When you have your blood pressure taken, you will see 2 numbers. Diastolic pressure is the lower number and systolic pressure is the higher number. The diastolic number represents the pressure exerted by the blood on the walls of your arteries between heartbeats. Low diastolic pressure -- diastolic hypotension -- is often, but not always, accompanied by low systolic pressure -- systolic hypotension. The number below which a person is considered to have diastolic hypotension varies depending on several factors, including a person's age. But in adults, a diastolic pressure below 60 millimeters of mercury is usually considered low diastolic pressure, according to PubMed Health.
Low Blood Volume
When the amount of blood within the body's arteries is less than normal, the pressure exerted on the arterial walls is reduced. Low blood volume is one of the most common causes of low diastolic pressure. The blood volume can be low because of actual loss of blood, as may occur with bleeding from an injury. Blood volume is also reduced when a person is dehydrated, as adequate intake of fluids is necessary to maintain a normal amount of blood within the body's blood vessels. Dehydration occurs when a person does not take in enough fluids, such as water or electrolyte-containing liquids. It is most common when people are too sick to drink normally or when they have excessive fluid loss, such as from sweating in hot weather, vomiting, diarrhea or excessive urination in some types of kidney disease.
Heart Problems and Pulmonary Embolus
When the heart is unable to pump a normal amount of blood to the rest of the body, the blood pressure will fall. Often the diastolic and systolic pressure will both be less than normal. A too slow heart rate, as well as a too fast heart rate, can cause low blood pressure. Abnormal heart rhythms such as atrial fibrillation, or A fib, can also produce hypotension. Low diastolic pressure can likewise occur with heart valve problems, a myocardial infarction or any other cause of heart failure. When the aortic heart valve is leaky -- called aortic insufficiency -- the diastolic pressure is low but the systolic pressure is generally normal.
A blood clot that blocks the flow of blood into the lungs -- known as a pulmonary embolus -- can lower the blood pressure when the blockage is severe. This occurs because the blood gets trapped in the right side of the heart and is thus unable to reach the left side of the heart from which it is pumped to the rest of the body.
Inadequate production of hormones by several endocrine glands can produce diastolic hypotension, which is often accompanied by systolic hypotension as well. These include:
- underactive thyroid gland -- hypothyroidism
- underactive parathyroid glands --
- underactive adrenal glands -- adrenal
insufficiency or Addison disease
Low blood pressure is a common sign of low blood sugar, or hypoglycemia. Some people with diabetes mellitus will develop hypotension even without hypoglycemia. This may be due to diabetes causing damage to the nerves that help maintain a normal blood pressure.
Medications and Other Substances
A large number of medications may cause low blood pressure. Any medicine normally used to treat high blood pressure can cause low diastolic pressure if its effects are excessive. Water pills, or diuretics, can cause hypotension if they produce dehydration. Many medications used to treat heart disease may also cause diastolic hypotension. A number of anti-anxiety medicines, painkillers, antidepressants and anti-Parkinson disease medications can produce hypotension. Erectile dysfunction medications, such as sildenafil (Viagra), can lead to hypotension, especially when combined with the heart medication nitroglycerine. Ingestion of excessive amounts of alcohol can also lower the blood pressure.
Allergic Reactions and Infections
Allergic reactions, especially severe reactions known as anaphylaxis, can cause hypotension. Usually the diastolic and systolic blood pressure are both reduced. Anaphylaxis is a life-threatening condition that generally includes other symptoms as well, such as difficulty breathing, itching, generalized red skin color, hives and a swollen face or throat. The red skin is caused by widening of blood vessels beneath the surface of the skin, allowing more blood to flow into the area. This widening leads to less pressure being exerted on the walls of the arteries, causing the blood pressure to fall.
A severe infection that enters the bloodstream, known as septicemia, can also cause low blood pressure. Septicemia is often accompanied by a fever and generalized red skin, and may lead to impaired function of body organs, such as the heart, lungs and kidneys. Very low blood pressure caused by septicemia is called septic shock, a life-threatening condition.
Both high and low body temperatures can cause hypotension through their effects on the heart rate, the heart's ability to pump blood or the width of blood vessels. Severe liver disease tends to cause blood vessels in the body to widen, lowering the diastolic blood pressure. Prolonged bed rest can also lead to diastolic hypotension.
Many neurologic conditions can lower the diastolic pressure. This is usually due to failure of normal nerve reflexes within the body that help maintain a normal blood pressure. Spinal cord or head injuries, strokes, Parkinson's disease and multiple sclerosis are all examples of neurologic conditions that can cause low blood pressure.
Seeking Medical Attention
Contact your doctor if your diastolic blood pressure is less than about 60 millimeters of mercury. Seek prompt medical attention if you notice symptoms of low blood pressure, such as weakness, dizziness or lightheadedness, nausea, confusion or excessive fatigue. Seek immediate medical care if you notice any of the following, as they may indicate a serious cause or effect of low blood pressure: fainting, chest pain, difficulty breathing, other symptoms of anaphylaxis, significant bleeding, very slow or very fast heart rate, irregular heart rhythm, very high or very low temperature, or any new neurologic symptoms such as weakness, numbness or difficulty speaking.
Reviewed by Mary D. Daley, M.D.