The Difference Between Systolic and Diastolic Blood Pressure
Diastolic pressure measures the force exerted by blood against vessel walls when the heart is relaxing between beats — that's why it's the lower of the two blood pressure numbers. With age, our arteries get a little stiff. As a result, they don't stretch out as much every time the heart beats, so more blood gets pushed right on through the larger vessels into smaller ones. Because there's less blood in the large arteries between heartbeats, diastolic pressure tends to decrease.
The top number (systolic) minus the bottom number (diastolic) gives you your pulse pressure. For example, if your resting blood pressure is 120/80 millimeters of mercury (mm Hg), your pulse pressure is 40 — which is considered a normal and healthy pulse pressure. Generally, a pulse pressure greater than 40 mm Hg is abnormal. Measuring your pulse pressure may help your doctor predict if you're at risk for a heart event, including a heart attack or stroke. If your pulse pressure is greater than 60 it's considered a risk factor for cardiovascular disease, especially for older adults. Stiffness of the body's largest artery, the aorta, is the leading cause of increased pulse pressure in older adults. High blood pressure or fatty deposits on the walls of the arteries (atherosclerosis) can make your arteries stiff. The greater your pulse pressure, the stiffer and more damaged the blood vessels are thought to be. Treating high blood pressure usually reduces pulse pressure. Following a healthy lifestyle is also important. Heart-smart strategies include getting regular exercise, not smoking, limiting alcohol and reducing the amount of salt in your diet.
Blood pressure is the pressure, measured in millimeters of mercury, within the major arterial system of the body. It is conventionally separated into systolic and diastolic determinations. Systolic pressure is the maximum blood pressure during contraction of the ventricles; diastolic pressure is the minimum pressure recorded just prior to the next contraction. The most accurate measurement of arterial blood pressure is obtained by direct methods that involve sophisticated and expensive equipment as well as cannulation of an artery (Kannel WB, Sorlie P., 1975). Although these methods are necessary in some settings, sphygmomanometric measurements are much easier and safer, and are accurate enough for most clinical situations. The standard blood pressure cuff must be of the proper size to minimize errors in blood pressure determinations. The width of the bladder ideally should be 40% of the circumference of the limb tested. Most standard cuffs have a bladder length that is twice its width. This ensures that the length is the recommended 80% of the limb circumference. Cuffs that are too small give results erroneously high; cuffs that are too large give results erroneously low. The patient should be comfortably seated and the deflated cuff applied with the bladder centered over the brachial artery. It should be high enough on the arm to allow the stethoscope to be placed in the antecubital fossa without touching the cuff. Pressure is then rapidly increased to at least 30 mm Hg higher than that which eliminates a palpable radial pulse (Janeway TC. 1915).
In short, because high blood pressure doesn’t cause symptoms, once you’ve been diagnosed with it, it’s critical to measure your blood pressure regularly. This is true even if you’re taking blood pressure medication. And whether you’ve got high or low blood pressure, tracking your systolic and diastolic numbers is a great way to gauge how well lifestyle changes or medications are working.
Fowler NO. Physiology of cardiac tamponade and pulsus paradoxus. I. Mechanisms of pulsus paradoxus in cardiac tamponade. Mod Concepts Cardiovasc Dis. 1978;47:109–13.
Janeway TC. Important contributions to clinical medicine during the past 30 years from the study of human blood pressure. Bull Johns Hopkins Hosp. 1915;26:341–50.
Kannel WB, Sorlie P. Hypertension in Framingham. In: Paul O, ed. Epidemiology and control of hypertension. New York: Stratton Intercontinental, 1975