title: Inspiratory resistance, cerebral blood flow velocity, and symptoms of acute hypotension.
abstract: BACKGROUND
  Symptoms of orthostatic intolerance, e.g., following prolonged bed rest and microgravity exposure, are associated with reductions in cerebral blood flow. We tested the hypothesis that spontaneously breathing through an impedance threshold device (ITD) would attenuate the fall in cerebral blood flow velocity (CBFV) during a hypotensive orthostatic challenge and reduce the severity of reported symptoms.
  
  
  METHODS
  While breathing through either an active ITD (-7 cm H2O inspiratory impedance) or a sham ITD (no impedance), 19 subjects performed a squat stand test (SST). Symptoms upon stand were recorded on a 5-point scale (1 = normal; 5 = faint) of subject-perceived rating (SPR). To address our hypothesis, only data from symptomatic subjects (SPR > 1 during the sham trial) were analyzed (N = 9). Mean arterial blood pressure (MAP) and mean CBFV were measured continuously throughout the SST and analyzed in time and frequency domains.
  
  
  RESULTS
  Breathing with the active ITD during the SST reduced the severity of orthostatic symptoms in eight of the nine symptomatic subjects (sham ITD SPR, 1.9 +/- 0.1; active ITD SPR, 1.1 +/- 0.1), but there was no statistically distinguishable difference in the reduction of mean CBFV between the two trials (sham ITD, -39 +/- 3% vs. active ITD, -44 +/- 3%). High frequency oscillations in mean CBFV, however, were greater during the active ITD trial (7.8 +/- 2.6 cm x s(-2)) compared with the sham ITD trial (2.5 +/- 0.9 cm x s(-2)).
  
  
  CONCLUSIONS
  Higher oscillations in CBFV while breathing with the active ITD may account for the reduction in symptom severity during orthostatic hypotension despite the same fall in absolute CBFV.
authors.affiliation: U.S. Army Institute of Surgical Research, Fort Sam Houston, TX 78234, USA. caroline.rickards@us.army.mil
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Rickards
authors.forename: Kenneth D
authors.initials: KD
authors.lastname: Cohen
authors.forename: Lindsey L
authors.initials: LL
authors.lastname: Bergeron
authors.forename: Lubrina
authors.initials: L
authors.lastname: Burton
authors.forename: Prateek J
authors.initials: PJ
authors.lastname: Khatri
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Lee
authors.forename: Kathy L
authors.initials: KL
authors.lastname: Ryan
authors.forename: William H
authors.initials: WH
authors.lastname: Cooke
authors.forename: Don F
authors.initials: DF
authors.lastname: Doerr
authors.forename: Keith G
authors.initials: KG
authors.lastname: Lurie
authors.forename: Victor A
authors.initials: VA
authors.lastname: Convertino
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006339:Heart Rate; D006801:Humans; D007024:Hypotension, Orthostatic / Q000503:physiopathology* / Q000517:prevention & control; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D012119:Respiration; D012138:Respiratory Therapy / Q000295:instrumentation / Q000379:methods*; D013575:Syncope; D014893:Weightlessness / Q000009:adverse effects; D018544:Weightlessness Countermeasures