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Hypotension is defined as systolic blood pressure <90mHg—or 40mmHg less than the patient's baseline blood pressure—in the absence of other reasons for hypotension.(1) One of the most disabling features of autonomic dysfunction is orthostatic hypotension. Criteria for this diagnosis include a postural decrease from the supine to the standing position of at least 20mmHg in systolic, or 10mmHg in diastolic BP sustained for at least three minutes; the latter criterion differentiates autonomic failure from sluggish baroreflex responses that are common in the elderly. Regulation of homeostatic functions is accomplished by the autonomic nervous system (ANS). An extensive peripheral innervation network combined with central vigilance provides rapid adjustments in vital physiologic mechanisms that are critical to survival.
The maintenance of arterial blood pressure in the standing position depends upon several factors; an adequate blood volume, unrestricted venous return, and an intact sympathetic nervous system. Significant postural hypotension may be a reflection of extracellular volume depletion or dysfunction of circulatory reflexes. Examples include diseases of the nervous system such as tabes dorsalis, syringomyelia, or diabetes mellitus, which may disrupt sympathetic reflexes and result in orthostatic hypotension.
The term idiopathic orthostatic hypotension refers to a group of degenerative diseases involving either pre- or postganglionic sympathetic neurons. Postural (orthostatic) hypotension with syncope occurs in patients who have a clinical defect in, or variable instability of, vasomotor reflexes. The fall in blood pressure upon assumption of an upright position is due to a loss of vasoconstriction reflexes in resistance and capacitance vessels of the lower extremities. Sudden arising from a recumbent position or standing quietly are precipitating circumstances. Postural hypotension can be a very disabling disorder. Patients often lose their source of livelihood as a result, and often home health support for patients and psychological help for caregivers is essential.
When evaluating a patient with hypotension, an adequate review of the patient's medications should be included. A number of medications may cause postural hypotension, including diuretics, antihypertensives, antidepressants, phenothiazines, ethanol, narcotics, insulin, barbiturates, and beta-adrenergic and calcium channel blockers. While these medications may cause hypotension, it is important to remember that exaggerated responses to drugs may be the first sign of an underlying autonomic disorder.
Postprandial hypotension occurs as blood is shunted to the splanchnic circulation after a meal. The importance of this process may have been underestimated, particularly in the healthy elderly population, hypertensive patients, and elderly patients in the nursing homes. The association between hypotension and diabetes, Parkinson's disease, renal failure and dialysis, cardiovascular disease, paraplegia, and autonomic failure has long been well accepted, however, the wisdom of administering cardiovascular medications that have hypotensive effects at meals should be reassessed.
The incidence of symptomatic hypotension during or immediately following dialysis ranges from 15% to 50%. The cause is usually multifactorial and includes ingestion of antihypertensive medications or food prior to or during dialysis, severe hypocalcemia, and high dialysate magnesium concentrations.
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