Comedian and actor Robin Williams, 57, last week postponed a planned 80-city tour of his one-man show, "Weapons of Self-Destruction" to undergo aortic valve replacement surgery. His announcement came just days after 83-year-old former first lady Barbara Bush left a Houston hospital after undergoing the same procedure.

The aortic valve is what keeps oxygenated blood flowing from our heart into the aorta, the largest artery in our body, and prevents it from washing back into the heart with each pump cycle. But as we age, the tricuspid (three-leafed) valve tends harden and thicken, forcing the heart work harder to keep blood flowing smoothly. Open-heart surgery is typically required to replace the valve if it thickens so much that it causes aortic stenosis, an abnormal narrowing and stiffening of the valve.

Although Williams is 26 years younger than Bush, the procedure is not particularly unusual for someone in his or her late 50s. Factors such as obesity and genetics can play role in hastening the decline of the valve's function, but heart infections, typically caused by rheumatic fever or drug use, can also lead to a form of sclerosis. Williams is well-known for his cocaine-infused youth.

To find out more we spoke with Rajendra Makkar, co-director of the Cedars–Sinai Medical Center Heart Institute's Cardiovascular Intervention Center in Los Angeles.

[An edited transcript of the interview follows.]

What is the aortic valve?
The aortic valve is the valve between the left ventricle—one of the heart's four chambers—and the aorta which provides oxygenated blood to the entire body. The heart pumps blood by contracting, and if this one-way valve was not present then when the heart relaxes then most of the blood it just pumped from the ventricle would leak back in, making the heart very inefficient.

Why would someone need a replacement?
The procedure is most commonly performed in the U.S. for a condition called aortic stenosis, where the valve fails to open properly. This is a fairly common condition in the aging population.

It begins as aortic sclerosis, a thickening of the aortic valve by calcification.  About 10 to 15 percent of the population above 55 has this, and about 25 percent of the population over 65 has it. That does not produce much of a problem unless it progresses to stenosis. Over 10 years, roughly 15 percent  of patients with sclerosis will progress to stenosis, and not all of those cases will be severe enough to lead to treatment.

About 2 percent of the population also has[is born with?] a bicuspid valve rather than a tricuspid valve, and this tends to wear out the heart much earlier, leading to stenosis. In the developing world, stenosis can be a consequence of rheumatic fever (caused by untreated strep infection) during childhood. Drug use can also lead to endocarditis, an infection of the heart. Anything that causes an infection of the valves can lead to stenosis.

The most common symptoms of aortic stenosis are related to the fact that the heart has to work much harder to pump blood out. These include chest pain and shortness of breath as well as fainting, known as syncope.

What does the surgery involve?
In the early days, surgeons would open up the entire sternum, put the person on a heart–lung machine, open up the aorta, and stitch in a new valve. These days the surgeons will make a smaller incision, but it is still open-heart surgery, which requires a whole team of people and lasts two to three hours.

Now, there is a technique we are testing, which involves changing the valve percutaneously (without opening the chest). We go from the femoral artery in the groin and work our way back almost 100 centimeters [40 inches] to the aortic valve. The whole procedure takes an hour and a half and is the way the whole field is going. We've done about 100 procedures as part of a clinical trial.

How do they make the replacement valves?

There are mechanical valves, and there are tissue valves. Tissue valves are the most commonly used valves, but they only have a life span of 10 to 15 years. They handcraft these valves from the covering layer of a cow or pig's heart, called the pericardium. From this, they make three leaflets and mount them in a ring. The biggest issue with tissue valves is that they get calcified over time, although companies have proprietary treatments to slow down this process.

Mechanical valves practically last forever, but the disadvantage is that patients have to take a blood thinner to prevent blood clots.

How risky is the traditional surgery?
The mortality of the procedure is about 2 percent, but it goes up in older patients with other significant medical problems. Often these patients require heart bypass surgery at the same time, which can double mortality. About half of patients with stenosis don't get treated, because they are considered too old. As we move forward, percutaneous aortic valve replacement will allow more of these patients to get treated.