Hypertension: Controlling the “silent killer”

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Are you at risk for hypertension?

Harvard Medical School – Essential hypertension has no known cause. As a result, identifying clear risk factors is difficult. Researchers have discovered a few patterns, however. Some factors you have no control over — for example, you can’t alter your genes. But others, like smoking and heavy drinking, are habits you can change.

Risk factors you can’t change

Even though you can’t control these risks, that doesn’t mean you can forget about them. Awareness of your risk factors can help you put your overall cardiovascular risk profile into perspective and may provide you with extra incentive to adopt healthier habits.

Race

Hypertension often develops earlier and with more ferocity in African Americans than in other races. African Americans are nearly twice as likely to suffer a fatal stroke, 1½ times more likely to die from heart disease, and four times more likely to suffer kidney failure than are whites. For black men, the picture is particularly disturbing — they face a death rate from disorders related to high blood pressure that’s more than three times that of the death rate in white men.

The high incidence of hypertension among African Americans may have a genetic explanation. Some researchers suspect that people who lived in equatorial Africa developed a genetic predisposition to being salt sensitive, which means their bodies retain more sodium. This condition increases blood volume, which, in turn, raises blood pressure. Salt sensitivity can be beneficial in a hot, dry climate because it allows the body to conserve water. Generations later, however, the American descendants of these individuals remain disproportionately salt sensitive (see Figure 2).

On a positive note, the 1997 Dietary Approaches to Stop Hypertension (DASH) study showed that a low-fat diet rich in fruits, vegetables, and fiber was particularly beneficial to African Americans, especially when it was combined with additional limits on salt consumption (see “How the DASH diet helps”).

Salt sensitivity and race

Family history

Hypertension, like many disorders, runs in families. In addition, a family history of heart attack, stroke, diabetes, kidney disease, or high cholesterol increases your risk of developing high blood pressure.

This doesn’t necessarily mean, however, that genetics always plays a role. Some of the similarities observed in families may be the result of environmental influences. Children’s eating patterns, coping skills, and propensity toward healthy and unhealthy habits are shaped by their parents’ behavior and the social climate in which they’re raised.

Research indicates that about 25% of cases of essential hypertension in families and up to 65% of cases of essential hypertension in twins may have a genetic basis. In addition, at least 10 genes have been found to influence blood pressure. So far, however, only a few studies have identified a link between particular genes and hypertension. For instance, a rare form of hypertension called Liddle’s syndrome, which develops in childhood and often leads to an early death from cardiovascular disease, results from a defective gene that causes the kidneys to retain too much sodium and water.

Age

Although aging doesn’t invariably lead to hypertension, high blood pressure becomes more common in later years. Diastolic pressure increases an average of 10 mm Hg up to age 55 in men and age 60 in women, and then begins to decline. Between ages 30 and 65, systolic pressure increases an average of 20 mm Hg, and it continues to climb after 70. This age-associated increase largely explains isolated systolic hypertension.

Sex

Up to about age 55, women have a lower incidence of hypertension and other cardiovascular diseases than men do. But women’s blood pressures, especially the systolic readings, rise more sharply with age. Indeed, after age 55, women are at greater risk for high blood pressure. This pattern may be partly explained by hormonal differences between the sexes. Estrogen tends to protect women against cardiovascular diseases, including hypertension, but as the production of estrogen drops with menopause, women lose its beneficial effects, and their blood pressures climb.

Controllable risk factors

Your health habits are key factors in determining your cardiovascular risk. In fact, you may be able to bring your blood pressure readings into a safe range simply by making changes in your lifestyle, such as quitting smoking and losing weight.

Smoking

Doctors have long known that smoking promotes heart disease, but for a long time smoking didn’t appear to have a direct connection to hypertension. Observations have revealed a crucial link that earlier studies missed because blood pressure is generally measured in doctors’ offices and clinics, where smoking is prohibited.

When researchers tested blood pressure while people smoked, they discovered that within five minutes of lighting up, the subjects’ systolic pressures rose dramatically — more than 20 mm Hg, on average — before gradually declining to their original levels over the next 30 minutes. This means the typical smoker’s blood pressure soars many times throughout the day. Like people with labile hypertension (in which blood pressure may jump frequently in response to daily stresses), smokers may suffer “part-time” hypertension. For example, smokers with a prehypertensive reading of less than 140/90 mm Hg may actually have stage 1 hypertension every time they puff a cigarette.

This increase occurs because nicotine, whether smoked or chewed, constricts small blood vessels, forcing the heart to work harder to circulate blood. As a result, the heart speeds up and blood pressure rises. Nicotine also interferes with some antihypertensive drugs, most notably beta blockers. The chemicals in tobacco smoke raise heart disease risk in other ways, too. They can reduce the body’s oxygen supply, lower levels of HDL (“good”) cholesterol, and make blood platelets more likely to stick together and form clots that can trigger a heart attack.

Excess salt

Doctors first noticed a link between hypertension and sodium chloride — the most common form of dietary salt — in the early 1900s, when they found restricting salt in patients with kidney failure and severe hypertension brought their blood pressures down and improved kidney function. When a massive effort began in the 1960s to educate the public about reducing the risk of heart disease, one recommendation was that all Americans decrease salt consumption to prevent hypertension.

Federal guidelines advise people to limit sodium intake to 2,300 milligrams (mg) per day — about the amount in 1 teaspoon of table salt. Yet Americans typically consume 1–3 teaspoons or as much as 7,200 mg a day (see Figure 3). This fact, coupled with the high prevalence of hypertension in the United States, led researchers to assume that salt overload was the culprit.
American salt consumption

As it turns out, this may or may not be true. Nearly 50% of people who have hypertension are salt sensitive, meaning eating too much sodium clearly elevates their blood pressure and puts them at risk for complications. In addition, people with diabetes, the obese, and older people seem more sensitive to the effects of salt than the general population. However, the question of whether high salt consumption also puts generally healthy people at risk for hypertension is the source of considerable debate (see “Consume less salt”). Regardless of whether high salt intake increases blood pressure, it does interfere with the blood pressure–lowering effects of antihypertensive medications.

Obesity

Excess weight and hypertension often go hand in hand because carrying even a few extra pounds forces your heart to work harder. People who are overweight or obese are also more likely to develop diabetes, heart disease, arthritis, gallstones, sleep apnea, gout, and some cancers.

High blood pressure is about six times more common in people who are obese than in those who are lean. Twenty-two pounds more weight boosts systolic blood pressure by 3 mm Hg and diastolic blood pressure by 2.3 mm Hg. These increases cause a 12% greater risk of heart disease and a 24% greater risk of stroke, according to a 2006 statement from the American Heart Association.

What’s a healthy weight for you? Let the body mass index (BMI) guide you. A BMI of 25 to 29 indicates that an individual is overweight, while a BMI of 30 or above designates obesity.

The body mass index (BMI) is an index of weight by height. The definitions of normal, overweight, and obese were established after researchers examined the BMIs of millions of people and correlated them with rates of illness and death. These studies found that the normal BMI range is associated with the lowest rate of illness and death.

Another thing to keep in mind is that it’s not weight alone that matters, but also where you carry your extra weight. People with excess fat in the abdominal area (see Figure 4) are not only at greater risk for hypertension, but also for high cholesterol and diabetes. So if your BMI is 25 or more, and especially if you have accumulated abdominal fat, the unfortunate reality is that you need to lose weight.

WHR

The waist to hip ratio (WHR) is one way to estimate how much weight a person is carrying around the abdomen versus around the hips. Men and women with a higher WHR (resembling an apple shape) have a higher risk for heart attack and stroke than men and women with a lower WHR (resembling a pear shape).

To determine your WHR:

With your abdomen relaxed, measure your waist at its narrowest (usually at the navel).
Measure your hips at the widest point (usually at the bony prominence).
Divide the waist measurement by the hip measurement to find your ratio (Waist measurement/hip measurement = WHR).

A healthy WHR for women is 0.8 or less (and a waist measurement of 35 inches or less), and a healthy WHR for men is 1.0 or less (and a waist measurement of 40 inches or less).

Sedentary lifestyle

Compared with the physically active, sedentary people are significantly more likely to develop hypertension and suffer heart attacks. Like any muscle, your heart gets stronger with exercise. A stronger heart pumps more blood more efficiently, with less force, through your body. Other cardiovascular benefits of exercise include losing excess weight, increasing levels of “good” HDL cholesterol, and making stroke-causing clots less likely.

Heavy drinking

Excessive drinking — having three or more drinks per day — is a factor in about 7% of hypertension cases. It can also interfere with antihypertensive medications, increase your risk of stroke, and lead to heart failure.

While moderate alcohol consumption (no more than one drink per day for women and two drinks a day for men) significantly lowers your risk of cardiovascular disease and has little effect on your blood pressure, heavier drinking has the opposite effect. How alcohol raises blood pressure is unknown, but it appears that once you go past two drinks per day, the more you drink, the higher your blood pressure. This effect becomes more pronounced as you age and occurs regardless of what type of alcohol you drink.

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Blood pressure basics

You can’t see your blood pressure or feel it, so you may wonder why this simple reading is so important. The answer is that measuring your blood pressure gives your doctor a peek into the workings of your circulatory system. A high number means that your heart is working overtime to pump blood through your body. This extra work can result in a weaker heart muscle and potential organ damage down the road. Your arteries also suffer when your blood pressure is high. The relentless pounding of the blood against the arterial walls causes them to become hard and narrow, potentially setting you up for stroke, kidney failure, and cardiovascular disease.

Having your blood pressure measured is a familiar ritual at most visits to the doctor’s office. The examiner inflates a cuff around your upper arm, listens through a stethoscope, watches a gauge while deflating the cuff, and then scribbles some numbers on your chart. You may be relieved if you learn your blood pressure is normal or alarmed if the examiner says “180 over 100.” But what do these numbers actually mean?

Understanding the numbers

Blood pressure is recorded as millimeters of mercury (mm Hg) because the traditional measuring device, called a sphygmomanometer, uses a glass column that’s filled with mercury (whose chemical symbol is Hg) and is marked in millimeters. A rubber tube connects the column to an arm cuff. As the cuff is inflated or deflated, mercury rises and falls within the column (see Figure 1). Although mercury gauges are still considered the gold standard for measuring blood pressure, newer mercury-free devices are available. Many modern instruments use a spring gauge with a round dial or a digital monitor, but even these are calibrated to give readings in millimeters of mercury.

Measuring blood pressure

A health care professional measures a patient’s blood pressure using a stethoscope and a cuff that is inflated until the pressure it exerts is greater than the patient’s systolic pressure (the pressure when the heart contracts). The cuff compresses the arm until the brachial artery is squeezed shut. At first, the artery walls will be closed, and the clinician will not hear anything through the stethoscope. As air is released from the cuff, he or she will hear a thump. This is the moment when the clinician records the systolic blood pressure — the first and higher of the two numbers in a person’s blood pressure. As the cuff pressure continues to drop below the level of systolic pressure, the artery will begin to open and close, and the clinician will hear a thumping noise. When the rhythmic sound disappears, he or she records the diastolic pressure — the second, lower figure. As the cuff pressure declines below the diastolic pressure in the artery (the pressure between heartbeats), the vessel remains open, and no further sounds are heard.

The top number, or systolic pressure, reflects the amount of pressure during the heart’s pumping phase, or systole. As the heart contracts with each beat, pressure in the arteries temporarily increases as blood is forced through them. The bottom number, or diastolic pressure, represents the pressure during the resting phase between heartbeats, or diastole. Hypertension is defined as having a systolic reading of at least 140 mm Hg or a diastolic reading of at least 90 mm Hg (see Table 1).
How high is high blood pressure?

The Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC), a group of physicians and researchers from across the United States, developed these guidelines for classifying blood pressure in 2003. The figures are based on extensive reviews of the scientific literature and are updated periodically to keep pace with new research.

To classify your blood pressure, a health professional averages two or more readings taken after you have been seated quietly for at least five minutes. For example, a patient with a measurement of 135/85 mm Hg on one occasion and 145/95 mm Hg on another has an average blood pressure of 140/90 mm Hg and is said to have stage 1 hypertension.

When systolic and diastolic pressures fall into different categories, the JNC advises physicians to rate overall blood pressure by the higher category. For example, 150/85 mm Hg is classified as stage 1 hypertension, not prehypertension. This is also an example of systolic hypertension — defined as a systolic pressure of 140 mm Hg or higher and a diastolic pressure below 90 mm Hg.

The JNC notes that people in the normal category — those with blood pressure below 120/80 mm Hg — have the lowest risk of developing cardiovascular disease. Patients in the “prehypertension” category have a greatly increased risk of developing hypertension and should make changes in their lifestyle to reduce the risk. Patients with stage 1 hypertension generally require medication, although aggressive changes in lifestyle sometimes eliminate the need for medication.

What does blood pressure measure?

Blood pressure reflects both how hard your heart is working and what condition your arteries are in. The formula is as simple as ABC — or actually, C × A = B. That is, cardiac output times arterial resistance equals blood pressure.

Cardiac output is the amount of blood your heart pumps per minute. With each beat, your heart propels about 5 ounces of blood into the arteries. That adds up to about 4 to 5 quarts over the course of a minute of normal activity. During strenuous activity, your heart must pump considerably more blood to meet your body’s increased demand for oxygen.

Arterial resistance is the pressure the walls of the arteries exert on the flowing blood. As blood pushes into the arteries with each heartbeat, it forces the artery walls to expand, much like an elastic waistband stretches to accommodate your body. When the blood flow ebbs, the vessel returns to its original shape. The less flexible the vessels are, the greater the arterial resistance. Narrowed, tightened, or inflexible vessels result in a higher pressure at any level of flow. As cardiac output or arterial resistance increases, so does blood pressure.

Natural blood pressure controls

Your blood pressure is never constant, nor should it be. Your body continually adjusts cardiac output and arterial resistance to deliver oxygen and nutrients to the tissues and organs that most need them — your muscles during a jog or your digestive system at mealtime, for example. Your blood pressure also varies according to the time of day. It’s highest in the morning and lowest at night during sleep.

Your body can make dramatic adjustments in blood pressure within seconds. A sprint for the elevator, the sound of breaking glass, or a confrontation with someone may send blood pressure soaring from an idling 110/70 mm Hg to a racing 180/110 mm Hg or higher.

These changes occur without conscious thought and are directed by complex interactions among your central nervous system, hormones, and substances produced in your blood vessels. The layer of cells lining the inner wall of blood vessels (known as the endothelium) produces an enormous number of vasodilators and vasoconstrictors — chemicals that cause the vessels to widen or narrow. The endothelium helps maintain the tone of your blood vessels by releasing these substances as your body’s needs change. As long as your blood pressure is in the normal range, healthy vessels tend to be dilated.

When blood pressure gets too high (such as during times of stress) or too low (when you’re dehydrated, for example), pressure-sensing nerve cells located throughout your circulatory system relay this information to your autonomic nervous system. The autonomic nervous system manages the involuntary activities of smooth muscles, including those in the intestines, sweat glands, airways, heart, and blood vessels. It responds by setting off a chain of events designed to restore blood pressure to normal levels.

A complex chain reaction

The autonomic nervous system is divided into two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system prepares the body for action by quickening heart rate and breathing, while the parasympathetic nervous system has the opposite effect. The sympathetic nervous system rules during times of stress or fear. The parasympathetic governs during sleep.

When your blood pressure drops suddenly, the sympathetic nervous system compensates by releasing two neurotransmitters, or chemical messengers, from nerve endings: norepinephrine and epinephrine (also called adrenaline). These substances stimulate your heart muscle and cause your blood vessels to tighten. This reaction speeds your heart, increases cardiac output, and raises your blood pressure. To lower the pressure, the parasympathetic nervous system releases acetylcholine, a neurotransmitter that slows the heart.

The autonomic nervous system can also trigger specific organs to release chemicals that regulate blood pressure. For example, when blood pressure drops, the sympathetic nervous system signals your kidneys to release an enzyme called renin into the circulatory system. Renin, in turn, triggers the production of angiotensin, a protein that helps increase pressure by constricting the walls of small arteries. Angiotensin also stimulates your adrenal glands to secrete the hormone aldosterone, which causes the kidneys to conserve sodium and water, thereby raising blood volume and blood pressure. Together, this sequence of events is called the renin-angiotensin-aldosterone cascade.

Given the many mechanisms the body uses to regulate blood pressure, there are a number of ways something could go wrong. Some researchers suggest, for instance, a lack of vasodilators — particularly nitric oxide, which is also known as endothelium-derived relaxing factor — or an overproduction of certain vasoconstrictors, such as endothelin, can cause some cases of hypertension, although this hasn’t been proved.