Hypertension is diagnosed when blood pressure readings are 140/90 mm Hg (millimeters of mercury, the height of mercury in the gauge) or higher on at least two doctor visits. At least 65 million Americans have hypertension, and nine out of 10 middle-aged people will eventually develop the condition. Although hypertension may not produce symptoms, it is a serious condition and is a primary cause of stroke, coronary heart disease, heart failure, kidney disease, and blindness. In most cases, hypertension is easily detected and usually controllable with lifestyle modifications—such as diet and exercise—and medication.
Blood pressure readings include two values: Systolic pressure, the higher number, corresponds with the peak pressure in the arteries when the heart contracts; diastolic pressure, the lower number, reflects the lowest pressure in the arteries as the heart relaxes. People with a blood pressure level less than 120/80 mm Hg have normal blood pressure. People with a systolic blood pressure between 120 and 139 mm Hg or a diastolic blood pressure between 80 and 89 mm Hg have a condition called prehypertension. These individuals are at increased risk for developing hypertension. Losing weight, eating a diet rich in fruits and vegetables, reducing salt intake, increasing physical activity, moderating alcohol consumption, and quitting smoking can help lower the risk. Most individuals with hypertension should aim to keep their blood pressure below 140/90 mm Hg. For those with diabetes or kidney disease, blood pressure should be maintained below 130/80 mm Hg.
Blood pressure, the amount of tension that blood exerts on the walls of blood vessels as it travels through the circulatory system, rises and falls with each heartbeat. Systolic pressure, the higher number, corresponds with the peak pressure in the arteries when the heart contracts; diastolic pressure, the lower number, reflects the lowest pressure in the arteries as the heart relaxes.
Blood pressure fluctuates throughout the day under the direct influence of the heart, the arteries, and the kidneys. During exercise, for example, the heart beats faster and more forcefully to raise blood pressure and deliver extra oxygen and nutrients to the muscles; blood pressure drops as the heart slows during sleep. Dilation of small arteries decreases blood pressure, while constriction of these arteries increases blood pressure. The kidneys affect blood pressure by increasing or decreasing the amount of sodium and water excreted in the urine, which affects the volume of blood in the arteries. The heart, arteries, and kidneys control blood pressure through an elaborate network of nerves and hormones.
Normally, this complex regulatory system allows blood pressure to rise and fall as needed while staying within a desirable range. In many people, however, abnormalities in this system lead to chronically elevated blood pressure, or hypertension.
Causes of Hypertension
In 90 percent to 95 percent of people, it is difficult to pinpoint the exact cause of hypertension. In these individuals, the condition is called primary hypertension. When hypertension has an identifiable cause, it is called secondary hypertension. About 5 percent of people with hypertension fall into this category.
Some people have white coat hypertension—high blood pressure readings that are present only when they are examined by a physician. Whether to treat white coat hypertension with antihypertensive medication is a controversial question. Many specialists believe that people with white coat hypertension who do not have other risk factors for cardiovascular disease (such as high cholesterol levels or diabetes) do not need to take medication. Instead, they should adopt lifestyle measures such as eating a healthy diet and exercising regularly. However, the general consensus is that people with white coat hypertension who have organ damage from hypertension (for example, kidney or heart disease) need treatment with medication.
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Primary hypertension often results from one or more of the following factors:
Dietary salt. Diets high in salt may raise blood pressure in two ways. One, by causing the body to retain water, salt increases blood volume and thus blood pressure. Two, salt causes vascular smooth muscle to constrict small blood vessels, which produces a greater resistance to blood flow.
Dietary potassium. A diet low in potassium tends to increase blood pressure. Conversely, increased potassium intake blunts the effects of dietary salt on blood pressure.
Other dietary factors. Diets low in fruits, vegetables, and dairy products and high in fat and cholesterol raise blood pressure. Alcohol intake beyond moderate levels (two drinks per day for men and a drink a day for women) raises blood pressure.
Metabolic syndrome. This cluster of health problems—obesity, hypertension, high triglyceride levels, low high-density lipoprotein (HDL) cholesterol levels, and elevated blood glucose (sugar) levels—occurs in about 1 in 4 Americans and in 40 percent of those age 60 and older. Obesity, especially a significant accumulation of fat in the abdomen, initiates the abnormalities of this syndrome by decreasing the body's ability to respond to the actions of insulin, a hormone that regulates blood glucose levels. To overcome this effect, the pancreas increases its production of insulin, and blood levels of insulin rise. Elevated blood insulin levels heighten the activity of the sympathetic nervous system and cause sodium retention by the kidneys—both of which raise blood pressure.
Genetics. Studies of twins and other members of the same family show that primary hypertension has a genetic component. In addition, researchers have identified a number of genetic mutations that result in a small number of cases of hypertension.
Lack of exercise. Physical inactivity can lead to hypertension in several ways. It increases the activity of the sympathetic nervous system, increases the stiffness of the arteries, decreases the release of hormones (such as nitric oxide) that cause arteries to dilate, and reduces the body's ability to respond to insulin.
Secondary hypertension can be caused by a number of health conditions and medications. It is important to identify secondary causes of hypertension because the resulting high blood pressure can often be cured or controlled by eliminating the underlying problem.
Kidney disorders. Kidney disease that progresses to kidney failure almost always results in hypertension owing to the excessive retention of sodium and water in the body. In addition, narrowing of the arteries that supply blood to one or both kidneys, and the resulting reduction in blood flow to the kidneys, causes a form of high blood pressure called renovascular hypertension. In many people, renovascular hypertension can be cured or controlled by surgical repair of the narrowed arteries.
Adrenal tumors. Three types of adrenal tumors can cause hypertension: primary aldosteronism, Cushing's syndrome, and pheochromocytoma. Treatment of primary aldosteronism and Cushing's syndrome is complicated and does not always lower blood pressure. In pheochromocytoma, the tumor secretes large amounts of epinephrine or norepinephrine, which can cause hypertension. Removal of the tumor may cure the hypertension.
Other hormone problems. Over- or underproduction of thyroid hormone (hyperthyroidism or hypothyroidism, respectively), excessive release of growth hormone by a tumor in the pituitary gland, or increased blood calcium levels due to a tumor in the parathyroid gland can all cause hypertension.
Coarctation of the aorta. In this condition, a portion of the aorta narrows, resulting in hypertension in the upper body and low blood pressure in the abdomen and legs. This disorder is the most common cause of secondary hypertension in young people and can be corrected with surgery.
Sleep apnea. People with this disorder stop breathing periodically during sleep. Studies show that people with sleep apnea are more likely to develop hypertension, and the risk rises with the severity of the apnea. Fortunately, treatment of sleep apnea with continuous positive airway pressure, a device that pumps air at high pressure through the nose to keep the airway open, can significantly reduce blood pressure.
Drugs. The following prescription medications can raise blood pressure: corticosteroids such as prednisone (Deltasone and other brands); cyclosporine (Sandimmune, Neoral); tacrolimus (Prograf); epoetin (Epogen and Procrit); and nonsteroidal anti-inflammatory drugs (NSAIDs) such as indomethacin (Indocin) and celecoxib (Celebrex). Some over-the-counter remedies can also elevate blood pressure. These include pain relievers like ibuprofen (Advil, Motrin, and other brands) and naproxen (Aleve), nasal decongestants, and weight loss products containing caffeine. Illegal drugs such as cocaine and amphetamines can increase blood pressure as well.
Complications of hypertension
Hypertension can damage both large and small arteries, leading to disease in the tissues and organs supplied by these damaged blood vessels. The tissues and organs most often affected by hypertension are the brain, heart, kidneys, and eyes. Controlling blood pressure can help prevent or slow the progression of many of the complications of hypertension.
Hypertension accelerates atherosclerosis—the buildup of deposits called plaques within the walls of large arteries. If the plaques partially obstruct blood flow in an artery that leads to the brain (for example, the carotid artery), the result could be a transient ischemic attack (a ministroke in which symptoms usually subside within five to 20 minutes). If a blood clot forms in a plaque-containing artery, it could completely block blood flow and cause an ischemic stroke. Hypertension can also weaken arteries, resulting in a sac-like bulge (aneurysm) in the artery's wall. Rupture of an aneurysm in an artery supplying blood to the brain can result in a hemorrhagic stroke. Hypertension is also associated with lesions in the brain that can impair mental functions such as memory.
Atherosclerosis in the coronary arteries, which carry blood to the heart, can lead to a type of chest pain called angina when blood flow to the heart is insufficient. Complete blockage of a coronary artery by a blood clot results in a heart attack.
In people with hypertension, the heart works harder to pump against the higher pressures in the arteries. This excess workload thickens and increases the size of the heart's left ventricle. Called left ventricular hypertrophy, this condition affects 30 percent of people with hypertension and increases the risk of angina, heart attack, heart failure, and cardiac arrest.
Hypertension can damage the kidneys in two ways: by promoting atherosclerotic narrowing of the main arteries supplying the kidneys and by damaging the small arteries within the kidneys. Both can lead to progressive loss of kidney function and, eventually, kidney failure.
Persistent elevation of blood pressure can damage the tiny arteries that supply blood to the retina (the light-sensitive layer of nerve tissue that lines the back of the eye), resulting in a condition called hypertensive retinopathy. In the early stages of this disorder, the arteries in the retina thicken and narrow. Eventually, these vessels may develop blockages or begin to leak blood and fluid into the surrounding tissue. In very severe cases, the optic nerve (the nerve that carries visual impulses to the brain) may swell and cause vision loss. Hypertensive retinopathy typically evolves gradually, and many years may pass before people notice any changes in their vision.
Content excerpted from the Johns Hopkins White Paper on Hypertension & Stroke.