Whilst high blood pressure is a common concern, low blood pressure (hypotension) can be just as serious.
Understanding the underlying causes is vital because, unlike high blood pressure, most forms of sudden hypotension are medical emergencies that require rapid treatment.
Medically, low blood pressure is defined as a systolic blood pressure (the top number) below 90 mmHg, or a mean arterial pressure (MAP) below 60 mmHg.
While some people naturally have low readings with no issues, for many, it leads to:
Dizziness or lightheadedness
Fainting (syncope)
Blurred vision
Fatigue or lack of concentration
Hypotension is generally categorized into five physiological groups. Identifying which group a patient falls into determines the life-saving treatment required.
1. Hypovolaemia (Fluid or Blood Loss)
This is the most common cause of low blood pressure. It occurs when there isn’t enough fluid in the “pipes” (the circulatory system) for the heart to pump effectively.
Dehydration: Caused by inadequate drinking, heat exhaustion, or excessive vomiting and diarrhoea.
Bleeding: Internal or external blood loss from trauma, surgery, or gastrointestinal ulcers.
Third Spacing: When fluid leaks out of the blood vessels into other body spaces, common in pancreatitis, burns, or severe liver disease (ascites).
2. Cardiac Causes (Heart Pump Failure)
If the heart “pump” isn’t working correctly, it cannot generate enough pressure to move blood to the brain.
Heart Attack (Myocardial Infarction): Damage to the heart muscle reduces its pumping power.
Heart Failure: Chronic weakening of the heart.
Arrhythmias: An irregular heartbeat (like Atrial Fibrillation or Tachycardia) can prevent the heart from filling properly between beats.
Medication: Beta-blockers or an overdose of blood pressure medication can slow the heart rate too much.
3. Vasodilation (Widening of Blood Vessels)
Even if you have enough fluid and a strong heart, your blood pressure will drop if your blood vessels suddenly “relax” or widen too much.
Sepsis (Septic Shock): This is a life-threatening emergency where a severe infection causes a total-body inflammatory response, leading to a massive drop in blood pressure.
Medication: ACE inhibitors, Alpha-blockers, and certain anesthetics can cause vessels to dilate.
Spinal Shock: A spinal cord injury can disrupt the signals that tell blood vessels to stay constricted.
4. Anaphylaxis (Severe Allergic Reaction)
Anaphylaxis is a specialized, life-threatening form of vasodilation. When the body encounters a severe allergen (like nuts or a bee sting), it releases a flood of chemicals that causes blood vessels to widen and leak fluid. This requires immediate treatment with adrenaline.
5. Hypoadrenalism (Addison’s Disease)
The adrenal glands produce hormones (like cortisol) that help maintain blood pressure. If these glands are underactive—due to Addison’s Disease or the sudden withdrawal of long-term steroid tablets—blood pressure can crash.
Medical Maxim: “No one should be allowed to die with hypotension without having had a dose of IV hydrocortisone.”
Orthostatic (Postural) Hypotension
This is a sudden drop in blood pressure that happens when you stand up quickly after sitting or lying down. It is very common in older adults and can be caused by dehydration, certain neurological conditions, or medications.
Pregnancy
During the first trimester, blood pressure characteristically drops as the circulatory system expands to support the baby. This is the only “non-serious” cause of hypotension, but it can still cause dizziness.
Neurological Disorders
Conditions that affect the Autonomic Nervous System (the body’s “fight-or-flight” controller) can lead to low blood pressure. Examples include:
Diabetic Neuropathy
Parkinson’s Disease
PoTS (Postural Tachycardia Syndrome)
Low blood pressure is often a sign of a serious underlying problem. If you experience persistent dizziness, fainting, or if your blood pressure is consistently below 90/60 mmHg, seek medical advice.
Causes and Investigation of High Blood Pressure
10 Common Blood Pressure Medications and Their Side Effects
Remember: If someone has low blood pressure and appears confused, cold, or clammy, they may be in shock—call emergency services immediately.
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