Aortic Stenosis: What a Narrowed Heart Valve Does and Why It Leads to Replacement

Aortic stenosis is a narrowing of the aortic valve, the one-way door that lets blood leave the heart for the rest of the body, and as the valve stiffens and stops opening fully the heart has to strain harder to push blood through. For about a year I called mine “getting old”. I was breathless on the stairs and I stopped halfway up the hill to the shops without knowing why. The word, when it finally came, was aortic stenosis, and this is the plain explanation of what it is, what causes it, and why, left to run on, it ends in a new valve.

What aortic stenosis is

Aortic stenosis is a narrowing of the aortic valve, so the valve cannot open fully and blood struggles to leave the heart. The aortic valve sits at the exit of the heart’s main pumping chamber, the left ventricle. With every beat it should swing wide open to let blood surge out into the aorta, then snap shut so none leaks back ¹.

When the valve narrows, that door only opens part way. The left ventricle has to generate far higher pressure to force the same amount of blood through the smaller gap. Over time the muscle thickens to cope, and eventually it starts to tire and fail. The problem is mechanical: a stiff, restricted opening where there should be a wide one ². Cardiologists call the narrowing severe once the valve’s opening has shrunk below 1.0 cm squared, roughly a quarter of a healthy valve’s area ³.

What causes it

By far the most common cause of aortic stenosis is age-related calcification, where calcium gradually deposits on the valve and stiffens it. It is essentially wear on a part that has opened and shut more than two billion times. The risk rises steeply with age, and aortic stenosis affects roughly 2 to 4% of people over 75 ¹.

There are other routes to a narrowed valve. Some people are born with a valve that has two leaflets instead of the usual three, a bicuspid valve, which tends to wear out and narrow earlier in life ². In parts of the world rheumatic fever remains a cause. But in older people in high-income countries, the everyday cause is calcium, which is why aortic stenosis has become steadily more common as populations age ⁴.

The symptoms, and why they are easy to miss

The classic symptoms of aortic stenosis are breathlessness on exertion, chest tightness, and dizziness or blackouts, though the valve can be quite narrow before any of them appear. The heart is remarkably good at compensating, so for years there may be nothing to feel at all ⁴.

When symptoms do arrive they tend to show up with effort first. Mine did. I was breathless on the stairs, I had to stop partway up the hill, and once, in the garden, the world went grey at the edges and I had to sit down on the wet grass. Those three, breathlessness, chest pain, and fainting or near-fainting, are the warning triad, and the last two matter especially because they signal the heart is struggling to get enough blood out during exertion ¹. The catch is how gently they creep up: many people have no symptoms at all until the valve area has already fallen below 1.0 cm squared, into the severe range ⁵, which is exactly why so many of us blame age and wait too long.

How severity is measured

Severity is not a guess; it is measured on an echocardiogram, and severe aortic stenosis has defined thresholds. The scan uses ultrasound to see the valve and measure how fast blood is forced through the narrowing, which reveals how tight it is ³.

Severe aortic stenosis is defined as a valve area below 1.0 cm squared, a mean gradient (the average pressure difference across the valve) above 40 mmHg, or a peak jet velocity above 4 m/s ⁵. Below those figures the narrowing is graded mild or moderate and is usually monitored with repeat scans rather than treated. These numbers are the same ones cardiologists across Europe and North America use, drawn from the 2020 ACC/AHA and 2021 ESC/EACTS valve guidelines, so the grade you are given means the same thing wherever you are seen ³.

Why severe aortic stenosis leads to valve replacement

Once aortic stenosis is severe and causing symptoms, the outlook without treatment is poor, and replacing the valve is the only thing that changes it. This is the hinge of the whole condition. While it stays mild or moderate, aortic stenosis can be watched for years. Once it becomes severe and symptomatic, survival is often quoted at around 50% at 2 years without valve replacement ³.

No tablet can reopen a calcified valve, so the definitive treatment is to replace it ². For a long time that meant open-heart surgery. It increasingly does not. Many people, especially those who are older or higher risk, now have the valve replaced through a catheter threaded up an artery, which is the subject of what TAVR is and how it works. Which route suits whom, the keyhole procedure or the operation, is a genuine decision with trade-offs on both sides, covered in TAVR versus open-heart surgery. That was the decision I faced at 74, and understanding the condition first is what let me follow it.

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General information, not medical advice. Aortic stenosis varies a great deal from person to person, and whether and how to treat it is a decision for your own cardiologist and heart team, who can examine you and read your scans.