title: Muscle oxygen saturation increases during head-up tilt-induced (pre)syncope.
abstract: OBJECTIVE
  To evaluate whether muscle vasodilatation plays a role for hypotension developed during central hypovolaemia, muscle oxygenation (SmO2) was examined during (pre)syncope induced by head-up tilt (HUT). Skin blood flow (SkBF) and oxygenation (SskinO2) were determined because evaluation of SmO2may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (ScO2) and middle cerebral artery mean blood flow velocity (MCAvmean).
  
  
  METHODS
  Twenty healthy male volunteers (median age 24 years; range 19-38) were subjected to passive 50° HUT for 1 h or until (pre)syncope. ScO2and SmO2(near-infrared spectroscopy), MCAvmean(transcranial Doppler) along with mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR) (Modelflow®) were determined.
  
  
  RESULTS
  (Pre)syncopal symptoms appeared in 17 subjects after 11 min (median; range 2-34) accompanied by a decrease in MAP, SV, CO and TPR, while HR remained elevated. During (pre)syncope, ScO2decreased [73% (71-76; mean and 95% CI) to 68% (65-71), P < 0.0001] along with MCAvmean[40 (37-43) to 32 (29-35) cm s-1, P < 0.0001]. In contrast, SmO2increased [63 (56-69)% to 71% (65-78), P < 0.0001], while SskinO2[64% (58-69) to 53% (47-58), P < 0.0001] and SkBF [71 (44-98) compared to a baseline of 99 (72-125) units, P = 0.020] were reduced.
  
  
  CONCLUSIONS
  We confirm that the decrease in MAP during HUT is associated with a reduction in indices of cerebral perfusion. (Pre)syncope was associated with an increase in SmO2despite reduced SskinO2and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT.
authors.affiliation: Department of Neuroanaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: A
authors.initials: A
authors.lastname: Lund
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: H
authors.initials: H
authors.lastname: Sørensen
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: T W
authors.initials: TW
authors.lastname: Jensen
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: M J
authors.initials: MJ
authors.lastname: Niemann
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.|Department of Neuroscience and Pharmacology, University of Copenhagen, Copenhagen, Denmark.
authors.forename: N D
authors.initials: ND
authors.lastname: Olesen
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: H B
authors.initials: HB
authors.lastname: Nielsen
authors.affiliation: Department of Neuroanaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.|Department of Neuroscience and Pharmacology, University of Copenhagen, Copenhagen, Denmark.
authors.forename: N V
authors.initials: NV
authors.lastname: Olsen
authors.affiliation: The Copenhagen Muscle Research Centre, Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.
authors.forename: N H
authors.initials: NH
authors.lastname: Secher
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D020896:Hypovolemia / Q000503:physiopathology*; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply*; D010100:Oxygen / Q000097:blood; D011187:Posture; D013575:Syncope / Q000503:physiopathology*; D014655:Vascular Resistance / Q000502:physiology; D014664:Vasodilation / Q000502:physiology; D055815:Young Adult
chemical_list: D010100:Oxygen
keywords: cerebral blood flow; head-up tilt; muscle oxygenation; near-infrared spectroscopy; skin blood flow; syncope