Hypotension (Low Blood Pressure)

How blood pressure is controlled

Blood pressure depends on three things: how much blood the heart pumps (cardiac output), how tight the blood vessels are (vascular resistance) and how much fluid is in the circulation (blood volume). The autonomic nervous system adjusts these second by second. Pressure sensors in the neck and chest (baroreceptors) detect a fall, for example when you stand, and trigger the heart to speed up and vessels to tighten. Hormones such as aldosterone and cortisol regulate salt and water over hours to days.

Hypotension develops when any of these fail: too little fluid, a weak or slow heart, vessels that are too relaxed, or reflexes that do not respond.

If you want a quick guide to symptoms and what to do when your pressure drops, see low blood pressure symptoms.

Types of hypotension

Postural orthostatic tachycardia syndrome (POTS) is related but different: heart rate rises by 30 beats a minute or more on standing, often without a large fall in blood pressure.

Causes

Low blood volume

• dehydration from heat, vomiting, diarrhea, fever or poor intake
• bleeding, including slow internal bleeding
• diuretics

Heart problems

• very slow heart rates (bradycardia, heart block)
• heart attack, heart failure, valve disease such as aortic stenosis
• fast abnormal rhythms that prevent the heart from filling

Blood vessels too relaxed

• medicines: antihypertensives, alpha-blockers, nitrates, PDE5 inhibitors, antidepressants, antipsychotics, opioids
• alcohol
• septic shock and anaphylaxis

Nervous system disorders

• autonomic neuropathy from diabetes, amyloidosis or autoimmune disease
• Parkinson's disease, multiple system atrophy, Lewy body dementia
• spinal cord injury

Hormonal causes

• Addison's disease and other adrenal insufficiency, including after stopping long-term steroids
• underactive thyroid
• low blood sugar

Pregnancy

Blood pressure falls in early and mid-pregnancy as blood vessels relax. Lying flat on the back late in pregnancy can compress the main vein and cause dizziness (supine hypotensive syndrome). Lying on the left side relieves it.

Diagnosis

1. Repeated blood pressure readings, including home or 24-hour ambulatory monitoring.
2. Lying and standing measurements: blood pressure and pulse after 5 minutes lying, then at 1 and 3 minutes standing.
3. History: symptoms, timing with meals or standing, medicines, fluid intake, weight loss, bowel and bladder changes (which suggest autonomic failure).
4. Blood tests: blood count for anemia, kidney function and electrolytes, glucose, thyroid function, and morning cortisol.
5. ECG and, if indicated, echocardiogram and heart rhythm monitoring.
6. Tilt-table testing and autonomic function tests for unexplained or recurrent symptoms.

Treatment

People with low readings and no symptoms need no treatment.

Treat the cause

• Replace fluids or blood.
• Stop, reduce or change the timing of medicines that lower blood pressure, with your doctor's guidance.
• Treat infection, heart rhythm problems, adrenal insufficiency or thyroid disease.

Non-drug measures for orthostatic and postprandial hypotension

• Drink 2 to 3 liters of fluid a day unless told otherwise. Drinking about 500 ml (2 cups) of water quickly raises blood pressure for about an hour, which helps before getting up or before activities.
• Increase salt if your doctor agrees.
• Stand up in stages and use counter-maneuvers: crossing legs, tensing thigh and buttock muscles, or squatting.
• Raise the head of the bed by 10 degrees, which reduces overnight fluid loss and morning symptoms.
• Wear waist-high compression stockings or an abdominal binder.
• Eat smaller, more frequent meals lower in refined carbohydrates. Avoid alcohol.
• Stay active with recumbent or water-based exercise to improve tolerance.

Medicines

• Midodrine: tightens blood vessels and raises standing blood pressure. Taken during the day, not before lying down.
• Fludrocortisone: increases salt and fluid retention.
• Droxidopa: for neurogenic orthostatic hypotension.
• Pyridostigmine in selected people.

These medicines can raise blood pressure too much when lying down (supine hypertension), so readings are monitored.

Shock

Shock is treated in hospital with IV fluids, blood products, medicines that raise blood pressure (vasopressors), and treatment of the cause, such as antibiotics for sepsis or epinephrine for anaphylaxis.

Complications and outlook

Symptomatic hypotension raises the risk of falls, fractures and head injury, especially in older adults. Orthostatic hypotension is also linked to higher cardiovascular risk and cognitive decline over time. Shock can cause organ damage and is life-threatening without rapid treatment. Most people with medication-related or dehydration-related hypotension improve fully once the cause is corrected. Neurogenic orthostatic hypotension is usually long-term but can be managed.

Frequently asked questions

What is hypotension?

Hypotension is abnormally low blood pressure, generally below 90/60 mmHg. It matters when it causes symptoms such as dizziness or fainting, or signals an underlying problem like dehydration, heart disease, infection or a hormone disorder.

What are the main types of hypotension?

The main types are orthostatic hypotension (on standing), postprandial hypotension (after meals), reflex or neurally mediated hypotension (vasovagal fainting), neurogenic hypotension from nerve damage, and severe hypotension in shock. Many healthy people also have chronic low pressure with no symptoms.

How is hypotension treated?

Treatment depends on the cause. It includes drinking more fluid, adding salt if safe, adjusting medicines, rising slowly, wearing compression stockings, and in persistent cases medicines such as midodrine or fludrocortisone. Shock needs emergency hospital treatment.

Is low blood pressure better than high blood pressure?

Low readings without symptoms are generally healthier than high blood pressure, which raises the risk of stroke and heart disease. But low blood pressure that causes dizziness or fainting, or that results from illness, needs attention.

Can hypotension be cured?

Hypotension caused by dehydration, medicines, infection or hormone problems usually resolves when the cause is treated. Chronic forms from nerve damage cannot be cured, but symptoms can be well controlled with lifestyle measures and medicines.