title: Cerebrovascular responses during lower body negative pressure-induced presyncope.
abstract: BACKGROUND
  Reduced orthostatic tolerance is commonly observed after spaceflight, occasionally causing presyncopal symptoms which may be due to low cerebral blood flow (CBF). It has been suggested that CBF decreases in early stages of exposure to orthostatic stress. The purpose of this study was to investigate cerebrovascular responses during presyncope induced by lower body negative pressure (LBNP).
  
  
  OBJECTIVE
  Although CBF decreases during LBNP exposure, blood pressure (BP) or heart rate (HR) contributes more to induce presyncopal conditions.
  
  
  METHODS
  Eight healthy male volunteers were exposed to LBNP in steps of 10 mm Hg every 3 min until presyncopal symptoms were detected. Electrocardiogram (ECG) was monitored continuously and arterial BP was measured by arterial tonometry. CBF velocity at the middle cerebral artery was measured by transcranial Doppler sonography (TCD). Cerebral tissue oxygenation was detected using near-infrared spectroscopy (NIRS). We focused our investigation on the data obtained during the final 2 min before the presyncopal endpoint.
  
  
  RESULTS
  BP gradually decreased from 2 min to 10 s before the endpoint, and fell more rapidly during the final 10 s. HR did not change significantly during presyncope. CBF velocity did not change significantly, while cerebral tissue oxygenation decreased prior to the presyncopal endpoint in concert with BP. Our results suggest that CBF is maintained in the middle cerebral artery during presyncope, while BP decreases rapidly.
  
  
  CONCLUSIONS
  Cerebrovascular hemodynamics are relatively well maintained while arterial hypotension occurs just prior to syncope.
authors.affiliation: Department of Health Administration, School of Health Sciences and Nursing, Faculty of Medicine, The University of Tokyo, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Kuriyama
authors.forename: T
authors.initials: T
authors.lastname: Ueno
authors.forename: R E
authors.initials: RE
authors.lastname: Ballard
authors.forename: P S
authors.initials: PS
authors.lastname: Cowings
authors.forename: W B
authors.initials: WB
authors.lastname: Toscano
authors.forename: D E
authors.initials: DE
authors.lastname: Watenpaugh
authors.forename: A R
authors.initials: AR
authors.lastname: Hargens
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001783:Blood Flow Velocity; D002560:Cerebrovascular Circulation; D006439:Hemodynamics; D006801:Humans; D007024:Hypotension, Orthostatic / Q000150:complications / Q000503:physiopathology*; D008150:Lower Body Negative Pressure / Q000009:adverse effects*; D008297:Male; D008875:Middle Aged; D019265:Spectroscopy, Near-Infrared; D013575:Syncope / Q000209:etiology*; D013997:Time Factors; D017585:Ultrasonography, Doppler, Transcranial