Home Health Hypotension: When the Blood Pressure is Low

Hypotension: When the Blood Pressure is Low



By Henrylito D. Tacio

Hypotension: When the Blood Pressure is Low. The phone rang at 7:30 that beautiful Sunday morning.  Darleen climbed out of bed feeling perfectly fine to answer the phone.  As she walked, she felt as she was going to pass out.

I must have gotten up too quickly, she wondered.  But on second thought, it happened several times already.  She told her husband about the recent incidents.  Sensing there must be something wrong with her; Johnny accompanied Darleen to consult a doctor. 

The physician’s verdict: hypotension.  “Your wife’s blood pressure is much lower than usual,” the doctor said.  In other words, Darleen is suffering from the exact opposite of hypertension.

Blood pressure is given as two numbers (140/90, for example) that describe the amount of pressure inside the arteries of the body. Arteries are blood vessels that carry blood from the heart to the rest of the body. Blood pressure is fairly easy to measure. The pressure is related to blood flow inside the arteries, much like the pressure inside a hose is related to water flowing through it.

“Normally, the body maintains the pressure of blood in the arteries within a narrow range,” explains the second home edition of The Merck Manual of Medical Information.  “If blood pressure is too high, it can damage a blood vessel and even rupture it, causing bleeding or other complications. 

“If blood pressure is too low, not enough blood reaches all parts of the body; as a result, cells do not receive enough oxygen and nutrients, and waste products are not adequately removed,” the Merck manual continues.

According to the Minnesota-based Mayo Foundation for Medical Education and Research, the normal blood pressure is less than 120/80 millimeters of mercury (mm Hg). “Unusually low blood pressure readings should be evaluated by a doctor,” it reminds. “Unlike high blood pressure, there are no clear-cut standards for the diagnosis of low blood pressure.”

The human body has several compensatory mechanisms that control blood pressure.  They involve changing the diameter of veins and small arteries (called arterioles), the amount of blood pumped from the heart (cardiac output), and the volume of blood in the blood vessels.  “These mechanisms return blood pressure to normal after it increases or decreases during normal activities, such as exercise or sleep,” the Merck manual notes.

Here’s the involvement of veins, according to the Merck manual: “Veins can widen (dilate) and narrow (constrict) to change how much blood they can hold (capacity).  When veins constrict, ther capacity to hold blood is reduced, forcing more blood into arteries.  As a result, blood pressure increases.  Conversely, when veins dilate, their capacity to hold blood is decreased, forcing less blood into the arteries.  As a result, blood pressure decreases.”

What about arterioles?  “They can also dilate and constrict,” the Merck manual states.  “The more constricted arterioles are, the greater their resistance to blood flow and the higher the blood pressure.  Constriction of arterioles (decreasing their diameter) increases blood pressure because more pressure is needed to force blood through the narrower space.  Conversely, dilation of arterioles reduces resistance to blood flow, thus reducing blood pressure.”

On the other hand, “the more blood pumped from the heart per minute (that is, the larger the cardiac output), the higher the blood pressure – as long as resistance to blood flow in the arteries remains constant. The body can change the amount of blood pumped during each heartbeat by making the heart rate slower or faster or by making each contraction weaker or stronger.”

Meanwhile, “the higher the volume of blood in the blood vessels, the higher the blood pressure – as long as resistance to blood flow in the arteries remains constant.  To increase or decrease blood volumen, the kidneys can vary the amount of fluid excreted in urine.”


Various disorders and drugs can cause the compensatory mechanisms to malfunction, and low blood pressure may result.  For instance, cardiac output may be reduced as a result of abnormal heart rhythms, heart muscle damage or malfunction (such as that due to a heart attack or viral infection), heart valve disorders, and pulmonary embolism.

Dilation of blood vessles may be due to alcohol, some antidepressants (like amitriptyline), antihypertensive drugs (such as calcium channel blockers, angiotensin-converting enzyme inhibitors, and angiostensin II receptor blockers), nitrates, bacterial infection, exposure to heat, and nerve damage (such as that due to diabetes or spinal cord injuries).

Blood volume can be reduced as a result of dehydration (due to vomiting, diarrhea or not getting enough fluids), bleeding, or kidney disorders.  “Some kidney disorders impair the kidneys’ ability to return fluid to blood vessels, resulting in the loss of large amounts of fluid in the urine,” the Merck manual points out.

“Other causes are also possible,” wrote Dr. Jacqueline A. Hart, of the Department of Internal Medicine at the Newton-Wellesley Hospital in Newton, Massachusetts.  “In some cases, no cause can be found. Some people always have a blood pressure that is low compared to other people, but normal for them.”

Symptoms, complications

“When the blood pressure is too low, there is inadequate blood flow to the heart, brain, and other vital organs,” warned Dr Hart.

The Merck manual explains: “When blood pressure is too low, the first organ to malfunction is usually the brain since it is located at the top of the body and blood has to fight gravity to reach the brain.  Consequently, most people with low blood pressure feel dizzy or light-headed when they stand, and some may even faint.  People who faint fall to the floor, usually bringing the brain to the level of the heart.  As a result, blood can flow to the brain without having to fight gravity, and blood flow to the brain increases, helping protect it from injury. However, if blood pressure is low enough, brain damage can still occur.”

Occasionally, low blood pressure causes shortness of breath or chest pain due to an inadequate blood supply to the heart muscle.  If blood pressure becomes sufficiently low and remains low, all organs start to malfunction, a condition called shock.

Some symptoms occur when the body’s compensatory mechanisms try to increase blood pressure that is low.  For instance, when arterioles constrict, blood flow to the skin, feet, and hands decreases.  These areas may become cold and turn blue.  When the heart beats more quickly and more forcefully, a person may feel palpitations (awareness of heart beats).


In most cases, treating the cause will correct the low blood pressure. Stopping or reducing the dose of a medication may end medication-related low blood pressure. Treating infections with antibiotics or surgery may stop low blood pressure from this cause. Blood transfusions may be needed to treat cases due to extensive blood loss.

Drinking water may help.  “I tell my patients to drink liberally,” Dr. Mar J. Rosenthal, associate professor of medicine and geriatrics at the University of California, told The Doctors Book of Home Remedies.  He recommends drinking about one glass (eight ounces) per hour; other doctors suggest eight glasses a day.

Walking also facilitates.  In one study of older people with low blood pressure after meals, walking afterward restored their blood pressure to normal.  “These findings support an old German proverb – ‘After meals, you should rest or walk a thousand steps,’” says Dr. Lewis A. Lipsitz, director of medical research at the Hebrew Rehabilitation Center for the Aged in Boston.

Meanwhile, extremely low blood pressure is often treated directly to try to raise the blood pressure. This is done to prevent organ damage from lack of proper blood flow. Fluids and medications are given through an intravenous line, a tube connected to a vein in the arm or other area.

Like other treatments, there are side effects.  For example, antibiotics may cause allergic reactions or stomach upset. Medications used to raise blood pressure may cause irregular heartbeats. Surgery carries a risk of bleeding, infection, and allergic reaction to the anesthesia. Blood transfusions carry a risk of allergic reaction and infection.

“Even so, having low blood pressure is generally better than having high blood pressure,” the Merck manual assures.  “Healthy people who have blood pressure that is low but still in the normal range (when measured at rest) tend to live longer than those who have higher normal blood pressure.” 

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