LOW BLOOD PRRESSURE (ORTHOSTATIC HYPOTENSION) - An excessive fall in BP (typically > 20/10 mm Hg) on assuming the upright posture. Orthostatic hypotension is not a specific disease but rather a manifestation of abnormal BP regulation due to various causes.
Etiology and Pathophysiology - The gravitational stress of sudden standing normally causes pooling of blood in the venous capacitance vessels of the legs and trunk. The subsequent transient decrease in venous return and cardiac output results in reduced BP. Baroreceptors in the aortic arch and carotid bodies activate autonomic reflexes that rapidly normalize BP by causing a transient tachycardia. These changes reflect primarily the sympathetic mediated increase in catecholamine levels, which augments vasomotor tone of the capacitance vessels, increases heart rate and myocardial contractility, and thereby enhances cardiac output; arterial and venous vasoconstriction are mediated by similar mechanisms. Vagal inhibition also increases the heart rate. With continued standing, ADH secretion and activation of the renin-angiotensin-aldosterone system cause Na and water retention and expansion of the circulating blood volume. (See Figure: Physiologic Maintenance of Blood Pressure Upon Assumption of the Upright Posture.)
Hypovolemia is the most common cause of symptomatic orthostatic hypotension. Hypovolemia is often induced by excessive use of diuretics (eg, loop diuretics such as furosemide, bumetanide, and ethacrynic acid); relative hypovolemia is due to vasodilator therapy with nitrate preparations and Ca blockers (verapamil, nifedipine, diltiazem, amlodipine) or with ACE inhibitors. The hypovolemia and diminished vasomotor tone caused by protracted bed rest are also often a cause of orthostatic hypotension. Orthostatic hypotension is more frequent in diabetic than nondiabetic patients treated with antihypertensive drugs and also occurs secondary to vasodilation during febrile illnesses.
Drugs that impair autonomic reflex mechanisms and reduce BP on standing, eg, excessive doses of antihypertensive drugs (methyldopa, clonidine, reserpine, ganglionic blocking drugs) and multiple drug use, are also frequent causes. b-Adrenergic blockers are a rare cause, but a-adrenergic blockers such as prazosin may be causative, especially at the initiation of therapy (first-dose effect). Drugs that provoke postural hypotension should be initiated in small doses with gradual upward titration. Other drugs that reversibly impair autonomic reflexes and reduce BP on standing (an important adverse effect) include many of those used to treat psychiatric disorders, such as monoamine oxidase inhibitors (isocarboxazid, phenelzine, tranylcypromine) used to treat depression; tricyclic antidepressants (nortriptyline, amitriptyline, desipramine, imipramine, protriptyline) or tetracyclic antidepressants; and phenothiazine antipsychotic drugs (chlorpromazine, promazine, thioridazine). Quinidine, levodopa, barbiturates, and alcohol may also produce orthostatic hypotension. The antineoplastic drug vincristine may produce severe long-lasting orthostatic hypotension due to neurotoxicity.
Symptoms, Signs, and Diagnosis - Faintness, light-headedness, dizziness, confusion, or visual blurring is evidence of a mild to moderate reduction in cerebral blood flow. With more severe cerebral hypoperfusion, syncope or generalized seizures may supervene (see also Syncope, below). Exercise or a heavy meal may exacerbate symptoms. Other associated phenomena usually relate to the underlying cause. Orthostatic hypotension is diagnosed when symptoms suggestive of hypotension and a marked reduction in measured BP are provoked by standing and relieved by lying down. An underlying cause must be sought based on the patient's presenting circumstances and associated phenomena.
Prognosis and Treatment - Prognosis depends on the underlying cause. Orthostatic hypotension due to hypovolemia or drug excess is rapidly reversed by correcting these problems. Anemia and electrolyte imbalance can be specifically treated. The orthostasis of protracted bed rest can be lessened by having patients sit up each day. Elderly patients should maintain adequate fluid intake, limit or avoid alcohol, and exercise regularly when feasible. The outlook in patients with a chronic underlying disorder is determined by the management of that disease; eg, postural hypotension appears to indicate a poor prognosis in diabetic patients with hypertension.
|
|
|
