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DEAR MAYO CLINIC:

What are the causes of an extremely low diastolic blood pressure?

ANSWER:

Two circumstances typically can result in extremely low diastolic blood pressure. The condition may be associated with severe hypotension, or it could be caused by profoundly stiff arteries that occur due to aging, diabetes or fatty buildup in the arteries (atherosclerosis).

A blood pressure reading, given in millimeters of mercury (mm Hg), has two numbers. The first, or upper, number measures the pressure in arteries when the heart beats (systolic pressure). The second, or lower, number measures the pressure in arteries between beats (diastolic pressure).

A systolic blood pressure reading of 90 mm Hg or less or a diastolic reading of 60 mm Hg or less is generally considered low blood pressure (hypotension). Normal blood pressure varies from person to person, though. If your diastolic blood pressure is consistently below 60, and you aren't experiencing problems as a result, treatment might not be required. If low blood pressure is causing noticeable symptoms -- such as dizziness, light-headedness or fainting -- more investigation may be required to uncover the possible cause and provide appropriate treatment.

The first condition that may result in extremely low diastolic blood pressure is severe hypotension (blood pressure less than 90/60 mm Hg). A number of medical conditions may cause severe hypotension. For example, heart problems such as extremely low heart rate (bradycardia), heart valve problems and heart failure can cause severely low blood pressure because they prevent the body from being able to circulate enough blood. An underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism) can also cause low blood pressure. In addition, endocrine disorders such as adrenal insufficiency (Addison's disease) and low blood sugar (hypoglycemia) can also trigger low blood pressure.

Some medications can also cause low blood pressure, including diuretics (water pills), alpha blockers, beta blockers, drugs for Parkinson's disease, certain types of antidepressants, and sildenafil (Viagra), particularly in combination with the heart medication nitroglycerine.

Other conditions, including dehydration, blood loss, significant infection and a severe allergic reaction may cause severe hypotension. But these disorders usually cause a sudden, dramatic and temporary drop in blood pressure, rather than a sustained low blood pressure reading over time.

The other possible cause for extremely low diastolic blood pressure is very stiff arteries. Healthy arteries are flexible and elastic, and blood moves through them easily. At roughly 55 years of age, arteries begin to lose some of that flexibility, decreasing diastolic blood pressure.

Arteries may also become stiff due to the effects of diabetes. Many people with diabetes have high blood pressure as a result. In some cases, the systolic blood pressure may be high while the diastolic reading is low. Atherosclerosis -- a condition in which fat (plaque) builds up in and on artery walls -- can stiffen blood vessels and have the same effect on blood pressure.

Thus, many older patients can have both a high systolic and a low diastolic blood pressure. This condition is known as high pulse pressure. The heightened difference between diastolic and systolic blood pressure is associated with a higher risk of stroke than an elevation of either systolic or diastolic blood pressure alone. Unfortunately, nothing can be done to raise diastolic pressure that does not also raise systolic blood pressure.

In some cases, stiff arteries can be a sign of vascular disease. If you chronically have very low diastolic blood pressure, talk to your doctor to ensure that you are controlling all vascular risk factors that can be modified, such as high blood pressure, smoking, cholesterol, diabetes and obesity.

-- John Graves, M.D., Nephrology/Hypertension, Mayo Clinic, Rochester, Minn.

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