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Initial orthostatic hypotension is unrelated to orthostatic tolerance in healthy young subjects

¹Department of Physiology, ²School of Physical Education, and ³Department of Medical and Surgical Sciences, University of Otago, Dunedin, New Zealand; and ⁴Department of Human Kinetics, Faculty of Health and Social Development, University of British Columbia Okanagan, Kelowna, British Columbia, Canada

Submitted 28 December 2008 ; accepted in final form 11 June 2009

The physiological challenge of standing upright is evidenced by temporary symptoms of light-headedness, dizziness, and nausea. It is not known, however, if initial orthostatic hypotension (IOH) and related symptoms associated with standing are related to the occurrence of syncope. Since IOH reflects immediate and temporary adjustments compared with the sustained adjustments during orthostatic stress, we anticipated that the severity of IOH would be unrelated to syncope. Following a standardized period of supine rest, healthy volunteers [n = 46; 25 ± 5 yr old (mean ± SD)] were instructed to stand upright for 3 min, followed by 60° head-up tilt with lower-body negative pressure in 5-min increments of –10 mmHg, until presyncope. Beat-to-beat blood pressure (radial arterial or Finometer), middle cerebral artery blood velocity (MCAv), end-tidal PCO₂, 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 ± 11 vs. 43 ± 12%; 36 ± 18 vs. 35 ± 13%; and 6 ± 5 vs. 4 ± 2%, respectively), the reduction in end-tidal CO₂ was greater (–7 ± 6 vs. –4 ± 3 mmHg) and was related to the decline in MCAv (R² = 0.4; P < 0.05). While MCAv pulsatility was elevated with IOH, it was reduced at presyncope (P < 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.

cerebral hypoperfusion; syncope; hypocapnia; circulatory collapse