Understanding Low Blood Pressure -- the Basics
What Is Low Blood Pressure?
Hypotension is the medical term for low blood pressure (less than 90/60).
A blood pressure reading appears as two numbers. The first and higher of the two is a measure of systolic pressure, or the pressure in the arteries when the heart beats and fills them with blood. The second number measures diastolic pressure, or the pressure in the arteries when the heart rests between beats.
Optimal blood pressure is less than 120/80 (systolic/diastolic). In healthy people, low blood pressure without any symptoms is not usually a concern and does not need to be treated. But low blood pressure can be a sign of an underlying problem -- especially in the elderly -- where it may cause inadequate blood flow to the heart, brain, and other vital organs.
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 lying down or sitting position to standing. This kind of low blood pressure is known as postural hypotension or orthostatic hypotension. Another type of low blood pressure can occur when someone stands for a long period of time. This is called neurally mediated hypotension.
Postural hypotension is considered a failure of the cardiovascular system or nervous system 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 risk of both low and high blood pressure normally increases with age due in part to normal changes during aging. In addition, blood flow to the heart muscle and the brain declines with age, often as a result of plaque buildup in blood vessels. An estimated 10% to 20% of people over age 65 have postural hypotension.
What Causes Low Blood Pressure?
The cause of low blood pressure isn't always clear. It may be associated with the following:
- Hormonal problems such as an underactive thyroid (hypothyroidism), diabetes, or low blood sugar (hypoglycemia)
- Some over-the-counter medications
- Some prescription medicines such as for high blood pressure, depression or Parkinson’s disease
- Heart failure
- Heart arrhythmias (abnormal heart rhythms)
- Widening, or dilation, of the blood vessels
- Heat exhaustion or heat stroke
- Liver disease
What Causes a Sudden Drop in Blood Pressure?
Sudden drops in blood pressure can be life-threatening. Causes of this type of hypotension include:
- Loss of blood from bleeding
- Low body temperature
- High body temperature
- Heart muscle disease causing heart failure
- Sepsis, a severe blood infection
- Severe dehydration from vomiting, diarrhea, or fever
- A reaction to medication or alcohol
- A severe allergic reaction, called anaphylaxis