Medical Edge Newspaper Column

Low Diastolic Blood Pressure

May 29, 2006
DEAR MAYO CLINIC:
What is the significance of low diastolic blood pressure? Mine is sometimes as low as 50. I take medication for heart disease and for high systolic blood pressure, which remains normal. -- Oklahoma City, Okla.

ANSWER:
Systolic blood pressure is the pressure in your aorta -- the large artery that receives blood coming out of your heart -- and in its main branches during each heartbeat as blood is pumped into the arteries. Diastolic blood pressure is the pressure in the aorta and its main branches between beats. Normal blood pressure is less than 120/80 millimeters of mercury (mm Hg). Systolic is the first number; diastolic is the second. The difference between the systolic and diastolic blood pressure is the pulse pressure. Normal pulse pressure is less than 40 mm Hg.

In most people, systolic and diastolic blood pressure change over time. Systolic blood pressure often rises throughout a person's life, whereas diastolic blood pressure tends to rise until midlife and then level off. After midlife, the diastolic blood pressure often falls. In people younger than 50, the primary blood pressure problem is elevated diastolic blood pressure. In people older than 50, the main abnormality is high systolic blood pressure, often associated with increased pulse pressure (a result of higher systolic blood pressure and lower diastolic blood pressure). Both elevated systolic blood pressure and elevated pulse pressure are associated with an increased risk for heart attacks and strokes.

The change in blood pressure as you age is caused by progressive stiffening of the aorta and its main branches. In young people, the aorta is very elastic and expands easily when blood enters it, so systolic pressure is low. The blood that stays in the aorta during the heartbeat moves out of it between heartbeats as the aorta returns to its original size before the next heartbeat. That blood flow contributes to the diastolic blood pressure level.

As the aorta stiffens with age, it expands less when blood from the heart enters it, causing higher systolic blood pressure. Also because of the stiffening, more of the blood in the aorta and its main branches is forced into other smaller blood vessels when the heart beats, thus, less is present between heartbeats to support the diastolic blood pressure, which becomes lower.

Treatment of high blood pressure in people older than 50 generally focuses on lowering the systolic blood pressure. The recommended goal is less than 140 mm Hg, with a goal of less than 130 mm Hg in people with diabetes or kidney disease. Usually, lower systolic blood pressure is also associated with lower pulse pressure and, thus, a reduced risk for heart attacks and strokes, despite an even lower diastolic blood pressure that might result from the treatment. So, even though your diastolic blood pressure is low, your doctor has taken the right approach by gearing your treatment toward bringing down your systolic blood pressure.

-- Gary L. Schwartz, M.D., Nephrology and Hypertension, Mayo Clinic, Rochester, Minn.