Orthostatic Hypotension (cont.)
What causes orthostatic hypotension?
Orthostatic hypotension has many potential causes, some affecting only one
part of the system that supplies blood to the brain, and others affecting two or
three.
Loss of fluid within the blood vessels is the most common reason to develop
the symptoms of orthostatic hypotension. The fluid may be water or blood
depending upon the cause.
- Dehydration occurs when fluid intake cannot match the
amount of fluid lost by the body.
Vomiting,
diarrhea,
fever, and
heat-related illnesses (for example,
heat exhaustion or
heat stroke) are common reasons
a person loses a significant amount of
fluid. Diuretics or water pills used to control
high blood pressure are also another cause of a decreased amount
of fluid in the body.
- Blood loss and other causes of
anemia decrease the number of
red blood cells that
carry oxygen in the bloodstream, and this may lead to the symptoms of
orthostatic hypotension. The bleeding may arise from one large event or may
occur slowly over a period of time. With slow bleeding, the body may be able to
compensate, replacing the lost volume of red blood cells with water in the
bloodstream. However, after a while the loss of oxygen-carrying capacity of the
blood will cause symptoms to develop. In addition to lightheadedness, there may
be weakness,
shortness of breath, or
chest pain.
- Medications that affect the autonomic nervous system may also cause
orthostatic hypotension.
- Beta blocker
medications such as
metoprolol (Inderal) block the
beta-adrenergic receptors in the body, preventing the heart from speeding up,
preventing the heart from contracting as forcefully, and
dilating blood vessels.
All three of these effects affect the ability of the body to react to position
changes. Aside from high blood pressure and
heart disease, these medications are
also used for headache control and
anxiety prevention.
- sildenafil (Viagra),
vardenafil (Levitra), and
tadalafil (Cialis)
dilate blood
vessels, and this class of medication may cause orthostatic hypotension. The
effect can be magnified if taken with nitrates, medications used to treat angina [for
example, nitroglycerin
(Nitrostat, Nitroquick, Nitrolingual, Nitro-Dur, Minitran, Nitro-Bid and others),
isosorbide mononitrate (Imdur,
Ismo, Monoket)], alcohol, or narcotic pain medications.
- Other medications used for high blood pressure control may be a potential
cause of orthostatic hypotension, even if taken as prescribed.
- Orthostatic hypotension is a side effect of many
psychiatric
medications, including tricyclic
antidepressants [amitriptyline
(Endep, Elavil), nortriptyline
(Pamelor, Aventyl),
phenothiazines (Thorazine, Mellaril, Compazine), and MAO inhibitors (Nardil,
Parnate)
A vasovagal episode is a condition that may occur when a
stimulus causes excess activation of the parasympathetic system, slowing the
heart rate and dilating blood vessels. Symptoms of lightheadedness or fainting
then occur due to the lowering of blood pressure and decrease in blood flow to
the brain. The stimulus may be pain from an injury such as a broken bone, or there may be a
psychologic trigger, such as a
medical student watching a first operation. The
vagus nerve that causes
this response may also in some cases be triggered by urinating (micturition
syncope) or by pushing hard to have a bowel movement.
Patients with diabetes may develop peripheral neuropathy
that can affect
nerves of the autonomic nervous system, and as a result, may develop orthostatic
hypotension. As well, patients with poorly-controlled diabetes have the
potential of becoming dehydrated.
Some patients may develop post-prandial lightheadedness,
meaning that symptoms occur after a heavy meal. In this case, the body dilates
blood vessels to the stomach and intestine to help with digestion, leaving less blood
available to flow to the brain.
Addison's Disease, or adrenal insufficiency, may also be associated with the
inability of the body to compensate for position change.
There are a variety of reasons that heart disease may
cause orthostatic hypotension. Abnormalities of the electrical conduction system
including heart rates that are too slow or too fast may cause changes in blood
pressure. Patients with heart valve disorders,
heart failure, and heart attack may all experience
orthostatic hypotension.
Disorders of the nervous system may also cause
orthostatic hypotension. Examples include
Parkinsonism, amyloidosis, and Shy-Drager Syndrome
(or multiple system atrophy).
Postural orthostatic tachycardia syndrome describes the
feeling of lightheadedness, nausea, fatigue, and weakness associated with an
elevated heart rate (greater than 120 beats per minute) that begins within 10
minutes of a heads-up tilt table test. It is
often seen in younger females between the ages of 12 and 50, and there may be a
relationship with chronic fatigue
syndrome.
Next: What are the risk factors for orthostatic hypotension? »
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