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

• Published in: Journal of applied physiology (1985) Vol. 107; no. 2; pp. 506 - 517
• Main Authors: Thomas, Kate N, Cotter, James D, Galvin, Sean D, Williams, Michael J. A, Willie, Chris K, Ainslie, Philip N
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Am Physiological Soc 01-08-2009; American Physiological Society
• Subjects: Adaptation, Physiological; Adult; Biological and medical sciences; Blood Flow Velocity; Blood Pressure; Carbon dioxide; Cardiovascular disease; Cerebrovascular Circulation; Female; Fundamental and applied biological sciences. Psychology; Hemodynamics; Humans; Hyperventilation - complications; Hyperventilation - physiopathology; Hypotension, Orthostatic - complications; Hypotension, Orthostatic - physiopathology; Lower Body Negative Pressure; Male; Middle Cerebral Artery - physiopathology; Nausea; Oxygen; Oxygen Consumption; Posture; Pulsatile Flow; Severity of Illness Index; Spectroscopy, Near-Infrared; Syncope - etiology; Syncope - physiopathology; Tilt-Table Test; Time Factors; Vasoconstriction; Young Adult; Human; cerebral hypoperfusion; circulatory collapse; Nervous system diseases; Consciousness impairment; Central nervous system; Carbon dioxide; Cardiovascular disease; Encephalon; Postural hypotension; Vertebrata; Mammalia; Syncope; Arterial hypotension; Hypocapnia; Diseases of the autonomic nervous system; Neurological disorder
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.91650.2008

1 Department of Physiology, 2 School of Physical Education, and 3 Department of Medical and Surgical Sciences, University of Otago, Dunedin, New Zealand; and 4 Department 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 P CO 2 , 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 CO 2 was greater (–7 ± 6 vs. –4 ± 3 mmHg) and was related to the decline in MCAv ( R 2 = 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
Address for reprint requests and other correspondence: P. N. Ainslie, Dept. of Human Kinetics, Faculty of Health and Social Development, Univ. of British Columbia Okanagan, Kelowna, Canada (e-mail: philip.ainslie{at}ubc.ca )