When a person assumes an upright posture, blood is shifted to the lower part of the body and cardiac output drops. When a drop in cardiac output or blood pressure occurs in a healthy individual, the autonomic nervous system responds with an increase in sympathetic output and a decrease in parasympathetic output. This autonomic nervous system reflex produces an increase in heart rate and peripheral vascular resistance, leading to an increase in both cardiac output and blood pressure, thereby allowing the individual to maintain an upright posture. If the autonomic response is insufficient to counter the drop in cardiac output, decreased cerebral perfusion and syncope may follow if the person remains upright. Symptom onset is usually within the first 3 min after assuming the upright posture, but may be more delayed in some patients. Symptoms are characteristic of decreased cerebral perfusion, with blurred vision, dizziness, and tunnel vision. Orthostatic hypotension is defined as a fall in systolic blood pressure of greater than 20 mmHg upon assuming the upright posture. Caution must be taken in diagnosing a patient with orthostatic syncope based on orthostatic blood pressure measurements alone, because 5 to 55 percent of patients with other causes of syncope have orthostatic hypotension on physical examination.5 To establish orthostasis as the cause of syncope, the patient should have recurrence of syncopal symptoms on orthostatic testing.
Serious orthostatic symptoms may be due to autonomic dysfunction, with failure of vasoconstriction during orthostatic stress. This may be a primary disease process or secondary to peripheral neuropathy, certain medications such as ganglionic blockers, spinal cord injury, and a variety of other neurologic diseases. More commonly, orthostatic symptoms are due to other conditions that lead to volume depletion, including gastrointestinal losses, bleeding, and diuresis. Medications commonly contribute to orthostatic syncope by blunting the chronotropic response of the heart to orthostatic stress or by leading to relative or absolute volume depletion. The elderly are more susceptible to orthostatic hypotension for many reasons, including use of diuretics and b-blocking agents, varying degrees of autonomic dysfunction, and other changes related to aging.
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