Harvard Heart Letter

Your blood pressure: How low should you go?

Too low may be a problem for people with clogged coronary arteries.

Lower is better" has become something of a mantra for protecting the heart. The definition of a healthy cholesterol level has been ratcheted downward over the years, and aggressive lowering of LDL (bad) cholesterol is now standard operating procedure. Blood pressure guidelines have followed a similar, though slower, trend.

There are physical limits to how low blood pressure can go. Problems like dizziness or fainting emerge if blood pressure falls too far. There's also some concern that aggressive blood pressure control may lower diastolic pressure (the bottom number of a blood pressure reading) too much, which could be harmful for people with narrowed or blocked coronary arteries.

Key points

  • Keeping your blood pressure under control is one of the most important things you can do for long-term health.

  • For people with high blood pressure, the goal is generally under 140/90. How much under that target is still being determined.

  • If you have coronary artery disease, keep an eye on your diastolic pressure to make sure it doesn't drift down too far.

Changing goals

Each contraction of the heart's main pumping chamber, the left ventricle, pushes a wave of blood into the arteries. The arteries push back. This dynamic creates blood pressure. It is influenced by several key factors: the force of the contractions, the amount of blood ejected with each beat, and the inner diameter and flexibility of the arteries.

Early in life the arteries are open and supple. They expand easily to allow a pulse of blood to pass, then contract behind it, helping push it along. Over time, smoking, fatty foods, lack of exercise, and other insults cause arteries to narrow and stiffen. As they lose their flexibility, they resist the wave of blood. This contributes to high blood pressure.

High blood pressure was once thought to be a normal, healthy part of aging, an adaptation that helped the heart distribute blood to the organs and tissues. John Hay, a British physician, summed up this idea in a 1931 article in the British Medical Journal. "The greatest danger to a man with high blood pressure lies in its discovery, because then some fool is certain to try and reduce it," he wrote. Diseases of the Heart, an influential textbook published in 1949, advised doctors that "people with mild benign hypertension with levels up to 210/110 need not be treated."

In the 1940s and 1950s, pioneering thinkers and doers began challenging the prevailing wisdom. Using diet (Dr. Walter Kempner's Rice Diet), surgery (a nerve-cutting operation), and various plant extracts and medications, researchers showed that lowering extremely high blood pressure prevented strokes and prolonged life. The development of safe and effective drug therapies showed similar benefits in people with moderately high blood pressure.

We now know that high blood pressure, also called hypertension, is hazardous in many ways. It injures the delicate endothelial cells that line the inside of the arteries. This damage can accelerate the stiffening process and further increase blood pressure. It also sets the stage for inflammation and atherosclerosis, two processes that contribute to heart attack and stroke. High blood pressure can weaken blood vessels and make them prone to springing leaks. When this happens in the brain, it causes a hemorrhagic (bleeding) stroke. Hypertension can also damage the kidneys, eyes, brain, and a host of other organs.

These discoveries shifted the goal posts of a healthy blood pressure. In the 1970s, doctors believed that a decent systolic blood pressure was your age plus 100. In the 1980s, national guidelines set normal at anything under 140/85. Today, a healthy blood pressure is under 120/80. People with high blood pressure are urged to get it below 140/90; those who also have kidney disease or diabetes are urged to go further, with goals under 130/80. How far under remains to be determined.

Blood pressure basics

Blood pressure is the force that a wave of blood propelled from the heart exerts on the arteries. It is measured at two points; each measurement is 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). It is the top number of a blood pressure reading.

Diastolic pressure reflects 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

Swing low

High blood pressure is silent, causing no symptoms or immediate problems unless it is sky-high. In fact, people who believe they can feel when their blood pressure is high are usually wrong. Low blood pressure, on the other hand, often causes noticeable symptoms. These include blurry vision, confusion, dizziness, fainting, light-headedness, and weakness. The symptoms are the same whether the low pressure is "natural" or whether it occurs as the result of drug therapy. These symptoms limit how aggressively blood pressure can be controlled.

For people with coronary artery disease, there may be another limit. When it comes to blood pressure, the heart differs from other organs in one very important way: the coronary arteries, which supply heart muscle cells with oxygen and nourishment, fill with blood when the heart relaxes between beats, not when it contracts. For people with healthy coronary arteries, even low diastolic pressure is enough to fill the supply lines. But when coronary arteries become clogged with plaque, filling them may take an extra push. That has led to worries that for people with coronary artery disease, aggressively lowering blood pressure might do harm as well as good.

There's some evidence this is more than a theoretical possibility. Dr. Franz Messerli, a hypertension expert at St. Luke's–Roosevelt Hospital in New York, and his colleagues have looked at the effect of reducing blood pressure in two large clinical trials. In both trials, lowering pressure was a good thing for people with coronary artery disease — up to a point. Participants whose diastolic pressures dropped into the 70s or 60s were more likely to have heart attacks than those whose diastolic pressures stayed above that level.

The two studies aren't smoking guns. Neither was done to explore the relationship between low diastolic pressure and cardiovascular risk. It is possible that other diseases or factors could account for the difference, and that diastolic pressure is a red herring.

A definitive answer about the best goal for diastolic blood pressure is several years down the road. The ongoing Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial is looking at the effect of lowering systolic blood pressure below 120 in people with diabetes, which will almost certainly drive diastolic pressure below 80. And the National Heart, Lung, and Blood Institute is planning the nine-year Systolic Blood Pressure Intervention Trial (SPRINT), which will randomly assign volunteers with high blood pressure to systolic pressure goals of under 140 or under 120.

What to do in the meantime? "It's reasonable for people with clear-cut coronary artery disease and high systolic blood pressure to exercise caution so they don't lower diastolic blood pressure too much," says Dr. Aram V. Chobanian, president emeritus of Boston University and past head of the panel that set federal blood pressure guidelines.

That means working hard to bring your systolic pressure under control (to 140 or so) while keeping an eye on your diastolic pressure. It's especially beneficial if you include exercise in doing this, since it offers improvements far beyond blood pressure.

Relax your way to lower blood pressure

The easiest way to lower blood pressure is by popping a pill. It takes just a few seconds and you are finished. But pills cost money and often have side effects. Fully relaxing your body and your mind for a few minutes a day could lower your systolic blood pressure by 10 points or more — at no cost, and with no side effects.

Researchers with the Benson-Henry Institute for Mind-Body Medicine at Harvard-affiliated Massachusetts General Hospital compared a stress management technique called the relaxation response with education about lifestyle changes such as sodium reduction, weight loss, and exercise. Blood pressure decreased more in the relaxation response group, and 32% of the volunteers in that group were able to eliminate one antihypertensive medication and still keep their blood pressure under control, compared with 14% of the lifestyle group.

The relaxation response, developed by Harvard's Dr. Herbert Benson, has been shown to reduce heart rate, blood pressure, breathing rate, and muscle tension. Here's how to do it:

  • Select a word (such as "one" or "peace"), a short phrase, or a prayer to focus on.

  • Sit quietly in a comfortable position and close your eyes.

  • Relax your muscles, progressing from your feet to your calves, thighs, abdomen, and so on, up to your neck and face.

  • Breathe slowly through your nose, silently saying your focus word, phrase, or prayer to yourself as you exhale.

  • When other thoughts come to mind, don't worry. Simply return your attention to your focus word, phrase, or prayer.

  • Do this for 10–20 minutes.

  • Sit quietly for a minute or so, then open your eyes.

  • Practice the relaxation response once or twice a day.

Focus on high

While low diastolic pressure may be a conundrum, high blood pressure is a problem of monumental proportions. An estimated 73 million Americans have high blood pressure, and one-third of them don't know it. The majority don't have it under control. That's worrisome, because high blood pressure is the cause of two-thirds of all strokes and half of heart attacks and other forms of coronary artery disease, and contributes to heart failure, kidney disease, vision loss, and even dementia.

Don't let the possibility of harm from low diastolic pressure divert you from the proven benefits of getting your systolic pressure under control.