Low Blood Pressure

What Is Low Blood Pressure?

Low blood pressure is a difficult clinical finding for a healthcare professional to address. While high blood pressure is known as the "silent killer," because it is associated with few acute symptoms, hypotension (hypo=low + tension=pressure) may be normal for a patient if it is without symptoms, but can be of great importance if it is associated with abnormal body function. Sometimes low is good, a goal to be achieved in keeping blood pressure under control. Sometimes low is bad because there is not enough pressure to provide blood flow to the organs of the body.

Blood pressure readings have two parts and are expressed as a ratio:

  • "Normal" blood pressure, for example is 120/80 (120 over 80) and measures the pressure within the arteries of the body.
  • Systolic pressure, the upper number, measures the pressure within the arteries when the heart is contracting (systole) to pump blood to the body.
  • Diastole pressure, the lower number, measures resting pressures within the arteries, when the heart is at rest.

You can think of the heart and the blood vessels (arteries and veins) as a system to pump blood, just like the oil pump in your car. Oil is pumped through rigid tubes. Pressure remains relatively constant throughout the pumping cycle unless the pump fails or there is an oil leak. Then oil pressure will fall.

The body is similar, except that the tubes have pliable walls, meaning that the space within the arteries can get bigger or smaller. If the space gets bigger, there is effectively less fluid, and pressure falls. If the space gets smaller, pressure goes up. Arteries have layers of muscles within their walls that can contract and narrow the artery, making less space inside the vessels. Alternatively, the muscles can relax and dilate the artery, making more room. These muscles are under the control of the autonomic nervous system, the body's automatic system that makes adjustments for moment-to-moment changes in the relationship of the body to the world. The autonomic nervous system has two pathways that balance each other.

The sympathetic nervous system uses adrenaline (epinephrine) to cause the muscles to contract (sympathetic tone). The nerves that help with this control are located in the sympathetic trunk, which is a group of nerves that runs alongside the spinal column. The parasympathetic system uses acetylcholine to make muscles in the blood vessel walls relax via the vagus nerve. As an example, when you stand up, the blood vessels have to narrow just a little to cause a slight increase in blood pressure, so that blood can travel uphill to the brain. Without that change, you might feel lightheaded or pass out.

Normal blood pressure depends on many factors including age and body size.

  • Infants and children have lower normal readings than adults.
  • Smaller or petite patients may have lower normal blood pressure ranges.
  • Based upon American Heart Association guidelines, any reading greater than 120/80 is considered pre-hypertension or early high blood pressure.

For low blood pressure to be a problem there needs to be a symptom associated with that low number. Readings below 120/80 may be normal depending upon the clinical situation. Many people have systolic blood pressures below 100, but some people develop symptoms with pressures that low. Symptoms of low blood pressure occur because one or more of the body's organs is not getting enough blood supply.

What Causes Low Blood Pressure?

If low blood pressure causes clinical symptoms, the cause will be in one of three general categories. Either the heart is not pumping with enough pressure, the artery walls are too dilated, or there is not enough intravascular fluid (intra=within + vascular= blood vessels) within the system.

Low Blood Pressure and the Heart

The heart is a muscle that works as a pump and is controlled by electrical signals. Problems with either the pump or the electricity can cause problems with low blood pressure.

  • If the heart beats too quickly, blood pressure may fall because there isn't enough time for the heart to refill in between each beat (diastole). If the heart beats too slowly, there may be too much time spent in diastole when blood is not flowing.
  • If the heart muscle has been damaged or irritated, there may not be enough pumping force to maintain blood pressure. In heart attack (myocardial infarction), enough heart muscle may be stunned so that the heart is too weak to pump effectively.
  • The valves of the heart allow blood to flow in only one direction. If a valve fails, blood can regurgitate backwards, minimizing the amount that will flow out to the body. If a valve becomes narrowed (stenotic), then blood flow may be decreased. Both situations may cause hypotension.
Low blood pressure can be an emergency situation depending on the cause.

Low Blood Pressure Symptoms

A Typical Night in the ER

Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD

The ambulance call comes in.

"We're running lights and siren with a 67 year old male. He's confused, lethargic, blood pressure 80 over 40, thready pulse and sweaty. We'll be at your door in a couple of minutes."

The nurses start setting up the equipment to care for the patient. We've been through this drill before when someone arrives at the ER with low blood pressure. I hope it's an easy diagnosis and something that we can treat quickly. IV's are set up, monitor pads ready. The respiratory tech arrives with her machines just in case they are needed.

The EMTS unload a man in obvious distress, sweaty, struggling to breathe, and limp.

"He's had a fever for a couple of days with a cough. Past history of COPD, heart problems, still smokes."

The diagnosis still could be easy, but there are a lot of things that could be causing this man's condition. He is in shock; his low blood pressure is not enough to provide blood flow and oxygen to his brain. That's why he is lethargic. No doubt the rest of his organs are at risk too. But why?

Low Blood Pressure and Intravascular Fluid

The fluid space within the blood vessels is made up of blood cells and serum (water, clotting factors, chemicals, and electrolytes).

  • Dehydration, the loss of water, decreases the total volume in the intravascular space (inside the blood vessels). This can be seen in any illness with increased water loss. Vomiting and diarrhea are visible signs of water loss.
  • Bleeding decreases the number of red blood cells in the blood stream and leads to a decreased amount of fluid in the intravascular space and low blood pressure.

Low Blood Pressure and the Arterial Wall

The space within the intravascular space is variable, based upon the muscle tension in the walls of the arteries. Adrenalin (epinephrine) increases tension and causes the arteries to narrow and support blood pressure. Acetylcholine dilates blood vessels and will lower pressure. Normally, the two are in balance.

  • Loss of the sympathetic tone may occur with injuries to the spine and damage to the sympathetic trunk, resulting in dilation of the blood vessels and lowered blood pressure.
  • Over stimulation of the vagus nerve can cause dilation of the blood vessels as well. A temporary, self-limiting situation called vasovagal syncope (passing out from low blood pressure due to overstimulation of the vagus nerve) is often seen when a patient receives a noxious stimulus. This can be physical, like a broken bone, or emotional like a medical student seeing their first operation.

Low Blood Pressure Medications

  • Diuretic medications [for example, hydrochlorothiazide (Hydrodiuril), furosemide (Lasix)] are used to control blood pressure by causing the kidneys to make more urine and decreasing the intravascular volume. If the patient loses too much water and becomes dehydrated, low blood pressure may result.
  • Beta blockers and calcium channel blockers are two commonly prescribed medications used for the treatment of high blood pressure. They may cause the heart to beat too slowly and thus cause hypotension. Any heart medication needs to be monitored by a healthcare provider to evaluate the body's response and to select the appropriate dose.
  • Medications like sildenafil (Viagra) in combination with nitroglycerin may cause blood vessel dilation and low blood pressure.

Low Blood Pressure and Pregnancy

Normal physiologic changes in pregnancy increase the intravascular space, especially in the first two trimesters, and can cause lower blood pressure.

Endocrine

The body maintains its normal function by a number of checks and balances and feedback loops. Hormones are the messengers that help make adjustments in the body's function. There are connections between the pituitary gland in the brain and glands in the body to help regulate function.

  • Low thyroid gland function (hypothyroidism) may be associated with low blood pressure.
  • Any adrenal gland abnormalities can cause a decrease in the cortisone hormone levels in the body. Low blood pressure is also seen in this situation.
  • Abnormally low pituitary function will also cause low blood pressure
  • People with diabetes may develop low blood pressure when their blood sugar drops too low. If the blood sugar gets too high (hyperglycemia), dehydration and low blood pressure may also occur through a different mechanism.

Allergic Reactions Can Cause Low Blood Pressure

Major allergic reactions (anaphylactic shock) can cause marked dilation of blood vessels and low blood pressure, associated with:

Orthostatic Hypotension

When standing quickly, it may take a second or two for the body to make adjustments to constrict blood vessels and push blood up to the brain. If there is a delay, then this time of relative low blood pressure may cause symptoms. It can be exacerbated or seen more often in patients who are pregnant, have diabetes, are dehydrated, or taking high blood pressure medications.

Diabetes and Low Blood Pressure

One of the complications of diabetes is damage to the nerves in the body, including those in the autonomic nervous system. In those people with diabetes who have autonomic dysfunction, orthostatic hypotension can occur. The blood vessels are unable to adjust to quick positional changes.

Micturition Syncope (Fainting During Urination or Bowel Movements)

One of the most common causes of syncope, (passing out or fainting), occurs when a person strains to urinate or have a bowel movement. This stimulates the vagus nerve, increasing acetylcholine in the body and dilating blood vessels, causing blood pressure to fall and decreasing blood supply to the brain. Though it is frightening and may result in a fall, micturition syncope is self-limiting, resolves quickly, and may need little treatment.

Low Blood Pressure Signs and Symptoms

If low blood pressure is the normal state for a patient, then there will be no symptoms.

If low blood pressure is symptomatic, then the patient may feel:

  • lightheaded,
  • dizzy and weak,
  • short of breath, or
  • have chest pain.

The symptoms will depend upon which organ in the body is lacking adequate blood flow.

Exams and Tests for Low Blood Pressure

The key to diagnosis is a good history and physical examination. If low blood pressure is found incidentally and no other symptoms exist, then documenting the lower readings will help remind the healthcare provider during future visits.

If the patient is symptomatic, documenting risk factors and exploring potential causes requires a detailed history of the situation; for example:

  • when symptoms occur,
  • associated complaints, and
  • a thorough review of past illnesses and medications.

Physical examination may include postural vital signs. The patient has the blood pressure and pulse rate taken when laying flat and again when standing (some may add a third set of measurements when sitting). If the blood pressure drops or the pulse rate increases, it may be an indicator of decreased intravascular volume from dehydration or bleeding. The rest of the examination will likely be directed by clues from the history, but may include palpation of the thyroid gland in the neck, listening to the heart and lungs, and examination of the abdomen and the extremities.

Blood tests may be done, again directed by findings in the history and physical examination.

An electrocardiogram (EKG or ECG) may be performed if the low blood pressure is thought to originate in the heart or if there is chest pain or shortness of breath associated with the low pressure.

Consideration for further testing will depend upon the potential underlying cause of low blood pressure.

What to Do for Low Blood Pressure

Usually, low blood pressure with no symptoms requires little intervention.

If low blood pressure is associated with chest pain, shortness of breath, or occurs because of active bleeding, treatment will occur at the same time as the diagnostic evaluation. These combinations may be truly life-threatening, and the healthcare provider may need to transfer the patient to an emergency department for further care. A patient with low blood pressure who is symptomatic may be considered to be in shock (a situation where organs can't function properly because of lack of blood supply).

Intravenous fluids and oxygen may be given, and heart monitoring may be necessary. Based upon the underlying complaints and potential diagnosis, specific therapy may be started even without a firm diagnosis. Examples include antibiotics for a patient with an infection, adrenaline and an antihistamine for a patient with an allergic reaction, or blood transfusion for a patient who is bleeding.

More often, a patient presents with a history of symptoms but is feeling normal upon presentation for care. In this circumstance, the healthcare provider has time to make a more specific diagnosis and match the treatment to the underlying cause of the low blood pressure.

If the blood pressure readings are abnormally low, observation in a hospital setting may be appropriate. This again will be dependent on the specific situation and patient presentation.

Low Blood Pressure Complications

If low blood pressure causes lack of blood flow to the organs of the body, then those organs will start to fail. This may result in stroke, heart attack, kidney failure, and bowel ischemia (decreased blood supply to the small and large intestine).

Shock and death are the end result of prolonged low blood pressure.

Summary

  • Low blood pressure in itself may be less important than the symptoms associated with it.
  • The healthcare provider will use a careful history and physical examination to find the cause.
  • Treatment will be based upon the cause of low blood pressure.
  • Sometimes emergency interventions will be needed if the symptoms suggest the risk of organ complications or shock.
  • If there are no symptoms, there may be no treatment needed.
Reviewed on 11/20/2017

Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care

REFERENCES

Kaufmann, Horatio, et al. "Mechanisms, causes, and evaluation of orthostatic hypotension." UptoDate. Updated Feb. 2, 2015.

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