Of 20 patients with idiopathic orthostatic hypotension (IOH), four manifested marked spontaneous fluctuations in arterial pressure when in the supine position (supine blood pressure). Blood pressure (intra-arterial recording) varied from 20 to 80 per cent between hemodynamic studies. The only significant difference between the lowest and highest pressure was in total peripheral resistance, which was 32 and 49 U · m2 respectively (P <0.01). In these patients, changes in blood pressure did not correlate with changes in blood volume, plasma renin activity, urinary sodium excretion or the aldosterone excretion rate. It is concluded that spontaneous changes in total peripheral resistance account for the marked variations in supine blood pressure in some patients with idiopathic orthostatic hypotension. The spontaneous vasoconstriction in the supine position is contrasted to the lack of vasoconstriction with upright tilt or standing in the same patients. The possible mechanism and clinical implications of the spontaneous alterations in total peripheral resistance in some patients with idiopathic orthostatic hypotension are briefly discussed.
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