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Following a standardized period of supine rest, healthy volunteers [ n = 46; 25 \u00b1 5 yr old (mean \u00b1 SD)] were instructed to stand upright for 3 min, followed by 60\u00b0 head-up tilt with lower-body negative pressure in 5-min increments of \u221210 mmHg, until presyncope. Beat-to-beat blood pressure (radial arterial or Finometer), middle cerebral artery blood velocity (MCAv), end-tidal Pco<jats:sub>2<\/jats:sub>, and cerebral oxygenation (near-infrared spectroscopy) were recorded continuously. At presyncope, although the reductions in mean arterial pressure, MCAv, and cerebral oxygenation were similar to those during IOH (40 \u00b1 11 vs. 43 \u00b1 12%; 36 \u00b1 18 vs. 35 \u00b1 13%; and 6 \u00b1 5 vs. 4 \u00b1 2%, respectively), the reduction in end-tidal CO<jats:sub>2<\/jats:sub>was greater (\u22127 \u00b1 6 vs. \u22124 \u00b1 3 mmHg) and was related to the decline in MCAv ( R<jats:sup>2<\/jats:sup>= 0.4; P &lt; 0.05). While MCAv pulsatility was elevated with IOH, it was reduced at presyncope ( P &lt; 0.05). The cardiorespiratory and cerebrovascular changes during IOH were unrelated to those at presyncope, and interestingly, there was no relationship between the hemodynamic changes and the incidence of subjective symptoms in either scenario. During IOH, the transient nature of physiological changes can be well tolerated; however, potentially mediated by a reduced MCAv pulsatility and greater degree of hypocapnic-induced cerebral vasoconstriction, when comparable changes are sustained, the development of syncope is imminent.<\/jats:p>","DOI":"10.1152\/japplphysiol.91650.2008","type":"journal-article","created":{"date-parts":[[2009,6,19]],"date-time":"2009-06-19T00:39:10Z","timestamp":1245371950000},"page":"506-517","source":"Crossref","is-referenced-by-count":74,"title":["Initial orthostatic hypotension is unrelated to orthostatic tolerance in healthy young subjects"],"prefix":"10.1152","volume":"107","author":[{"given":"Kate N.","family":"Thomas","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"James D.","family":"Cotter","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sean D.","family":"Galvin","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michael J. 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