What is low blood pressure?
Blood pressure is the force exerted by circulating blood on the walls of
blood vessels. It constitutes one of the critically important signs of life or
vital signs which include heart beat, breathing, and temperature. Blood pressure
is generated by the heart pumping blood into the arteries modified by the
response of the arteries to the flow of blood.
An individual's blood pressure is expressed as systolic/diastolic blood
pressure, for example, 120/80.The systolic blood pressure (the top number)
represents the pressure in the arteries as the muscle of the heart contracts and
pumps blood into them. The diastolic blood pressure (the bottom number)
represents the pressure in the arteries as the muscle of the heart relaxes
following its contraction. Blood pressure always is higher when the heart is
pumping (squeezing) than when it is relaxing.
The range of systolic blood pressure for most healthy adults falls between 90
and 120 millimeters of mercury (mm Hg). Normal diastolic blood pressure ranges
between 60 and 80 mm Hg. Current guidelines define normal blood pressure range
as lower than 120/80. Blood pressures over 130/80 are considered high.
High blood pressure increases the risk of developing:
Low blood pressure (hypotension) is pressure so low it causes symptoms or
signs due to the low flow of blood through the arteries and veins. When the flow
of blood is too low to deliver enough oxygen and nutrients to vital organs such
as the brain, heart, and kidney, the organs do not function normally and may be
temporarily or permanently damaged.
Unlike high blood pressure, low blood pressure is defined primarily by signs
and symptoms of low blood flow and not by a specific blood pressure number. Some
individuals routinely may have blood pressures of 90/50 with no symptoms and
therefore do not have low blood pressure. However, others who normally have
higher blood pressures may develop symptoms of low blood pressure if their blood
pressure drops to 100/60.
How is blood pressure generated?
During relaxation of the heart (diastole) the left ventricle of the heart
fills with blood returning from the lungs. The left ventricle then contracts and
pumps blood into the arteries (systole). The blood pressure in the arteries
during contraction of the ventricle (systolic pressure) is higher because blood
is being actively ejected into the arteries. It is lower during relaxation of
the ventricle (diastolic pressure) when no blood is being ejected into the
arteries. The pulse that we can feel when we place our fingers over an artery is
caused by the contraction of the left ventricle and the ejection of blood.
Blood pressure is determined by two factors: 1) The amount of blood pumped by
the left ventricle of the heart into the arteries, and 2) the resistance to the
flow of blood caused by the walls of the arterioles (smaller arteries).
Generally, blood pressure tends to be higher if more blood is pumped into the
arteries or if the arterioles are narrow and/or stiff. (Narrow and/or stiff
arterioles, by resisting the flow of blood, increase blood pressure.) Arterioles
may become narrower when the muscles surrounding them contract. Arterioles may
become stiff and narrow when older patients develop atherosclerosis.
Blood pressure tends to be lower if less blood is being pumped into the
arteries or if the arterioles are larger and more flexible and, therefore, have
less resistance to the flow of blood.
How does the body maintain normal blood pressure?
The body has mechanisms to alter or maintain blood pressure and blood flow.
There are sensors that sense blood pressure in the walls of the arteries and
send signals to the heart, the arterioles, the veins, and the kidneys that cause
them to make changes that lower or increase blood pressure.
There are several
ways in which blood pressure can be adjusted - by adjusting the amount of
blood pumped by the heart into the arteries (cardiac output), the amount of
blood contained in the veins, the arteriolar resistance, and the volume of
blood.
- The heart can speed up and contract more frequently and
it can eject more blood with each contraction. Both of these responses increase
the flow of blood into the arteries and increase blood pressure.
- The veins can expand and narrow. When veins expand,
more blood can be stored in the veins and less blood returns to the heart for
pumping into the arteries. As a result, the heart pumps less blood, and blood
pressure is lower. On the other hand, when veins narrow, less blood is stored in
the veins, more blood returns to the heart for pumping into the arteries, and
blood pressure is higher.
- The arterioles can expand and narrow. Expanded
arterioles create less resistance to the flow of blood and decrease blood
pressure, while narrowed arterioles create more resistance and raise blood
pressure.
- The kidney can respond to changes in blood pressure by
increasing or decreasing the amount of urine that is produced. Urine is
primarily water that is removed from the blood. When the kidney makes more
urine, the amount (volume) of blood that fills the arteries and veins decreases,
and this lowers blood pressure. If the kidneys make less urine, the amount of
blood that fills the arteries and veins increases and this increases blood
pressure. Compared with the other mechanisms for adjusting blood pressure,
changes in the production of urine affect blood pressure slowly over hours and
days. (The other mechanisms are effective in seconds.)
For example, low blood volume due to bleeding (such as a bleeding ulcer in
the stomach or from a severe laceration from an injury) can cause low blood
pressure. The body quickly responds to the low blood volume and pressure by the
following adjustments which all increase blood pressure:
- The heart rate increases and the forcefulness of the heart's
contractions increase thus pumping more blood through the heart.
- Veins narrow to return more blood to the heart for pumping.
- Blood flow to the kidneys decreases to reduce the formation of
urine and thereby increases the volume of blood in the arteries and veins.
- Arterioles narrow to increase resistance to blood
flow
These adaptive responses will keep the blood pressure in the normal range
unless blood loss becomes so severe that the responses are overwhelmed
Is low blood pressure bad for your health?
People who have lower blood pressures have a lower risk of stroke, kidney
disease, and heart disease. Athletes, people who
exercise regularly, people who maintain ideal body weight, and
non-smokers tend to have lower blood pressures. Therefore, low blood pressure is
desirable as long as it is not low enough to cause symptoms and damage to the
organs in the body.
What are low blood pressure signs and symptoms?
When blood pressure is not sufficient to deliver enough blood to the organs
of the body, the organs do not work properly and can be temporarily or
permanently damaged. For example, if insufficient blood flows to the brain,
brain cells do not receive enough oxygen and nutrients, and a person can feel
lightheaded, dizzy, or even faint.
Going from a sitting or lying position to a standing position often brings
out symptoms of low blood pressure. This occurs because standing causes blood to
"settle" in the veins of the lower body, and this can lower the blood pressure.
If the blood pressure is already low, standing can make the low pressure worse,
to the point of causing symptoms. The development of lightheadedness,
dizziness, or
fainting upon standing
caused by low blood pressure is called
orthostatic
hypotension. Normal individuals are able to compensate rapidly for the low
pressure created by standing with the responses discussed previously and do not
develop orthostatic hypotension.
When there is insufficient blood pressure to deliver blood to the coronary
arteries (the arteries that supply blood to the heart's muscle), a person may
develop
chest pain (a symptom of
angina) or even a
heart attack.
When insufficient blood is delivered to the kidneys, the kidneys fail to
eliminate wastes from the body, for example, urea (BUN) and
creatinine, and increases in their levels in the blood occur.
Shock is a
life-threatening condition where persistently low blood pressure causes organs
such as kidney(s), liver, heart, lung, and brain to fail rapidly
What are the causes of low blood pressure?
Conditions that reduce the volume of blood, reduce cardiac output (the amount
of blood pumped by the heart), and medications are frequent reasons for low
blood pressure.
- Dehydration is common
among patients with prolonged nausea, vomiting, and diarrhea. Large amounts
of water are lost when vomiting and with diarrhea, especially if the person does
not drink adequate amounts of fluid to replace the depleted water.
Other causes of dehydration include exercise, sweating, fever, and heat exhaustion, or heat stroke. Individuals with mild dehydration may experience only
thirst and dry mouth. Moderate to
severe dehydration may cause orthostatic hypotension (manifested by
lightheadedness, dizziness, or fainting upon standing). Prolonged and severe
dehydration can lead to shock, kidney failure, confusion, acidosis (too much acid in the blood), coma, and even death.
- Moderate or severe bleeding can quickly deplete an
individual's body of blood, leading to low blood pressure or orthostatic
hypotension. Bleeding can result from trauma, surgical complications, or from
gastrointestinal abnormalities such as ulcers, tumors, or diverticulosis. Occasionally, the bleeding may be so severe and rapid
(for example, bleeding from a ruptured aortic aneurysm) that it
causes shock and death rapidly.
- Severe inflammation of organs inside the body such as
acute pancreatitis can cause low
blood pressure. In acute pancreatitis, fluid leaves the blood vessels to enter
the inflamed tissues around the pancreas as well as the abdominal cavity,
concentrating blood and reducing its volume.
Causes of low blood pressure due to heart disease
- Weakened heart muscle can cause the heart to fail and
reduce the amount of blood it pumps. One common cause of weakened heart muscle
is the death of a large portion of the heart's muscle due to a single, large
heart attack or repeated smaller heart attacks. Other examples of conditions
that can weaken the ability of the heart to pump blood include medications that
are toxic to the heart, infections of the muscle of the heart by viruses (myocarditis), and diseases of the heart's valves such as aortic stenosis.
- Pericarditis is an inflammation of the pericardium (the
sac surrounding the heart). Pericarditis can cause
fluid to accumulate within the pericardium and compress the heart, restricting
the ability of the heart to fill and pump blood.
- Pulmonary embolism is a condition in which a blood clot
in a vein (deep vein thrombosis)
breaks off and travels to the heart and eventually the lung. A large blood clot
can block the flow of blood into the left ventricle from the lungs and severely
diminish the blood returning to the heart for pumping. Pulmonary embolism is a life-threatening emergency.
- A slow heart rate (bradycardia) can decrease the amount
of blood pumped by the heart. The resting heart rate for a healthy adult is
between 60 and 100 beats/minute. Bradycardia (resting
heart rates slower than 60 beats/minute) does not always cause low blood
pressure. In fact, some highly trained athletes can have resting heart rates in
the 40s and 50s (beats per minute) without any symptoms. (The slow heart rates
are offset by more forceful contractions of the heart that pump more blood than
in non-athletes.) But in many patients bradycardia can lead to low blood
pressure, lightheadedness, dizziness, and even fainting.
Several common reasons for bradycardia include: 1) sick sinus syndrome, 2)
heart block, and 3) drug toxicity. Many of these conditions occur in the
elderly.
- Sick sinus syndrome: Sick sinus syndrome occurs when
the diseased electrical system of the heart cannot generate signals fast enough
to maintain a normal heart rate.
- Heart block: Heart block occurs when the specialized
tissues that transmit electrical current in the heart are damaged by heart
attacks, degeneration from atherosclerosis, and medications. Heart block
prevents some or all of the electrical signals from reaching parts of the heart,
and this prevents the heart from contracting as well as it otherwise would.
- Drug toxicity: Drugs such as digoxin (Lanoxin) or beta blockers for
high blood pressure, can slow the transmission of electricity in the heart
chemically and can cause bradycardia and hypotension (see section below
"Medications that cause low blood pressure").
- An abnormally fast heart rate (tachycardia) also can
cause low blood pressure. The most common example of tachycardia causing low
blood pressure is atrial fibrillation.
Atrial fibrillation is a disorder of the heart characterized by rapid and
irregular electrical discharges from the muscle of the heart causing the
ventricles to contract irregularly and (usually) rapidly. The rapidly
contracting ventricles do not have enough time to fill maximally with blood
before the each contraction, and the amount of blood that is pumped decreases in
spite of the faster heart rate. Other abnormally rapid heart rhythms such as
ventricular tachycardia also can produce low blood pressure, sometimes even
life-threatening shock.
Medications that cause low blood pressure
- Medications such as calcium channel
blockers, beta blockers, and
digoxin (Lanoxin) can slow the rate at which the heart contracts. Some elderly
people are extremely sensitive to these medications since they are more likely
to have diseased hearts and electrical conduction tissues. In some individuals,
the heart rate can become dangerously slow even with small doses of these
medications.
- Medications used in treating high blood pressure (such as ACE inhibitors, angiotensin receptor
blockers, beta blockers, calcium channel blockers, and alpha-blockers) can
excessively lower blood pressure and result in symptomatic low blood pressure
especially among the elderly.
- Alcohol and narcotics also can cause low blood
pressure.
Other conditions that cause low blood pressure
- Vasovagal reaction is a common condition in which a
healthy person temporarily develops low blood pressure, slow heart rate, and
sometimes fainting. A vasovagal reaction typically is brought on by emotions of
fear or pain such as having blood drawn, starting an intravenous infusion, or by
gastrointestinal upset. Vasovagal reactions are caused by activity of the
involuntary (autonomic) nervous system, especially the vagus nerve, which releases hormones that slow the heart and widen
the blood vessels. The vagus nerve also controls digestive tract function and
senses activity in the digestive system.
Thus, some people can have a vasovagal reaction from straining at a bowel
movement or vomiting.
- Postural (orthostatic) hypotension is a sudden drop in
blood pressure when an individual stands up from a sitting, squatting, or supine
(lying) position. When a person stands up, gravity causes blood to settle in the
veins in the legs so that less blood returns to the heart for pumping, and, as a
result, the blood pressure drops. The body normally responds automatically to
the drop in blood pressure by increasing the rate at which the heart beats and
by narrowing the veins to return more blood to the heart. In patients with postural hypotension, this compensating reflex fails to occur,
resulting low blood pressure and its symptoms. Postural hypotension can occur in
persons of all ages but is much more common among the elderly, especially in
those on medications for high blood pressure and/or diuretics. Other causes of
postural hypotension include dehydration, adrenal insufficiency (discussed
later), prolonged bed rest, diabetes that has caused
damage to the autonomic nerves, alcoholism with damage to
the autonomic nerves, and certain rare neurological syndromes (for example, Shy-Drager syndrome) that damage the autonomic nerves.
- Another form of postural hypotension occurs typically in
young healthy individuals. After prolonged standing, the individual's heart
rate and blood pressure drop, causing dizziness, nausea, and often fainting. In
these individuals, the autonomic nervous
system wrongly responds to prolonged standing by directing the heart to slow
down and the veins to dilate.
- Micturition syncope is a temporary drop in blood
pressure and loss of consciousness brought about by urinating. This condition
typically occurs in elderly patients and may be due to the release by the
autonomic nerves of hormones that lower blood pressure.
-
Adrenal insufficiency, for example, due to
Addison's disease, can cause low blood pressure. Addison's disease is
a disorder in which the adrenal glands (small glands next to the kidneys) are
destroyed. The destroyed adrenal glands can no longer produce sufficient adrenal
hormones (specifically cortisol) necessary to maintain normal bodily functions.
Cortisol has many functions, one of which is to maintain blood pressure and the
function of the heart. Addison's disease is characterized by weight loss, muscle
weakness,
fatigue, low blood
pressure, and, sometimes, darkening of the skin.
- Septicemia is a
severe infection in which bacteria (or other infectious organisms such as fungi)
enter the blood. The infection typically originates in the lungs (as pneumonia), bladder, or in the abdomen due to diverticulitis or gallstones. The bacteria
then enter the blood where they release toxins and cause life-threatening and
profound low blood pressure (septic shock), often
with damage to several organs.
- Anaphylaxis
(anaphylactic shock) is a potentially fatal allergic reaction to medications such as penicillin, intravenous iodine used in some X-ray studies, foods such as peanuts, or bee stings (insect stings). In addition to a severe drop in blood
pressure, individuals may also experience hives as well as wheezing
due to constriction of the airways, and a swollen throat which cause difficulty
breathing. The shock is caused by enlargement of blood-containing blood vessels
and escape of water from the blood into the tissues.
-
How is low blood pressure diagnosed and evaluated?
In some individuals, particularly relatively healthy ones, symptoms of
weakness, dizziness, and fainting raise the suspicion of low blood pressure. In
others, an event often associated with low blood pressure, for example a heart
attack has occurred to cause the symptoms.
Measuring blood pressure, in both the lying (supine) and standing positions
usually is the first step in diagnosing low blood pressure. In patients with
symptomatic low blood pressure, there often is a marked drop in blood pressure
upon standing, and patients may even develop orthostatic symptoms. The heart
rate often increases greatly. Once low blood pressure has been identified as the
cause of symptoms, the goal is to identify the cause of the low blood pressure.
Sometimes the causes are readily apparent (such as loss of blood due to trauma,
or sudden shock after receiving x-ray dyes containing iodine). At other times,
the cause may be identified by testing:
- CBC (complete blood count). CBC may reveal anemia from blood loss or
elevated white blood cells due to infection.
- Cortisol levels can be measured to diagnose adrenal
insufficiency and Addison's disease.
- Blood and urine cultures can
be performed to diagnose septicemia and bladder infections,
respectively.
- Electrocardiograms
(EKG) can detect abnormally slow or rapid heart beats, pericarditis, and heart muscle damage from either previous heart
attacks or a reduced supply of blood to the heart muscle that has not yet caused
a heart attack.
- Holter monitor recordings are used to diagnose
intermittent episodes of abnormal heart rhythms. If abnormal rhythms occur
intermittently, a standard EKG performed at the time of a visit to the doctor's
office may not show the abnormal rhythm. A Holter monitor is a
continuous recording of the heart's rhythm for 24 hours that often is used to
chart and diagnose intermittent episodes of bradycardia or
tachycardia.
- Patient-activated event recorder. If the episodes of
bradycardia or tachycardia are infrequent, a 24-hour Holter recording may not
capture these sporadic episodes. In this situation, a patient can wear a
patient-activated event recorder for up to four weeks. The patient presses a
button to start the recording when he or she senses the onset of an abnormal
heart rhythm or symptoms possibly caused by low blood pressure. The doctor then
analyzes the recordings at a later date to identify the abnormal
episodes.
- Echocardiograms are examinations of the structures and
motion of the heart using ultrasound. Echocardiograms can detect pericardial fluid due to pericarditis,
the extent of heart muscle damage from heart attacks, diseases of the heart
valves, and rare tumors of the heart.
- Tilt-Table tests are used to evaluate patients
suspected of having postural hypotension or syncope due to abnormal function of
the autonomic nerves. During a tilt-table test, the
patient lies on an examining table with an intravenous infusion administered
while the heart rate and blood pressure are monitored. The table then is tilted
upright for 15 minutes to 45 minutes. Heart rate and blood pressure are
monitored every few minutes. The purpose of the test is to try to reproduce
postural hypotension. Sometimes a doctor may administer epinephrine (Adrenalin, Isuprel) intravenously to induce postural
hypotension.
How is low blood pressure treated?
Low blood pressure in healthy subjects without symptoms or organ damage needs
no treatment. However, all patients with symptoms possibly due to low blood
pressure should be evaluated by a doctor. (Patients who have had a major drop in
blood pressure from their usual levels even without the development of symptoms
also should be evaluated.) The doctor needs to identify the cause of the low
blood pressure; remedies will depend on the cause. For example, if a medication
is causing the low blood pressure, the dose of medication may have to be reduced
or the medication stopped
, though only after consulting the doctor.
Self-adjustment of medication should not be done.
- Dehydration is treated with fluids and minerals
(electrolytes). Mild dehydration without nausea and vomiting can be treated with
oral fluids and electrolytes. Moderate
to severe dehydration usually is treated in the hospital or emergency room with
intravenous fluids and electrolytes.
- Blood loss can be treated with intravenous fluids and
blood transfusions. Continuous and severe bleeding needs to be
treated immediately.
- Septic shock is an emergency and is treated with
intravenous fluids and antibiotics.
- Blood pressure medications or diuretics are adjusted,
changed, or stopped by the doctor if they are causing low blood pressure
symptoms.
- Bradycardia may be due to a medication. The doctor may
reduce, change or stop the medication. Bradycardia due to sick sinus syndrome or
heart block is treated with an implantable
pacemaker.
- Tachycardia is treated depending on the nature of the
tachycardia. Atrial fibrillation can be treated with oral medications,
electrical cardioversion, or a
catheterization procedure called pulmonary vein
isolation. Ventricular tachycardia can be controlled with medications or
with an implantable
defibrillator.
- Pulmonary embolism and deep vein thrombosis is treated
with blood thinners, intravenous initially with heparin, and oral warfarin (Coumadin)
later.
- Pericardial fluid can be removed by a procedure called
pericardiocentesis.
- Postural hypotension can be treated with changes in
diet such as increasing water and salt intake*, increasing intake of caffeinated
beverages (because caffeine constricts
blood vessels), using compression stockings to compress the leg veins and reduce
the pooling of blood in the leg veins, and in some patients, the use of a
medication called midodrine
(ProAmatine). The problem with ProAmatine is that while it increases blood
pressure in the upright position, the supine blood pressure may become too high,
thus increasing the risk of strokes. Mayo Clinic researchers found that a
medication used to treat muscle weakness in myasthenia gravis called
pyridostigmine (Mestinon) increases upright blood pressure but not
supine blood pressure. Mestinon, an anticholinesterase medication, works on the
autonomic nervous system, especially when a person is standing up. Side effects
include minor abdominal cramping or increased frequency of bowel movements.
*Note: Increasing salt intake can lead to heart failure in
patients with existing heart disease and should not be undertaken without
consulting a doctor.
- Postprandial hypotension refers to low blood pressure
occurring after meals. Ibuprofen (Motrin) or
indomethacin (Indocin) may be beneficial.
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