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Blood Pressure Explained and Measured Accurately

NEW YORK (Mar 1, 2012)

Haleh Milani, M.D., F.A.C.C.
Haleh Milani, M.D.,
F.A.C.C.

From Haleh Milani, M.D., F.A.C.C. –

Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs.

During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. BP decreases as the circulating blood moves away from the heart through arteries. It drops most rapidly along the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins. Gravity, valves in veins, and pumping from contraction of skeletal muscles are some other influences on BP at various places in the body.

Blood pressure is typically measured on the inside of an elbow at the brachial artery, which is the upper arm's major blood vessel that carries blood away from the heart. A person's BP is usually expressed in terms of the systolic pressure over diastolic pressure (mmHg), for example 140/90.

Various factors, such as age and gender influence a person's BP. In children, the normal ranges are lower than for adults and depend on height. As adults age, systolic pressure tends to rise and diastolic tends to fall. In the elderly, BP tends to be above the normal adult range, largely because of reduced flexibility of the arteries. Also, an individual's BP varies with exercise, emotional reactions, sleep, digestion, and time of day.

Differences between left and right arm BP measurements tend to be random and average to nearly zero if enough measurements are taken. However, in a small percentage of cases there is a consistent difference greater than 10 mmHg which may need further investigation, e.g. for obstructive arterial disease.

When measuring BP, an accurate reading requires that one not drink coffee, smoke cigarettes, or engage in strenuous exercise for 30 minutes before taking the reading. A full bladder may have a small effect on BP readings; if the urge to urinate exists, one should do so before the reading. For five minutes before the reading, one should sit upright in a chair with one's feet flat on the floor and with limbs uncrossed. The BP cuff should always be against bare skin, as readings taken over a shirt sleeve are less accurate. During the reading, the arm that is used should be relaxed and kept at heart level, for example, by resting it on a table.

Since BP varies throughout the day, measurements intended to monitor changes over longer time frames should be taken at the same time of day to ensure that the readings are comparable.

Haleh Milani, M.D., F.A.C.C., is the Director of Noninvasive Cardiovascular Diagnostics at NewYork-Presbyterian/Columbia University Medical Center and an Assistant Professor of Medicine at Columbia University College of Physicians and Surgeons.

Blood Pressure Chart

For reference: a blood pressure chart created by the American Heart Association.

Blood Pressure
Category
Systolic
mm Hg (upper #)
 Diastolic
mm Hg (lower #)
Normal
 
less than 120andless than 80
Prehypertension120 - 139or80 - 89
High Blood Pressure
(Hypertension) Stage 1
140 - 159or90 - 99
High Blood Pressure
(Hypertension) Stage 2
160 or higheror100 or higher
Hypertensive Crisis
(Emergency care needed)
Higher than 180orHigher than 110

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