valve means some of that blood goes the wrong direction, and the consequences ripple through the entire circulatory system.
How Blood Flows Backward
The mitral valve sits between the left atrium and the left ventricle. It opens to let blood drop down into the ventricle, then snaps shut when the ventricle contracts so blood can only exit forward into the aorta. In mitral regurgitation, the valve flaps don’t seal completely. With every heartbeat, a portion of blood slips back into the left atrium instead of heading out to the body.
This backward jet can range from a small trickle to a major leak. In severe cases, the regurgitant volume reaches 60 milliliters or more per beat, meaning a significant fraction of each heartbeat’s output is wasted, cycling between two chambers instead of delivering oxygen to organs. The regurgitant fraction (the percentage of blood that leaks back compared to the total pumped) can exceed 50% in severe disease.
Less Blood Reaches the Body
Because some blood reverses course with every heartbeat, the effective amount pumped forward to the aorta drops. This is sometimes called reduced “forward cardiac output.” The body doesn’t get the full benefit of each contraction. During exercise, this gap widens dramatically. Research comparing heart failure patients with and without significant mitral regurgitation found that those with the leak had 29% lower cardiac output at peak exercise. Their bodies compensated by extracting 17% more oxygen from each unit of blood passing through the tissues, but that only partially made up the difference, resulting in a 15% reduction in exercise capacity.
Resting oxygen levels in the blood typically remain normal (around 97%), so mitral regurgitation doesn’t cause low oxygen saturation the way lung disease does. The problem is volume: not enough blood reaches the muscles and organs per minute, which is why fatigue and exercise intolerance are hallmark symptoms.
The Left Atrium Stretches and Enlarges
The left atrium was never designed to handle the extra volume that comes rushing back through a leaky valve. Over months and years, the repeated overload causes the atrium to stretch and enlarge. This enlargement reflects both how severe the leak is and how long it has been present. A normal left atrium is roughly 30 to 40 millimeters across. In chronic mitral regurgitation, it can balloon to 50, 55, or even 60 millimeters.
That enlargement matters. Research from the American Heart Association found that patients whose left atrium reached 55 millimeters or larger had significantly higher mortality, independent of symptoms or how well the ventricle was pumping. Each additional millimeter of atrial size increased the risk of death by about 8%. Patients with the most enlarged atria also had higher pressures in the blood vessels of their lungs and larger, weaker ventricles.
The Left Ventricle Works Harder, Then Weakens
The left ventricle initially compensates for the leak by pumping a larger total volume with each beat. If 30% of the blood leaks backward, the ventricle simply pushes out more to maintain adequate forward flow. For a while, this works. The ventricle dilates to hold more blood, and the ejection fraction (the percentage of blood squeezed out per beat) stays normal or even looks high, often above 60%.
This is where the numbers can be misleading. An ejection fraction of 60% sounds healthy, but in mitral regurgitation, it’s actually the expected minimum. Because the ventricle is emptying into two directions (one low-pressure, one high-pressure), it takes less effort to achieve a high percentage. When the ejection fraction drops below 60%, it signals that the heart muscle is genuinely weakening, even though 60% would be perfectly normal in someone without a leaky valve. European cardiology guidelines flag an ejection fraction at or below 60% as a trigger to consider surgical repair, even in patients who feel fine.
Over time, the ventricle’s compensatory stretching becomes permanent. The muscle walls thin, contractile strength fades, and the chamber dimensions grow. In patients with severe atrial enlargement, the left ventricle’s relaxed diameter averaged nearly 65 millimeters compared to 57 millimeters in those with less atrial stretch.
Pressure Backs Up Into the Lungs
The chain of backflow doesn’t stop at the left atrium. As atrial pressure rises, that pressure transmits backward into the pulmonary veins, which drain blood from the lungs into the left atrium. The pulmonary veins and capillaries experience higher pressure than they’re built for.
When this pressure becomes high enough, fluid begins to seep through the thin walls of the lung capillaries into the air sacs. This process, sometimes called alveolar-capillary stress failure, leads to pulmonary edema: fluid in the lungs that causes shortness of breath, especially when lying flat or during exertion. In patients with severe atrial enlargement, the estimated pressure in the lung arteries averaged about 49 mmHg, compared to roughly 40 mmHg in those with less enlargement. Sustained elevation of these pressures can eventually cause pulmonary hypertension, making the right side of the heart work harder too.
Blood Stagnation and Clot Risk
An enlarged, sluggish left atrium creates conditions for blood to pool rather than flow briskly through. This stagnation is especially pronounced in a small pouch called the left atrial appendage. When blood sits still, it’s more likely to clot. If a clot forms and breaks free, it can travel to the brain and cause a stroke.
This risk increases substantially if mitral regurgitation triggers atrial fibrillation, an irregular heart rhythm that’s common when the atrium stretches. The combination of a large, fibrillating atrium and reduced flow velocity through the appendage creates ideal conditions for clot formation. Research published in the BMJ’s Heart journal found that blood stasis in the left atrial appendage identified patients at higher stroke risk regardless of how severe their mitral regurgitation was.
Acute vs. Chronic: Two Different Stories
Not all mitral regurgitation develops gradually. A sudden valve rupture, from a heart attack or torn valve tissue, creates an acute emergency that the heart has no time to adapt to. The left atrium is still normal-sized and stiff, so it can’t absorb the sudden backward surge of blood. Pressure spikes immediately, flooding the lungs with fluid. Blood pressure tends to be significantly higher in acute cases (averaging 148/86 mmHg vs. 124/66 mmHg in chronic cases), because the body is in a compensatory crisis state.
In chronic mitral regurgitation, the heart remodels over months to years. The left atrium gradually stretches and becomes more compliant, absorbing the extra volume without as sharp a pressure spike. The ventricle enlarges to maintain forward output. The valve leaflets themselves can stretch to partially compensate for the forces pulling them apart. This adaptation buys time, but it has limits. Eventually the compensatory mechanisms fail, the ventricle weakens, and the patient transitions from a compensated to a decompensated state, often marked by worsening shortness of breath, fatigue, and fluid retention.
What the Leak Sounds Like
The turbulent jet of blood flowing backward through the leaky valve produces a distinctive sound called a holosystolic murmur, audible through a stethoscope during the entire contraction phase of the heartbeat. It’s typically heard best at the bottom tip of the heart, on the left side of the chest. Doctors grade murmurs on a scale of one to six based on loudness. In severe cases, the turbulence is strong enough that a doctor can feel a vibration (called a “thrill”) by placing a hand over the chest. The loudness of the murmur doesn’t always correlate perfectly with severity, but it’s often the first clue that blood isn’t flowing where it should.