title: Aortic Wave Reflection During Orthostatic Challenges: Influence of Body Position and Venous Pooling.
abstract: BACKGROUND
  Aortic wave reflection (augmentation index; AIx) decreases during orthostatic challenges despite increased peripheral resistance, which is thought to be due to venous pooling. The purpose of this study was to examine if the decrease in AIx during an orthostatic challenge is due to venous pooling alone or body position manipulation.
  
  
  METHODS
  Twenty-three young, healthy adults (11F/12M) participated in 3 separate orthostatic challenges (5 minutes each); 60° head-up tilt (HUT), 60° HUT with bilateral rhythmic blood pressure (BP) cuff inflation on calves (75 mm Hg) to minimize venous pooling, and lower body negative pressure (LBNP; -30 mm Hg) for venous pooling independent of body position. High-fidelity radial artery pressure waveforms using applanation tonometry were recorded at minutes 2:30 and 5:00 during each condition. Aortic BP and wave reflection were analyzed from a synthesized aortic BP waveform.
  
  
  RESULTS
  Compared to resting (baseline) measurements, AIx did not significantly decrease at minutes 2:30 or 5:00 of HUT conditions (HUT 0 ± 2% vs. -3 ± 3%, 0 ± 2%; HUT w/cuffs 0 ± 2% vs. -4 ± 2%, 0 ± 2%). Conversely, LBNP substantially reduced AIx at minutes 2:30 and 5:00 (1 ± 2% vs. -15 ± 2% and -12 ± 2%; P < 0.01). When standardized to heart rate (AIx@75), AIx@75 increased relative to baseline during HUT conditions (P < 0.05).
  
  
  CONCLUSIONS
  In contrast to previous studies, AIx did not decrease during passive HUT, yet decreased substantially during LBNP. Despite being well matched for peripheral hemodynamics, it appears that LBNP elicits a greater effect on central hemodynamics, relative to passive HUT. Collectively, changes in body position alone do not explain differences in AIx during orthostatic conditions.
authors.affiliation: Department of Physical Therapy and Rehabilitation Science, Carver College of Medicine, University of Iowa, Iowa City, Iowa, USA.
authors.forename: William E
authors.initials: WE
authors.lastname: Hughes
authors.forename: Darren P
authors.initials: DP
authors.lastname: Casey
mesh_terms: D000328:Adult; D001011:Aorta / Q000503:physiopathology*; D062186:Arterial Pressure / Q000502:physiology*; D004562:Electrocardiography; D015444:Exercise / Q000502:physiology; D005260:Female; D005500:Follow-Up Studies; D064368:Healthy Volunteers; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006896:Hydroxyethylrutoside; D006973:Hypertension / Q000503:physiopathology*; D008150:Lower Body Negative Pressure / Q000379:methods*; D008297:Male; D011187:Posture / Q000502:physiology*; D063177:Pulse Wave Analysis / Q000379:methods*; D012146:Rest; D059289:Vascular Stiffness / Q000502:physiology*; D055815:Young Adult
chemical_list: D006896:Hydroxyethylrutoside
keywords: aortic wave reflection; augmentation index; blood pressure; hypertension; orthostatic challenge; total peripheral resistance.