Hypertension, or high blood pressure, is characterized by a persistent increase in the force that the blood exerts upon the walls of the arteries. It is normal for this force to increase with stress or physical exertion, but with hypertension the patient’s blood pressure is high even at rest.

Blood pressure is measured with two numbers: systolic (the top number in a reading) and diastolic (the bottom number). It is measured in millimeters of mercury (abbreviated mm Hg) using a device called a sphygmomanometer. Systolic pressure refers to the force of blood against the walls of the arteries when the heart contracts to pump blood to the rest of the body. Diastolic pressure refers to the pressure within the arteries as the heart relaxes and refills with blood (which explains why the diastolic number is always lower than the systolic measurement).

Hypertension is defined as systolic pressure greater than 140 mm Hg or diastolic pressure greater than 90 mm Hg; optimal blood pressure is less than 120/80 mm Hg.

Some 60 million Americans have high blood pressure, but only about half of them know it, primarily because it so rarely causes any noticeable symptoms and is usually detected only incidentally during a routine physical examination. But left untreated, high blood pressure promotes atherosclerosis (narrowing of the arteries) and increases the risk of heart attack, stroke, kidney damage, and destruction of tiny blood vessels in the eye, which can result in vision loss.

For these reasons high blood pressure is often called “the silent killer.” Fortunately, if detected early and treated properly, the prognosis is good.