• Systolic pressure. The first (top) number in a blood pressure reading, it reflects the pressure the heart generates when pumping blood through the arteries to the rest of your body.
• Diastolic pressure. The second (bottom) number in a blood pressure reading, it reflects the pressure in the arteries when the heart is resting in between beats.
A blood pressure reading is considered normal if it is equal to or lower than 120/80 mm Hg.
Blood pressure readings vary considerably in a short amount of time according to the kind of activity the body is doing: sometimes between heartbeats, with body position, breathing rhythm, stress level, physical condition, medications, what you eat and drink, and even time of day. Blood pressure is usually lowest at night and rises sharply on waking.
What's considered low blood pressure for one person may be normal for another.
Low blood pressure is generally defined as readings lower than 90 mm Hg systolic or 60 mm Hg diastolic. If only one value falls in the low range, then blood pressure is considered lower than normal.
A sudden drop in blood pressure also can be dangerous. For example, a decrease of 20 mm Hg can deprive the brain from adequate blood and this may result in dizziness and fainting. Conditions associated with significant and life0threatening drop in blood pressure include uncontrolled bleeding, severe infections or allergic reactions (anaphylaxis).
Athletes and people who exercise regularly tend to have lower blood pressure and a slower heart rate than do people who aren't as fit. The same applies to nonsmokers and people who eat a healthy diet and maintain a normal weight.
In rare scenarios low blood pressure can be associated with serious conditions.
Categories of Low Blood Pressure:
• Low blood pressure on standing up (orthostatic, or postural, hypotension). Defined a sudden drop in blood pressure upon standing up from a sitting position or after lying down.
Normally, gravity causes blood to pool in the legs with upright posture. The body compensates for this by increasing heart rate and constricting (narrowing) blood vessels to ensure that enough blood returns to the brain. Individuals who have orthostatic hypotension do not have an adequate compensating mechanism. As a result, blood pressure falls, leading to dizziness, lightheadedness, blurred vision and even fainting.
Orthostatic hypotension can be due to dehydration, prolonged bed rest, pregnancy, diabetes, heart problems, burns, excessive heat, large varicose veins and certain neurological disorders.
Some medications also can cause orthostatic hypotension, particularly drugs used to treat high blood pressure — diuretics, beta blockers, calcium channel blockers and angiotensin-converting enzyme (ACE) inhibitors — as well as antidepressants and drugs used to treat Parkinson's disease and erectile dysfunction.
• Low blood pressure after eating (postprandial hypotension). Postprandial hypotension is a sudden drop in blood pressure after eating. It affects mostly older adults.
It is explained by the hypothesis that says that after meals, a large amount of blood flows to the digestive system; this is coupled with an increased pulse and constriction of certain blood vessels to help maintain normal blood pressure. But in some people these mechanisms fail, leading to dizziness, faintness and falls.
Postprandial hypotension is more likely to affect people with high blood pressure or autonomic nervous system disorders.
• Low blood pressure from faulty brain signals (neurally mediated hypotension). This disorder causes blood pressure to drop after standing for long periods, leading to signs and symptoms such as dizziness, nausea and fainting.
The main population to be affected by this condition is young people; it is explained by a miscommunication between the heart and the brain: when standing up for extended periods, blood pressure falls as blood pools in the legs. Normally, the body responds by making adjustments to keep blood pressure within normal range. Nerves in the heart's left ventricle of individuals with neurally mediated hypotension paradoxically signal the brain that blood pressure is too high, rather than too low.
As a result, the brain lowers the pulse, decreasing blood pressure even further. This causes more blood to pool in the legs and less blood to reach the brain, leading to lightheadedness and fainting.
Common medical conditions that can cause low blood pressure include:
• Dehydration. This is when the body loses more water than it takes in. Conditions associated with fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can all lead to dehydration. Initially, the heart responds by increasing blood pressure to meet the needs of the rest of the body, but if fluid replenishment does not occur properly, blood pressure eventually falls.
• Pregnancy. Pregnancy is associated with blood vessel dilation (expansion), so blood pressure usually drops with conception.
• Heart problems. Some heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), heart valve problems, heart attack and heart failure.
• Blood loss. Losing a significant amount blood from a major injury or internal bleeding leads to a major drop in blood pressure.
• Severe infection (septicemia). Septicemia occurs when an infection invades the entire body after having been confined to one area. This condition can lead to a life-threatening drop in blood pressure called septic shock.
• Severe allergic reaction (anaphylaxis). Anaphylaxis is a severe form of allergic reaction and is potentially life-threatening.
• Nutrient deficiency. Deficiency the vitamins B-12 and folate can cause a condition in which your body doesn't produce enough red blood cells (anemia), causing low blood pressure.
Some medications you may take can also cause low blood pressure, including:
• Diuretics (water pills), such as furosemide (Lasix) and hydrochlorothiazide (Microzide)
• Alpha blockers, such as labetalol
• Beta blockers, such as atenolol, propranolol (Inderal)
• Drugs for Parkinson's disease,
• Certain types of antidepressants (tricyclic antidepressants),
• Sildenafil (Viagra) or tadalafil (Cialis), particularly in combination with the heart medication nitroglycerin