Circulatory Control in Idiopathic Orthostatic Hypotension (Shy-Drager Syndrome)

Abstract

Of three patients with Shy-Drager syndrome, two presented with syncope related to exercise and, in all three, exercise stress clearly defined the element of orthostatic hypotension. Autonomic cardiovascular regulatory function was absent in two, but some residual autonomic function was present in the form of heart rate response in one patient who had milder postural hypotension. Hemodynamic studies were carried out with subjects supine and tilted 45° before and after exercise, following plasma volume expansion, and during the effects of isoproterenol and atropine. Despite disordered venoconstrictor responses demonstrated during exercise, hyperventilation, and after the Valsalva maneuver, decrease in central blood volume and right atrial pressure were not excessive with tilt, and total systemic vascular resistance remained essentially unchanged as did heart rate. Cardiac index may have decreased excessively (–24%), suggesting a cardiac component in production of hypotension. Absence of sympathetic cardiac effects, both chronotropic and inotropic, may play a significant role in orthostatic hypotension by limiting heart rate and preventing shift of the Frank-Starling curve to the left. Amelioration of postural effects by plasma volume expansion was noted along with increased cardiac output, stroke volume, and pulse pressure indicating that myocardial heterometric regulation was operative. Atropine had little effect, eliminating the possibility that parasympathetic depressor effects were responsible for postural hypotension. Exercise, by decreasing systemic vascular resistance even though venous return was adequate, produced hypotension even in recumbency and aggravated postural effects. Similarly, isoproterenol caused marked hypotension in supine subjects. Exercise, by reducing systemic vascular resistance, has a deleterious effect despite increased venous return and may precipitate postural symptoms. Exercise stress is of value in the evaluation of patients with syncope or cerebral ischemia secondary to autonomic dysfunction where orthostatic hypotension is of doubtful significance at rest.