Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

U N I T 5

Aging. Weakness and dizziness on standing are common complaints of elderly persons. Orthostatic tolerance is usually well maintained until the age of 70 years, after which there is an increasing tendency toward arterial pressure instability and postural hypotension. Although orthostatic hypotension may be either systolic or dia- stolic, hypertensoin associated with aging seems more often to be systolic. 68 Several deficiencies in the circula- tory response may predispose the elderly to this prob- lem, including diminished ability to produce an adequate increase in the heart rate, ventricular stroke volume, or peripheral vascular resistance; decreased function of the skeletal muscle pumps; and decreased blood volume. Because cerebral blood flow primarily depends on sys- tolic pressure, persons with impaired cerebral circula- tion may experience symptoms of weakness, ataxia, dizziness, and syncope when their arterial pressure falls even slightly. This may occur in older persons who are immobilized for even brief periods or whose blood vol- ume is decreased owing to inadequate fluid intake or overzealous use of diuretics. Postprandial blood pressure often decreases in elderly persons. 68,69 The greatest postprandial changes occur after a high-carbohydrate meal. Although the mechanism responsible for these changes is not fully understood, it is thought to result from glucose-mediated impairment of baroreflex sensitivity and increased splanchnic blood flow mediated by insulin and vasoactive gastrointestinal hormones. Bed Rest and Immobility. Prolonged bed rest promotes a reduction in plasma volume, a decrease in venous tone, failure of peripheral vasoconstriction, and weakness of the skeletal muscles that support the veins and assist in returning blood to the heart. Physical deconditioning fol- lows even short periods of bed rest. After 3 to 4 days, the blood volume is decreased. Loss of vascular and skeletal muscle tone is less predictable but often becomes maxi- mal after approximately 2 weeks of bed rest. Orthostatic intolerance is a recognized problem of space flight—a potential risk after reentry into the earth’s gravitational field. Disorders of Autonomic Nervous System Function. The sympathetic nervous system plays an essential role in adjustment to the upright position. Sympathetic stimulation increases heart rate and cardiac contractil- ity and causes constriction of peripheral veins and arte- rioles. Orthostatic hypotension caused by altered ANS function is common in peripheral neuropathies associ- ated with diabetes mellitus, after injury or disease of the spinal cord, or as the result of a cerebral vascular accident in which sympathetic outflow from the brain stem is disrupted. The American Autonomic Society and the American Academy of Neurology have distin- guished three forms of primary ANS dysfunction: (1) pure autonomic failure, which is defined as a sporadic, idiopathic cause of persistent orthostatic hypotension and other manifestations of autonomic failure such as urinary retention, impotence, or decreased sweating; (2) Parkinson disease with autonomic failure; and (3)

Blood pressure returns to normal

Increased venous return

Increased cardiac output

Increased heart rate

Vasoconstriction

Baroreceptors

Compression of veins

Drop in blood pressure

Decreased cardiac output

Activation of skeletal muscle pumps

Decreased venous return to the heart

Pooling of blood in lower body

Skeletal muscle

Baroreceptor

Assumption of the upright position

output may be adequate when a person is in the recum- bent position, it often decreases to the point of caus- ing weakness and fainting when the person assumes the standing position. Common causes of orthostatic hypotension related to hypovolemia are excessive use of diuretics, excessive diaphoresis, loss of gastrointestinal fluids through vomiting and diarrhea, and loss of fluid volume associated with prolonged bed rest. Drug-Induced Hypotension. Antihypertensive drugs and psychotropic drugs are a common cause of chronic orthostatic hypotension. In most cases, the orthostatic hypotension is well tolerated. If postural hypotension is severe enough to cause light-headedness or dizziness, it is recommended that the dosage of the drug be reduced or a different drug be used. FIGURE 18-16. Skeletal muscle pump (blue) and baroreceptor mechanism (red) for blood pressure control on assumption of the upright posture.