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Clarify What Low Blood Pressure Is

May 22, 2008
Low blood pressure is a difficult clinical finding for a healthcare provider to address. While high blood pressure is known as the "silent killer," because it is associated with few acute symptoms, hypotension (hypo=low + tension=pressure) may be normal for a patient if it is without symptoms, but can be of great importance if it is associated with abnormal body function.

Sometimes low is good, a goal to be achieved in keeping blood pressure under control. Sometimes low is bad because there is not enough pressure to provide blood flow to the organs of the body.

For low blood pressure to be a problem there needs to be a symptom associated with that low number. Readings below 120/80 may be normal depending upon the clinical situation. Many people have systolic blood pressures below 100, but some people develop symptoms with pressures that low. Symptoms of low blood pressure occur because one or more of the body's organs is not getting enough blood supply.

If low blood pressure is the normal state for a patient, then there will be no symptoms. If low blood pressure is symptomatic, then the patient may feel lightheaded, dizzy and weak, short of breath, or have chest pain. The symptoms will depend upon which organ in the body is lacking adequate blood flow.

Blood pressure readings have two parts and are expressed as a ratio. "Normal" blood pressure, for example is 120/80 (120 over 80) and measures the pressure within the arteries of the body. Systolic pressure, the upper number, measures the pressure within the arteries when the heart is contracting (systole) to pump blood to the body. Diastole pressure, the lower number, measures resting pressures within the arteries, when the heart is at rest.

Normal blood pressure depends on many factors including age and body size. Infants and children have lower normal readings than adults. Smaller or petite patients may have lower normal blood pressure ranges.

According to American Heart Association guidelines, any reading greater than 120/80 is considered pre-hypertension or early high blood pressure.

You can think of the heart and the blood vessels (arteries and veins) as a system to pump blood, just like the oil pump in your car. Oil is pumped through rigid tubes. Pressure remains relatively constant throughout the pumping cycle unless the pump fails or there is an oil leak. Then oil pressure will fall.

The body is similar, except that the tubes have pliable walls, meaning that the space within the arteries can get bigger or smaller. If the space gets bigger, there is effectively less fluid, and pressure falls. If the space gets smaller, pressure goes up. Arteries have layers of muscles within their walls that can contract and narrow the artery, making less space inside the vessels.

Alternatively, the muscles can relax and dilate the artery, making more room. These muscles are under the control of the autonomic nervous system, the body's automatic system that makes adjustments for moment-to-moment changes in the relationship of the body to the world. The autonomic nervous system has two pathways that balance each other.

The sympathetic nervous system uses adrenaline (epinephrine) to cause the muscles to contract (sympathetic tone). The nerves that help with this control are located in the sympathetic trunk, which is a group of nerves that runs alongside the spinal column. The parasympathetic system uses acetylcholine to make muscles in the blood vessel walls relax via the vagus nerve.

As an example, when you stand up, the blood vessels have to narrow just a little to cause a slight increase in blood pressure, so that blood can travel uphill to the brain. Without that change, you might feel lightheaded or pass out.

If low blood pressure causes clinical symptoms, the cause will be in one of three general categories. Either the heart is not pumping with enough pressure, the artery walls are too dilated, or there is not enough intravascular fluid (intra=within + vascular= blood vessels) within the system.

The heart is an electrical pump. Problems with either the pump or the electricity can cause problems with low blood pressure.

The valves of the heart allow blood to flow in only one direction. If a valve fails, blood can regurgitate backwards, minimizing the amount that will flow out to the body. If a valve becomes narrowed (stenotic), then blood flow may be decreased. Both situations may cause hypotension.

If the heart beats too quickly, blood pressure may fall because there isn't enough time for the heart to refill in between each beat (diastole). If the heart beats too slowly, there may be too much time spent in diastole when blood is not flowing.

If the heart muscle has been damaged or irritated, there may not be enough pumping force to maintain blood pressure. In heart attack (myocardial infarction), enough heart muscle may be stunned so that the heart is too weak to pump effectively.

Diuretic medications [for example, hydrochlorothiazide (Hydrodiuril), furosemide (Lasix)] are used to control blood pressure by causing the kidneys to make more urine and decreasing the intravascular volume. If the patient loses too much water and becomes dehydrated, low blood pressure may result.

Beta blockers and calcium channel blockers are two commonly prescribed medications used for the treatment of high blood pressure. They may cause the heart to beat too slowly and thus cause hypotension. Any heart medication needs to be monitored by a healthcare provider to evaluate the body's response and to select the appropriate dose.

Medications like sildenafil (Viagra) in combination with nitroglycerin may cause blood vessel dilation and low blood pressure.

Normal physiologic changes in pregnancy increase the intravascular space, especially in the first two trimesters, and can cause lower blood pressure. One of the complications of diabetes is damage to the nerves in the body, including those in the autonomic nervous system.

In those people with diabetes who have autonomic dysfunction, orthostatic hypotension can occur. The blood vessels are unable to adjust to quick positional changes.

If low blood pressure causes lack of blood flow to the organs of the body, then those organs will start to fail. This may result in stroke, heart attack, kidney failure, and bowel ischemia (decreased blood supply to the small and large intestine). Shock and death are the end result of prolonged low blood pressure.
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