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Blood pressure is a measurement of the force of blood moving through the circulatory system. A blood pressure reading is composed of measurements of force at two distinct intervals: when the heart is pumping, called systolic pressure, and when the heart is resting between beats, called diastolic pressure. A low diastolic blood pressure can be caused by many factors, some temporary and others chronic. A low diastolic blood pressure is usually defined by a diastolic reading under 60 millimeters of mercury (mmHg).
It is important to understand that blood pressure fluctuates somewhat throughout the day. Eating, exercising, and even sleeping can all temporarily raise or lower blood pressure. Low diastolic blood pressure becomes a concern when it is significantly lower than a normal reading of 80 mmHg, or when it remains low for an extended period of time. Even so, low diastolic blood pressure may be related to temporary conditions that will correct naturally.
Early pregnancy is a common cause of low diastolic blood pressure. According to some studies, women in the first two trimesters of pregnancy may drop around 10 mmHg below normal due to the the fast expansion of circulatory paths. Usually, pressure will increase during the final trimester and return to normal following birth. Pregnant women with unusually or significantly lower than normal blood pressure may be at some risk for dizzy spells, fainting, and falls, and are thus carefully monitored.
Dehydration is a common cause of low blood pressure and may be due to below average fluid consumption or the effects of certain medications, such as diuretics. The reduction of fluids in the body decreases the volume of blood, lowering its capacity to circulate effectively. A sudden, severe dehydration, such as strenuous exercise while already somewhat dehydrated, can lead to a condition known as hypovolemic shock, in which blood pressure drops rapidly and severely and can lead to death within minutes if not quickly treated.
Heart conditions that slow or weaken heart rate can play a part in the development of chronic low diastolic blood pressure. Some conditions associated with this risk include damaged heart valves or a history of heart attacks. Some people may also simply have a lower than normal heart rate, called bradycardia, which may result in low diastolic blood pressure.
Symptoms of low diastolic pressure may include dizziness, nausea, fatigue, a sensation of faintness when changing body position, and weakness in the arms and legs. In some cases, low diastolic pressure may be a result of a severe viral or bacterial infection, and should be treated immediately by medical professionals. If symptoms of low blood pressure appear for a long period of time, visiting a doctor for examination may be recommended.