Mechanisms underlying the impairment in orthostatic tolerance after nocturnal recumbency in patients with autonomic failure

In the present study, we have assessed in patients with neurogenic orthostatic hypotension the haemodynamics underlying the reduced tolerance to standing after prolonged recumbency at night. In 10 patients with neurogenic orthostatic hypotension (age 33-68 years), of which seven were being treated with fludrocortisone and/or sleeping in the 12° head-up tilt position, 24h continuous non-invasive finger blood pressure was recorded by a Portapres device. Beat-to-beat blood pressure, heart rate, stroke volume, cardiac output and total peripheral vascular resistance obtained by pulse contour analysis were assessed during 5min of standing in the evening (at 22.30 hours) and in the morning (at 06.30hours). On average, the inverse of the normal 24h blood pressure profile was found, with a large diversity in blood pressure profiles among patients. Supine blood pressure values were similar, but standing blood pressure values were lower in the morning than in the evening (P < 0.01). This resulted from larger falls in stroke volume and cardiac output upon standing in the morning compared with the evening, while total peripheral resistance did not change. There was no relationship between the decrease in body weight during the night (mean 0.9kg; range 0.2-1.6kg) and the evening-morning difference in standing blood pressure. We conclude that, in patients with neurogenic orthostatic hypotension, the impaired tolerance to standing in the morning is due to larger falls in stroke volume and cardiac output. Not only nocturnal polyuria, but also a redistribution of body fluid, are likely mechanisms underlying the pronounced decreases in stroke volume and cardiac output after prolonged recumbency at night.