What causes low blood pressure (hypotension)?
Definition of low blood pressure
- Low blood pressure (hypotension), is defined as a systolic blood pressure (SBP) below 90 mmHg (or a mean arterial pressure (MAP) below 60 mmHg).
Low blood pressure might cause no symptoms that you notice. Or it might cause dizziness and fainting. Sometimes, low blood pressure can be life-threatening.
Almost all causes of low blood pressure are serious and need treating soon.
In this article, we will review the common causes of low blood pressure.
5 groups of causes of low blood pressure
Hypotension can be broadly categorised into five main groups:
- Hypovolaemia (fluid loss; e.g. dehydration, bleeding, burns, pancreatitis)
- Cardiac (poor heart function)
- Vasodilation (widening of blood vessels) – especially sepsis (severe infection); and medication
- Anaphylaxis (severe allergy)
- Hypoadrenalism (underactive adrenal glands, e.g. caused by Addison’s Disease, or withdrawal of steroid tablets).
1. Hypovolaemia hypotension
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Blood loss: Trauma, surgery, gastrointestinal bleeding, can lead to significant blood loss, resulting in hypotension
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Dehydration: Inadequate fluid intake, excessive fluid loss (e.g. diarrhoea, vomiting), or increased insensible losses – e.g. fever, burns or pancreatitis (the ‘internal burn’) – can cause hypovolemic hypotension
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‘Third spacing’: Sequestration of fluid into third spaces, such as ascites or pleural effusions, can reduce effective circulating volume.
2. Cardiac hypotension
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Myocardial infarction: Acute myocardial infarction can lead to decreased left ventricular function, resulting in hypotension
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Heart failure: Acute or chronic heart failure can cause decreased cardiac output, leading to hypotension
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Arrhythmias: Certain arrhythmias, such as atrial fibrillation or ventricular tachycardia, can decrease cardiac output and lead to hypotension
- Medication: Certain drugs for high blood pressure (hypertension) – e.g. beta-blockers.
Note. Too much of any blood pressure tablet can cause low blood pressure.
3. Vasodilation
Sepsis and other problems can vasodilation (widening of blood vessels) leading to a decrease in systemic vascular resistance. Common causes include:
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Septic shock: This is the most important cause in this group. It is a life-threatening condition that occurs when the body’s response to an infection becomes uncontrolled
- Medication: Several groups of drugs can cause low blood pressure by this mechanism – e.g. anaesthesia, drugs for high blood pressure (hypertension; ACE/ARBs, calcium antagonists and alpha-blockers)
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Spinal cord injury (‘spinal shock’): Spinal cord injury can cause loss of sympathetic tone, leading to vasodilation and hypotension.
4. Anaphylaxis
Anaphylaxis, a severe allergic reaction, can cause low blood pressure.
Anaphylaxis can cause widespread vasodilation, leading to hypotension. So it is really in group 3.
But it is so important (as it is life threatening but easy to treat) we are giving it its own group.
5. Hypoadrenalism
This means underactive adrenal glands. It can be caused by Addison’s Disease, or withdrawal of steroid tablets, and other conditions. It is another important cause of low blood pressure, as its easy to treat. This has led to the maxim,
No one should be allowed to die with hypotension, not having had 200mg of IV hydrocortisone”
Other causes of low blood pressure
Pregnancy
Low blood pressure can occur during pregnancy, especially in the first trimester. This is normal and not a disease. It is the only non serious cause of low blood pressure.
Orthostatic hypotension
This is also called postural hypotension. It is a rapid drop in blood pressure when standing up after sitting or lying down. Causes include dehydration, long-term bed rest, pregnancy, certain medical conditions and some medicines. This type of low blood pressure is common in older adults.
Other neurological disease
- Multiple system atrophy with orthostatic hypotension. This rare condition affects the autonomic nervous system
- Autonomic nervous system dysfunction (due to diabetic autonomic neuropathy), can cause low blood pressure. Postural tachycardia syndrome (PoTS) can also cause low blood pressure.
Note. The autonomic nervous system sends the ‘fight-or-flight’ signal that, depending on the situation, tells the heart and other systems in the body to increase or decrease blood pressure.
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