What Is High Blood Pressure?

What is blood pressure and how is it measured?

Blood pressure is measured as millimeters of mercury (mm Hg), though few people directly refer to these units. More often, you'll hear blood pressure described as two numbers, called the systolic (top) and diastolic (bottom) readings. These readings are often recorded as a fraction, with systolic over diastolic. For example: 120/70.

The systolic reading is a measurement of the force that is created as the heart pushes blood through the aorta (a large artery coming from the left side of the heart) and into the circulatory system.

The diastolic reading is a measurement of the force exerted by the walls of the arteries as they resist blood flow.

Blood pressure measurements vary during the course of the day, depending on your activity level and even your emotions. Blood pressure is lower when you sleep and usually rises when you exercise.

How do I know if I have high blood pressure?

In most instances, high blood pressure does not have warning signs. So the best way to screen for it is to have your blood pressure checked regularly at your doctor's office. Your doctor can tell you how often.

A doctor diagnoses high blood pressure after several high blood pressure readings are recorded. High blood pressure is classified as follows:

Category	Systolic reading* (mm Hg)		Diastolic reading* (mm Hg)	Management
Normal	Less than 120	and	Less than 80	Lifestyle modification encouraged	Indications for drug therapy
Prehypertension	120-139	Or	80-89	Definitive modifications	Drug therapy not indicated. unless compelling indication* exists.
Hypertension Stage 1	140-159	or	90-99	Definitive modifications	Drug therapy often indicated. Compelling indications have specific medications.
Hypertension Stage 2	Greater than 160	or	Greater than 100	Definitive modifications	Drug therapy indicated. Compelling indications have specific medications.
*Compelling indications are high-risk situations such as coronary heart failure, heart attack, coronary heart disease, diabetes, kidney disease and stroke. Patients with chronic kidney disease or diabetes are treated to BP goal of less than 130/80 mm Hg.
From the Seventh Report of the Joint National Committee on Detection, Evaluation and Treatment of High Blood Pressure, National Institutes of Health. May 2003.

What causes high blood pressure and how common is it?

In 90 percent to 95 percent of the cases, the cause is unknown. This type of high blood pressure is known as essential hypertension. In rarer cases, high blood pressure can result from other illnesses like kidney or adrenal gland problems. That type of high blood pressure is called secondary hypertension.

As many as 65 million Americans 20 and older have high blood pressure. The prevalence of hypertension is 17 percent among white women, 26 percent among white men, 37 percent among black women, and 44 percent among black men 35 to 45 years old.

In people older than 65, the incidence of high blood pressure is almost the same among men and women. In this age group, about 60 percent of whites and almost 80 percent of blacks will develop the condition.

Related Articles Risk Factors for Hypertension Complications of Untreated Hypertension Treating Hypertension Only at the Doctor's Office: High Blood Pressure and White Coat Syndrome

External Sources National Institutes of Health. The Seventh Report of the Joint National Committee on Detection, Evaluation and Treatment of High Blood Pressure. May 2003 The American Heart Association

This article was reviewed and updated June 2007.

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