Causes of Low Blood Pressure (Hypotension)


In the medical fraternity, low blood pressure is referred to as hypotension. Chronic low blood pressure without any symptoms is rarely serious. However, if you experience symptoms, like dizziness or fainting, you must take medical opinion.

There are various causes of low blood pressure some serious and some not so serious. Some cause a sudden drop of blood pressure while some do it over a time. Some are accompanied by a fast heart rate while some are accompanied by slow heart rate. Some causes are related to low blood pressure in women during pregnancy.  All these are explained below

You could read the previous post on what is low blood pressure and its range of readings so that you will know what we are talking about.

A healthy desired reading of blood pressure is 120/90 mm of Hg. Any blood pressure reading below 90/60 is considered low.

One risk factor of low blood pressure is advancing age. As you age, the blood flow to the heart and the brain reduces. This comes about due to the clogging of the arteries by cholesterol plaque buildup.

What can be the causes of low blood pressure and who is more prone?

Broadly, there are four major reasons, which cause low blood pressure:

1. Reduction of Blood Volume.

2. Certain Medicines.

3. Cardiac causes.

4. Other Causes.

1. Reduction of blood volume

Loss of blood from the body can be either due to severe bleeding, externally or internally.

External bleeding can be due to

  • Trauma or
  • An accident.

Internal bleeding can be due to

  • Surgical  complications arising after surgery can cause low blood pressure due to excessive loss of blood.
  • A bleeding  ulcer  such as a peptic ulcer or tumor
  • A ruptured aortic aneurysm causes severe loss of blood and death.
  • An inflamed organ like the pancreas causes the fluid to separate from the blood and accumulate in the abdomen or around the inflamed tissue.

2. Certain medicines can cause low blood pressure

  • Medicines given for heart conditions like calcium channel blockers, digoxin, and beta blockers, nitrates reduce the heart rate and the blood pressure.
  • Medicines for high blood pressure if taken in a higher doses and in the elderly can give rise to low blood pressure — medicines like diuretics (lasix), alpha-blockers, ace inhibitors.
  • Alcohol
  • Certain narcotics: Drugs used in psychiatric treatment especially for depression can cause blood pressure to fall. Examples include (tricyclic antidepressants), including doxepin (Silenor), imipramine (Tofranil), protriptyline (Vivactil) and trimipramine (Surmontil)
  • Drugs used in Parkinsonism like pramipexole (Mirapex) or those containing levodopa

3.  Cardiac Causes of low blood pressure

Weakness of the myocardium (heart muscle)

Weakness of the myocardium happens due to

  • a massive heart attack or
  • a viral infection of the myocardium (myocarditis) or
  • stenosis of the aortic valve.

All these factors lead to insufficient pumping of blood into the blood vessels of the body causing reduced blood pressure in the arteries. In aortic stenosis, the heart tries to keep the blood flow normal by pumping hard. As a result, it gets overworked and this leads to weakening of the heart muscle.

Pulmonary embolism

Pulmonary embolism is a condition wherein a clot in a vein is dislodged and travels through the blood into the heart and subsequently is lodged into the lungs. It causes a block and the blood flow from the lungs to the heart is blocked. This is a medical emergency and more often than not, results in the death of the patient.

Causes of low heart rate (or slow pulse) and low blood pressure

Pulse is an indicator of your heart rate. If your heart beats at 72 beats per minute, your pulse will be 72 per minute.

A low heart rate or bradycardia, as referred to in medicine, can cause your blood pressure to fall. This is caused by medications like Digoxin and Beta-blockers. A low heart rate with low blood pressure can also be caused by conditions called Sick Sinus Syndrome or Heart Block, wherein the sensory motor impulses, which control the heart rate, are disturbed.

Causes of increased heart rate and low blood pressure

Increased heart rate (tachycardia) due to atrial fibrillation or ventricular tachycardia is a condition where the heart beats at a very rapid rate. Due to the rapid rate, ventricles do not fill completely and the amount of blood pumped out, is less. This causes low blood pressure.

How pericarditis causes low blood pressure?

Pericarditis is a condition where the sac (pericardium) covering the heart is inflamed and thickened and is filled with fluid. This presses on the heart and the heart does not have the flexibility to relax and contract as it normally does because the pericardium is covering it tightly. This results in less blood flowing out of the heart causing low blood pressure.

4. Other Causes of Low Blood Pressure

To briefly innumerate them:

  • Postural hypotension is seen in about 20 % of people above the age of 65 years. It is a condition where the individual develops low blood pressure on getting up after being sitting or lying down for a long period. This type of low blood pressure happens usually in the morning. Due to gravity, the blood does not return from the legs to the heart in sufficient quantities.  Normally, in such conditions the body responds by increasing the heart rate and the peripheral arteries to contract, but in patients of postural hypotension, this backup fails causing the blood pressure to fall temporarily. The common causes of postural hypotension are drugs and conditions that cause hypovolaemia (blood or fluid loss), diuretic or vasodilator therapy.
  • Malnutrition is a common cause of low blood pressure. This is because the deficiency of certain vitamins in the body, especially Vitamin B-12 and folate inhibit the production of red blood cells, which become deficient. This leads to fall in the blood pressure.
  • A syncopal attack resulting from micturation causes sudden low blood pressure. A syncope is a medical name for fainting. It is usually temporary and is caused by the momentary reduction in blood flow and, therefore, a lack of oxygen to the brain.
  • Adrenal  gland  damage due to some pathology results in lack of secretion of the hormone, cortisol, from the adrenal glands. Cortisol is responsible for controlling the heart and the blood pressure. Blood pressure, therefore, falls.
  • Septicemia is a condition wherein the blood gets infected with the bacteria, which enter the blood from lungs, bladder or the intestines. Septicemia results in shock and low blood pressure. This drop in blood pressure can be life-threatening.
  • Anaphylactic shock caused by allergy to injections of medicines like penicillin and vitamin B1 causes blood pressure to fall.
  • A vasovagal reaction is a reaction triggered by fear and pain. The vagus nerve belongs to the involuntary autonomic system, which controls the heart rate and the blood pressure. The vagus nerve slows down the heart rate and this causes low blood pressure.
  • Dehydration and loss of electrolytes due to diarrhea and/or vomiting can cause your blood pressure to fall.
  • Hormonal problems such as an underactive thyroid (hypothyroidism) can cause low blood pressure.
  • Heat exertion or a heat stroke
  • Prolonged bed rest or hospitalization due to a prolonged illness

Causes of low blood pressure in women during pregnancy

It is not uncommon to see low blood pressure levels in women during pregnancy. There is consensus that the blood volume increases significantly during pregnancy and again due to hormonal changes during pregnancy, the blood vessels dilate.

This blood volume expansion and dilation of the blood vessels can cause the blood pressure to drop. This drop in blood pressure starts somewhere during the beginning of the first trimester and is lowest during the middle of the second trimester.

This is a normal phenomenon and the blood pressure returns to normal once the baby is delivered. Below is a table showing what is considered low blood pressure during pregnancy.

Severity                              Systolic                                 Diastolic 

Low BP (mild)                      60 mm Hg                             40 mm Hg
Borderline low BP               90 mm Hg                              60 mm Hg
Extremely low BP               50 mm Hg                              33 mm Hg

Causes of sudden low blood pressure

Of the above causes, the following can cause a sudden fall in blood pressure:

  • Loss of blood
  • Sepsis – severe infection of blood
  • Medication and/or a reaction to a drug
  • Too much alcohol causing an allergic reaction
  • Anaphylaxis. This is a severe allergic reaction to an injected medication.
  • Heart muscle disease that leads to heart failure
  • Severe dehydration due to vomiting or diarrhea