Mitral Stenosis: A Common Heart Condition During Pregnancy

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Heart disease is not so common during pregnancy, but mitral stenosis is one of the most common heart conditions to affect pregnant women, and the most common major heart valve condition among this group of people. We have discussed heart conditions in recent posts, so you may be wondering why I’m devoting an entire post to mitral stenosis in particular? More generally, you may be wondering what a valve is in the heart, what can go wrong, and why it makes a difference if something does go wrong, before you are pregnant and once pregnancy begins.

In the heart, a valve is a structure through which blood is supposed to pass easily in one direction, while flow in the opposite direction is stopped. The heart has four valves. Channeling blood flow out of the ventricles are the pulmonic valve, through which blood passes from the right ventricle into the pulmonary artery, and the aortic valve, through which blood passes from the left ventricle to the aorta, and from there throughout the body. The other two valves —the tricuspid and the mitral (also called the bicuspid) valves— channel blood into the ventricles. Blood moves from the right atrium (right upper chamber) through the tricuspid valve into the right ventricle, through the pulmonic valve, the pulmonary artery, and lungs, then through the pulmonary veins into the left atrium. From there, blood moves through the mitral valve into the left ventricle and through the aortic valve.

In terms of things going wrong, you hear much more about the two valves on the left side of the heart (the mitral and aortic) than about the two valves on the right side (tricuspid and pulmonic). This is partly because blood on the left side of the heart is under higher pressure, so the valves have to work better to maintain health, while the valves on the right can get away with being a little less perfect. Of the two valves on the left, the mitral valve is more commonly affected in young and middle age adults, in a way that causes it to become narrowed, what doctors call stenotic. A person who has a narrowed, or stenotic, mitral valve is said to have mitral stenosis. The most common reason for this is rheumatic heart disease, resulting from rheumatic fever. This, in turn, results from an A Streptococcus infection (the group of bacteria that causes strep throat among other things), when it is not treated early enough with antibiotics. Today, this is rare in the United States and other western countries, but rheumatic heart disease is still fairly common throughout the world. The commonality of rheumatic fever in many countries, the vulnerability of the mitral valve to long-term damage, and the timing of this process are the reasons why mitral stenosis is the most common major problem of heart valves in pregnant women.

So why is mitral stenosis such a problem? Normally, the mitral valve has a wide opening. This allows a very large volume of blood to pass easily from the left atrium into the left ventricle, while the ventricles are relaxing, the part of the heart cycle that doctors call diastole. Blood passes so easily through a normal mitral valve that there is essentially no difference between the blood pressure just upstream from the valve in the atrium and the pressure just downstream from the valve in the ventricle, prior to the valve snapping shut, when the ventricle contracts. In mitral stenosis, however, there is a pressure gradient, meaning a higher pressure on the atrial side and a lower pressure on the ventricular side. The worse the stenosis (the narrower the space through the valve), the higher the gradient, the more sick the person feels. When the ventricle contracts during systole, the valve may close just fine, but the ventricle has not received as much blood as it needed to receive. This is because less blood can move through a narrower opening in any given amount of time. Not having enough blood in the left ventricle at the start of systole means that the ventricle will not pump as much blood as it should pump through the aortic valve. Such a ventricle has what doctors call a low stroke volume, an abnormally low volume of blood pumped out with each heart beat.

Meanwhile, there are problems upstream from the valve. The pressure in the left atrium is elevated, causing the pressure to elevate back through the lungs and all the way back to the right side of the heart. When mitral stenosis is severe, this can cause fluid to accumulate in the lungs and cause problems in the right side of the heart. The disruption on both side of the heart also can cause heart rhythm problems, such as atrial fibrillation, meaning that the two atria are only quivering, not pumping. This can cause clots to form in the heart, which can produce emboli (traveling clots) that can cause clots in other parts of the body. Problems with the pressures and other effects all get worse during pregnancy, because the volume of blood increases dramatically as pregnancy advances.

For doctors to treat mitral stenosis, the main strategy is to increase the stroke volume, the amount of blood that is pumped from the heart with each beat. As noted earlier, this depends on the amount of blood that can pass through the mitral valve during each beat. This can be increased by extending the time of each beat; in other words, by slowing down the heart with medications. There are different families of medications for doing this. Some can be used safely during pregnancy, while others cannot. Meanwhile, doctors will also use another tactic, which is to give medicines called diuretics that make you excrete fluid. This helps keep fluid from building up in the lungs. Doctors also may give such a woman low molecular weight heparin (LMWH) to prevent blood clots. Generally, all of this will be managed by cardiologists and obstetricians who specialize in high risk pregnancies.

If your mitral valve problem is severe, you may be counseled to avoid pregnancy, or to terminate a pregnancy that has begun. However, if you still wish to pursue the pregnancy, the next option is to perform a procedure to improve the function of the valve. Surgery is complicated to perform on a pregnant woman, but there is a technique called valvuloplasty, which does not require surgeons to open the chest, and often works well for a stenotic mitral valve. The technique is performed by an interventional cardiologist using instruments passed through blood vessels, right into the heart. The other option is called mitral valve replacement, which means that an artificial valve is inserted to replace the failing valve. This strategy can be used as an alternative to valvuloplasty in a valve that is only stenotic, and is the only option when the valve also has other things going wrong, such as regurgitation (it doesn’t close all the way). If required during pregnancy, usually such procedures are performed preferably during the second trimester.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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