Why is the blood pressure in my right arm different from that of my left?

Ask the doctor

Q. Why are the blood pressure readings in my right and left arm different even when they're taken within a minute or two of each other?

A. The answer depends on how big of a difference you're talking about. A difference of just a few points (that is, a few millimeters of mercury, or mm Hg) is nothing to worry about. It's actually quite normal, even when both arms are checked almost simultaneously.

But a large difference in pressure — about 10 points or more — suggests the presence of artery-clogging plaque in the vessel that supplies blood to the arm with lower blood pressure. If the difference is 20 points or more, then the likelihood of a blockage somewhere in the arm circulation is much higher. Such plaque is a signal of peripheral artery disease (PAD), which refers to cholesterol-clogged arteries anywhere in the body other than the heart. When PAD is present, there's a good chance the arteries serving the heart and the brain are also clogged, boosting the odds of having a heart attack or stroke.

Checking for this potential problem is the reason cardiologists usually measure a person's blood pressure in both arms — if not every time, then at least during an initial consultation. If you don't have a cardiologist, you should ask your primary care physician to check your blood pressure in both arms. If the pressure in one arm is higher, that arm should be the one upon which to base any treatments and to check your blood pressure in the future.

If the difference is 10 points or more, your doctor can use a special cuff to measure the blood pressure in your ankle to calculate your ankle-brachial index, or ABI (the brachial artery in the upper arm is where blood pressure is normally measured). The ABI is simply the ratio between the blood pressures in your ankle and your arm. Normally, blood pressure in the legs is the same or a little higher than in the arm, which means the ratio is 1 or higher. A lower ratio (0.90 or less) means blood is not moving well in the lower half of your body, which is a classic sign of PAD.

About one in seven people over age 60 has PAD. But doctors don't routinely screen for this condition, despite the fact that the ABI test is easy, painless, and inexpensive. If you notice a difference of 10 points of more between your left and right arm blood pressure readings at home, ask your doctor about getting evaluated for PAD.

— by Deepak L. Bhatt, M.D., M.P.H.
Editor in Chief, Harvard Heart Letter

Image: © Mindaugas Kurmis/Getty Images