Essential or primary hypertension, the most common form of hypertension, is elevated blood pressure that develops without apparent cause. Genetic factors, however, appear to play role in increasing the risk of developing the disorder.
Normal blood pressure refers to a range of values rather than a specific set of numbers and varies with factors such as age, race, and gender. However, a blood pressure reading greater than 140/90 mm Hg (millimeters of mercury pressure) is generally considered to be elevated. In this measurement, 140 refers to the systolic pressure (the maximum pressure in the arteries when the heart contracts). The 90 refers to the diastolic pressure (the lowest pressure in the arteries when the heart is between contractions).
More than 95% of all elevated blood pressure can be classified as essential hypertension. When a disease, other physical problems, medications, or even temporary physical exertion or stress cause high blood pressure, the condition is called secondary hypertension.
Blood pressure refers to the force exerted by blood against the interior walls of the body's blood vessels. There are three categories of blood pressure, corresponding to the three types of blood vessels: arterial, capillary, and venous. In individuals with hypertension, arterial pressure (recorded as two numbers: systolic and diastolic pressure) is the most important measurement to obtain. The reason is that because of their relative proximity to blood flowing forcefully from the heart, arteries must withstand the highest pressures of all the body's blood vessels.
The body requires a relatively constant blood pressure level to ensure adequate passage of nutrients and oxygen to organs and tissues. To maintain a constant level of pressure, the body must balance and react to a number of factors such as these:
- volume of blood in the circulatory system
- amount of blood ejected by the heart (stroke volume)
- heart rate
- thickness of the blood (viscosity)
- elasticity of the arteries
When the systolic or diastolic pressure is elevated for an extended period of time, such as months or years, the heart has to work harder and may become damaged, along with the blood vessels. If it remains untreated, high blood pressure can lead to a variety of serious health problems, including heart disease, stroke, and kidney failure.
Studies suggest that some people with essential hypertension may inherit abnormalities of the sympathetic nervous system—the part of the nervous system that controls heart rate, blood pressure, and the diameter of blood vessels. It is estimated that the risk of developing essential hypertension is increased two- to four-fold if one or both parents are diagnosed with the disorder.
Researchers have identified the chromosomes (11 and 18) that house the genes responsible for blood pressure regulation, although narrowing down the range of specific genes involved in hypertension is more difficult.
Genes under intense study are those that regulate a group of hormones known as the angiotensin-reninaldosterone system. This system influences all aspects of blood pressure control, including blood vessel contraction, sodium and water balance, and cell development in the heart.
When blood pressure drops, the kidneys release an enzyme called renin, which initiates a chain reaction to bring blood pressure back up. Renin acts on angiotensinogen (a plasma protein) to produce the hormone, angiotensin I (an inactive form), which is then converted to angiotensin II (an active form of the hormone) by the angiotensin-converting enzyme (ACE). Angiotensin II then stimulates the adrenal glands to release the hormone aldosterone, which decreases kidney sodium excretion, thereby causing blood vessels to constrict. When blood vessels constrict, blood pressure goes up.
Researchers believe that this angiotensin-reninaldosterone system evolved millions of years ago to protect humans. By retaining salt and water and narrowing blood vessels, the body was ensured an adequate blood flow and the ability to repair injured tissue. Over time, however, this system outlived its original protective function and led to serious health complications.
It is estimated that one in four Americans have high blood pressure; it is also estimated that one in three people who have high blood pressure are unaware of the problem. Also, hypertension is much more common among African-Americans and Mexican-Americans than in Caucasian populations. Low levels of nitric oxide, which have been observed in individuals—particularly African-Americans—with elevated blood pressure, may be an important factor in the development of essential hypertension.
The prevalence of essential hypertension increases with age until at least the age of 80. Statistics indicate that more than half of all Americans over the age of 65 have hypertension. In those under the age of 55, essential hypertension is more common in males than females. Over age 55, there is an equal distribution among males and females.
Signs and symptoms
Essential hypertension may cause no symptoms for years. For this reason, high blood pressure is often called the "silent killer." The first symptom may be a heart attack or stroke. However, many people with hypertension may experience one or more of the following symptoms:
- blurred vision
- irregular or rapid heartbeat
Although genetic studies hold hope for detecting, evaluating, and treating hypertension in the future, there are no reliable genetic screening tests for the disorder yet. Thus, essential hypertension is a condition that cannot be diagnosed until it has developed; it is often diagnosed during a routine physical or medical examination.
Blood pressure is measured by an instrument called a sphygmomanometer. A cloth-covered rubber cuff is wrapped around the upper arm and inflated. When the cuff is inflated, an artery in the arm is squeezed to momentarily stop the flow of blood. Then the air is let out of the cuff, while a stethoscope placed over the artery is used to detect the sound of the blood spurting back through the artery. This first sound is the systolic pressure. The last sound heard as the rest of the air is released is the diastolic pressure. Both sounds are recorded on the mercury gauge of the sphygmomanometer.
Because a number of factors such as pain, stress, or anxiety can cause a temporary increase in blood pressure, hypertension is not diagnosed on the basis of one elevated reading. Also, blood pressure results may be different depending on which arm is used. Thus, if a blood pressure reading is 140/90 or higher for the first time, the physician will have the individual return for another blood pressure check. Diagnosis of essential hypertension is usually made based on two or more readings after the first visit.
A typical physical examination to evaluate hypertension includes:
- medical and family history (especially important to determine a genetic contribution)
- physical examination
- examination of the blood vessels in the eye
- chest x ray
- electrocardiograph (EKG)
- blood and urine tests
Treatment and management
There is no complete cure for essential hypertension because unlike secondary hypertension, there is no single cause of the problem; it is a complex disorder only determined, in part, by genes. Environmental (lifestyle) factors interact with genetic factors to produce hypertension.
However, essential hypertension can be treated and managed effectively, even if an individual has a genetic predisposition to the disorder. If essential hypertension is mildly or even moderately high, it may be possible to bring it down to a normal level without medication. Weight loss, changes in diet, and exercise may be the only treatment necessary. General nonpharmacologic recommendations include:
- reducing the amount of salt (sodium) and fat in the diet
- exercising regularly
- maintaining a healthy weight
- limiting alcohol and caffeine consumption
- quitting smoking
- reducing stress through stress management techniques, relaxation exercises, or counseling
If lifestyle changes are not effective in lowering blood pressure to a normal level, medication may be prescribed. There are many types of drugs available to treat essential hypertension. The main categories of drugs include:
- diuretics (help kidneys eliminate excess salt and water from the body's tissues and blood, thereby reducing swelling and lowering blood pressure)
- beta-blockers, alpha-blockers, and alpha/beta blockers (act on nervous system to slow heart rate and reduce the force of the heart's contractions
- angiotensin-converting enzyme (ACE) inhibitors (block the production of substances that constrict blood vessels and reduce salt and water build-up in the tissues)
- calcium channel blockers (block the entry of calcium into muscle cells in artery walls, making arteries more relaxed)
- vasodilators (relax artery walls and lower blood pressure rapidly)
- peripheral acting adrenergic antagonists (act of nervous system to relax arteries and reduce the force of the heart's contractions)
- Centrally acting agonists (act on nervous system to relax arteries)
When a blood pressure medication is prescribed, it is important to:
- take the medication regularly, exactly as prescribed
- report any side effects immediately
- have regular follow-up visits with a physician
It may take weeks or even months to find the most effective pharmacologic treatment. Once an effective drug or combination of drugs is found, individuals with high blood pressure may require treatment for the rest of their lives.
The higher the blood pressure, the worse the prognosis. However, most serious complications of essential hypertension can be delayed or even avoided by getting regular blood pressure checks and by treating the disorder as soon as it is diagnosed.
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American Society of Hypertension. 515 Madison Ave., Suite 1212, New York, 10022. (212) 644-0600. <http://www.ash-us.org>.
Heart Information Network. <http://www.heartinfo.org>.
Genevieve T. Slomski, PhD