The sudden drops in blood pressure that can be lifethreatening are those associated with collapse of the cardiovascular system, caused by such things as:
- Loss of blood from severe bleeding, either internally or externally
- Extremely low body temperature
- Extremely high body temperature
- Heart muscle disease causing heart failure
- Sepsis, a severe blood infection, sometimes called blood poisoning
- Severe dehydration from vomiting, diarrhea, or fever
- A reaction to medication or alcohol
- A severe allergic reaction, called anaphylaxis
Chronic low blood pressure with no symptoms is almost never serious. But health problems can occur when blood pressure drops suddenly, and the brain is deprived of an adequate blood supply. This can lead to dizziness or lightheadedness. Sudden drops in blood pressure most commonly occur in someone who's rising from a prone or sitting position to standing. This kind of low blood pressure is known as postural hypotension, orthostatic hypotension, or neurally mediated orthostatic hypotension.
Postural hypotension is considered a failure of the autonomic nervous system -- the part of the nervous system that controls involuntary vital actions, such as the heartbeat -- to react appropriately to sudden changes. Normally, when you stand up, some blood pools in your lower extremities. Uncorrected, this would cause your blood pressure to fall. But your body normally compensates by sending messages to your heart to beat faster and to your blood vessels to constrict. This offsets the drop in blood pressure. If this does not happen, or happens too slowly, postural hypotension results.
The cause of low blood pressure isn't always clear, but it may be associated with the following:
- Hormonal problems such as an underactive thyroid (hypothyroidism), overactive thyroid (hyperthyroidism), diabetes, or low blood sugar (hypoglycemia)
- Over-the-counter medications
- Overdose of high blood pressure drugs
- Heart failure
- Heart arrhythmias (abnormal heart rhythms)
- Widening, or dilation, of the blood vessels
- Heat exhaustion or heat stroke
- Liver disease
Postural hypotension, which is low blood pressure when standing up suddenly, can happen to anyone for a variety of reasons, such as dehydration, lack of food, prolonged standing in the heat, or being overly fatigued. It can also be influenced by genetic make-up, aging, medication, dietary and psychological factors, and acute triggers, such as infection and allergy.
Postural hypotension occurs most frequently in people who are taking drugs to control high blood pressure (hypertension). It can also be related to pregnancy, strong emotions, hardening of the arteries (atherosclerosis), or diabetes. The elderly are particularly affected, especially those who have high blood pressure or autonomic nervous system dysfunction.
Hypotension after meals is a common cause of dizziness and falls after eating. It’s believed to be caused by blood pooling into he vessels of the stomach and intestines.
Several drugs are commonly associated with postural hypotension. These medications can be divided into two major categories:
- Drugs used to treat high blood pressure, such as diuretics, beta-blockers, calcium-channel blockers, and angiotensin-converting enzyme (ACE) inhibitors.
-Drugs that have hypotension as a side effect, including nitrates, drugs for Parkinson's disease, antipsychotics, neuroleptics, anti-anxiety agents, sedative-hypnotics, and tricyclic antidepressants.
Common causes of naturally occurring postural hypotension include:
- Dehydration and electrolyte loss, which may result from diarrhea, vomiting, excessive blood loss during menstruation, or other conditions
- Age associated decline in blood pressure regulation, which may be worsened by certain health conditions or medications
Certain diseases can cause postural hypotension. These include:
- Central nervous system disorders, such as Shy-Drager syndrome or multiple system atrophy
- Nerve problems, such as peripheral neuropathy or autonomic neuropathy
- Cardiovascular disorders
- Nutritional diseases