Low Blood Pressure (Hypotension): Symptoms and Causes

What is considered low blood pressure

Blood pressure has two numbers. The top number (systolic) is the pressure when the heart beats. The bottom number (diastolic) is the pressure between beats. A reading below 90/60 mmHg is usually called low. Some people normally run at 85/55 and feel completely well, so doctors focus on symptoms and on any sudden drop from a person's usual level.

Orthostatic (postural) hypotension means a drop of at least 20 mmHg systolic or 10 mmHg diastolic within 3 minutes of standing up.

Low blood pressure symptoms

• Dizziness or lightheadedness, especially on standing
• Fainting or near-fainting
• Blurred or dimmed vision
• Nausea
• Tiredness and weakness
• Difficulty concentrating
• Cold, pale, clammy skin
• Fast, shallow breathing
• A fast, weak pulse

The last four suggest the brain and organs are not getting enough blood, which can mean shock.

Causes of low blood pressure

Common causes

• Orthostatic hypotension: blood pools in the legs when you stand, and the body is slow to compensate. Common in older adults, after bed rest, in hot weather, and with dehydration or certain medicines.
• Dehydration: from vomiting, diarrhea, fever, heavy sweating or not drinking enough.
• Medicines: blood pressure medicines, diuretics, nitrates, alpha blockers for prostate problems, some antidepressants, Parkinson's medicines, erectile dysfunction medicines (especially combined with nitrates), and opioids.
• Vasovagal fainting: a reflex triggered by pain, the sight of blood, standing for long periods or heat, causing a sudden drop in heart rate and blood pressure.
• Pregnancy: blood pressure normally falls in the first and second trimesters.
• After meals (postprandial hypotension), especially in older adults.

Hypovolemia: low blood volume

Hypovolemia means there is not enough fluid in the blood vessels. It is one of the main mechanisms of low blood pressure. Causes include bleeding (injury, stomach ulcers, ruptured ectopic pregnancy), severe dehydration and burns. When the loss is large, it leads to hypovolemic shock.

Serious causes

• Sepsis: the body's extreme response to infection widens blood vessels and leaks fluid, causing septic shock.
• Peritonitis: infection or inflammation of the lining of the belly, for example after a burst appendix or perforated ulcer, can cause low blood pressure through sepsis and fluid shifting into the abdomen. Symptoms include severe belly pain, a rigid belly, fever, a fast pulse and dizziness.
• Anaphylaxis: a severe allergic reaction that widens blood vessels.
• Heart problems: heart attack, very slow or very fast heart rhythms, heart valve disease, heart failure.
• Pulmonary embolism.
• Hormone problems: Addison's disease (adrenal insufficiency), severe hypothyroidism, low blood sugar.
• Nervous system disorders: Parkinson's disease, diabetic nerve damage and multiple system atrophy affect blood pressure control.
• Poisoning and overdose: carbon monoxide, sedatives, and blood pressure medicine overdose.

Can cooking gas poisoning cause low blood pressure?

Natural gas and propane themselves are not very toxic, but faulty or poorly ventilated gas stoves, heaters and boilers can produce carbon monoxide. Carbon monoxide poisoning causes headache, dizziness, nausea, confusion and, in severe cases, low blood pressure, chest pain and collapse. Leave the area immediately, get into fresh air and call 911 or your local emergency number. Install carbon monoxide alarms near sleeping areas.

How doctors find the cause

A clinician will check blood pressure lying down and after standing for 1 and 3 minutes, pulse, temperature, and signs of dehydration or bleeding. They will review all medicines. Tests depend on the situation and can include:

• Blood count, electrolytes, kidney function and glucose
• ECG and heart monitoring
• Echocardiogram
• Morning cortisol for adrenal insufficiency
• Tilt table testing for recurrent fainting
• Blood cultures, lactate and imaging for suspected sepsis

Treatment and self-care

Treatment depends on the cause. Emergencies such as shock, sepsis and anaphylaxis need IV fluids, medicines that raise blood pressure, epinephrine or treatment of bleeding in hospital.

For mild or long-term low blood pressure:

• Stand up slowly: sit on the edge of the bed for a minute before standing. Flex your calf muscles before rising.
• Drink enough fluids: around 2 to 2.5 liters a day unless your doctor advises otherwise.
• Salt: your doctor may suggest more salt in your diet. Do not do this if you have heart failure, kidney disease or high blood pressure readings at other times.
• Eat smaller, more frequent meals, and limit alcohol.
• Compression stockings reduce blood pooling in the legs.
• Raise the head of the bed slightly for orthostatic hypotension.
• If you feel faint, sit or lie down with your legs raised. Crossing your legs and tensing your muscles can stop a faint.
• Medicine review: your doctor may lower doses or change medicines.
• Prescription medicines such as fludrocortisone or midodrine are used for some people with persistent symptoms.

Frequently asked questions

What are the symptoms of low blood pressure?

Common symptoms are dizziness or lightheadedness, especially when standing, fainting, blurred vision, nausea, tiredness and poor concentration. Cold, clammy skin, confusion, fast shallow breathing and a weak rapid pulse suggest shock and need emergency care.

What does hypovolemia mean?

Hypovolemia means low blood volume: not enough fluid in the blood vessels. It is caused by bleeding, severe dehydration from vomiting or diarrhea, or burns. Because there is less fluid to pump, blood pressure falls and the heart beats faster. Severe hypovolemia leads to shock.

Why does peritonitis cause low blood pressure?

Peritonitis is inflammation of the abdominal lining, usually from infection. Infection can trigger sepsis, which widens blood vessels, and fluid shifts out of the bloodstream into the abdomen. Both reduce blood pressure. Low blood pressure with severe belly pain and fever is an emergency.

Is low blood pressure dangerous?

Not usually, if you feel well. Many healthy people have low readings. It is dangerous when it causes fainting, falls or confusion, or when it drops suddenly from your usual level, which can signal dehydration, bleeding, infection, heart problems or an allergic reaction.

What should I do if my blood pressure is low and I feel dizzy?

Sit or lie down straight away and raise your legs. Drink water if you can. Get up slowly once you feel better. Call 911 or your local emergency number if dizziness comes with chest pain, breathlessness, confusion, fainting or signs of bleeding.