What is blood pressure?
Blood pressure is a measure of the force of blood against the walls of your arteries (or blood vessels) as blood flows through the body. Blood pressure is given as two numbers. The first or top number is the systolic pressure—the pressure of the blood in the vessels as the heart beats. The second or bottom number is the diastolic pressure—the pressure in the vessels as the heart relaxes between beats. Blood pressure is measured in millimeters of mercury (mm Hg), reflecting how much the pressure in your arteries would raise a column of mercury. A blood pressure of 120 mm Hg systolic and 80 mm Hg diastolic is written 120/80 ("120 over 80").
Blood Pressure Categories
|Classification of Blood Pressure for Adults1|
|Category||Systolic (mm Hg)||Diastolic (mm Hg)|
|Normal||Less than 120||and||Less than 80|
|Stage 1 high blood pressure||140-159||or||90-99|
|Stage 2 high blood pressure||160 or higher||or||100 or higher|
For information on blood pressure measurement, click here.
What is high blood pressure?
Normal blood pressure is less than 120 mm Hg systolic and 80 mm Hg diastolic. For adults, high blood pressure ( hypertension) is when blood pressure is higher than 140 mm Hg systolic and/or 90 mm Hg diastolic. High blood pressure is dangerous because it makes the heart work too hard and leads to hardening of the arteries, increasing your risk of heart disease and stroke.
Many older people develop a form of high blood pressure called isolated systolic hypertension (ISH). This occurs when the top (systolic) number is high (more than 160 mm Hg) but the bottom (diastolic) number is normal (less than 90 mm Hg). Since stroke is most common in the elderly, controlling this type of blood pressure is very important.
What is prehypertension?
If your blood pressure is higher than 120/80 but lower than 140/90, you have prehypertension. This means that you don't have high blood pressure yet, but are very likely to develop it in the future.
How does high blood pressure affect stroke risk?
High blood pressure is the most important modifiable risk factor for both blocked-vessel (ischemic) and bleeding (hemorrhagic) stroke. High blood pressure is estimated to be responsible for 60% to 70% of all strokes. Approximately 3 of 4 people who experience a first stroke have high blood pressure, and two-thirds have blood pressure higher than 160/95.2 For each rise in blood pressure of 20 mm Hg systolic (top) or 10 mm Hg diastolic (bottom), the risk of dying of a stroke doubles in both women and men.3 Women and men with high blood pressure at age 50 develop heart disease or have a stroke 7 years earlier and die on average 5 years earlier than people with normal blood pressure at this age.4
High blood pressure increases your risk of stroke in a number of ways. It contributes to the buildup of fatty plaque in the arteries and makes your artery walls stiffer, thicker, and less flexible, making it harder for blood to flow through them. The high pressure can cause a piece of plaque to break off and travel to the brain, causing a blocked-vessel stroke. Increased pressure on the artery walls can also weaken parts that are already thin or damaged, causing them to rupture and resulting in a bleeding stroke.