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What's the best target for blood pressure when it is high?

AUG 2011

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Whether it is best to aim for "normal" or float a bit above remains to be determined.

Blood pressure isn't a bad thing. Quite the contrary — the heart needs a certain amount of pressure to deliver oxygen-rich blood to the farthest reaches of the body and the brain. Blood pressure becomes a problem only when it rises too high or falls too low.

High blood pressure wreaks havoc in the circulatory system. It does its dirty work silently and over the course of many decades. High blood pressure (also known as hypertension) is a key contributor to heart attack and stroke. It promotes the development of abdominal aortic aneurysm and can damage the kidneys, eyes, and other organs.

Low blood pressure is harmful in more immediate ways. It can cause blurry vision, confusion, dizziness, weakness, fainting, and falls.

A graph of the connection between blood pressure and health forms a curve that looks like the letter "U." The bottom of the curve represents the safest blood pressure range. As blood pressure rises above that range, so does cardiovascular risk. As blood pressure falls below that range, the hazards of low blood pressure increase.

Among healthy people, the curve bottoms out at a systolic blood pressure (see "Blood pressure basics") around 120 and a diastolic pressure around 80. These are the current "optimal" values for blood pressure. It's doubtful these targets will ever get much lower, given the physical problems that come with low blood pressure. But there's a lingering — and important — question about the low point of the curve for people with high blood pressure, especially those with clogged coronary arteries (coronary artery disease), diabetes, or kidney disease. Should they aim to get down to the 120/80 mark, or is it better to float somewhere above it?

Almost every tissue in the body gets a fresh pulse of oxygenated blood when the heart contracts. The coronary arteries are the exception — they fill with blood when the heart relaxes between beats. Healthy coronary arteries can become filled even when the diastolic pressure is pretty low. But coronary arteries clogged with plaque may need a diastolic pressure higher than 80 to fill completely. Poor filling could make the heart work harder and less efficiently.

Some experts worry that lowering systolic blood pressure too far will compromise blood flow to the heart muscle by dropping the diastolic pressure.

Hints of problems due to low diastolic pressure have been seen in clinical trials conducted for other reasons. Researchers are now testing it directly.

Blood pressure basics

Blood pressure is the force exerted on the arteries by a wave of blood propelled from the heart. It is given as two numbers, each measurement recorded in millimeters of mercury (mm Hg).

Systolic pressure gauges the pressure in the arteries at systole (SIS-tuh-lee), the instant when the heart contracts and pushes a wave of blood along the arterial tree (think "s" for squeeze). Systolic pressure is the top number of a blood pressure reading. Diastolic pressure is the pressure during diastole (die-AS-tuh-lee), the brief period of relaxation between beats. It is the bottom number of a blood pressure reading.

Hypertension is the formal name for high blood pressure.

Blood pressure categories

Systolic

Diastolic

Normal (optimal)

less than 120

and

less than 80

Prehypertension

120–139

or

80–89

Hypertension

140 or higher

or

90 or higher

Balancing act

The Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial tested two systolic blood pressure targets — 140 or lower versus 120 or lower — among people with type 2 diabetes. During the five-year trial, there were just as many heart attacks and cardiovascular deaths among the volunteers who took extra medications and aimed for a systolic blood pressure of 120 as there were among those aiming for a systolic pressure of 140 (New England Journal of Medicine, Apr. 29, 2010). A Japanese trial conducted among elderly men and women with high blood pressure had similar results.

The Systolic Blood Pressure Intervention Trial (SPRINT) is a U.S. trial that hopes to identify the best target pressure for people with high blood pressure. The nine-year, $114 million trial will compare the effects of lowering systolic blood pressure to 140 or below or to 120 or below in people with heart disease or at high risk for it.

What should you do until SPRINT's results are in? Work with your doctor to get your systolic pressure down to at least 140 while keeping an eye on your diastolic pressure. You could also take part in the trial (details are available at www.health.harvard.edu/171).

Most of the 70 million Americans with high blood pressure don't have it under control. An as-yet unproven harm from low diastolic pressure shouldn't keep anyone from the proven benefits of getting his or her systolic pressure under control.