title: Effect of lower-body negative pressure on cerebral blood flow velocity during 21 days head-down tilt bed rest.
abstract: BACKGROUND
  The purpose of this study was to investigate cerebral blood flow (CBF) velocity in humans during 21 days of head-down tilt (HDT) bed rest with and without lower-body negative pressure (LBNP).
  
  
  METHODS
  Twelve healthy male volunteers were exposed to -6 degrees HDT bed rest for 21 days. Six subjects received -30 mmHg LBNP sessions for 1 h per day from the 1st to the 7th day and from the 15th to the 21st day of HDT, and six others served as controls. CBF velocity was measured by use of the transcranial Doppler technique in the right middle cerebral artery before and during HDT.
  
  
  RESULTS
  In the control group, mean and systolic CBF velocities decreased on day 1 of HDT compared with the pre-HDT value, and dropped further on day 3 of HDT, then remained significantly below the pre-HDT baseline on days 7 and 10 of HDT, and reached a minimum value on day 21 of HDT. In the LBNP group, mean and systolic CBF velocities decreased significantly on day 1 of HDT compared with the pre-HDT value, and remained lowered throughout HDT. Diastolic CBF showed no significant change throughout HDT in both groups. There were no significant differences in these parameters between the two groups.
  
  
  CONCLUSIONS
  The results of this study suggest that CBF velocity is reduced during 21 days of HDT, and brief daily LBNP sessions used in the first and last weeks of 21-day HDT bed rest does not improve CBF velocity.
authors.affiliation: Department of Aerospace Biodynamics, Faculty of Aerospace Medicine, Fourth Military Medical University, Xi'an, PR China. sunxiqing@hotmail.com
authors.forename: Xi-Qing
authors.initials: XQ
authors.lastname: Sun
authors.forename: Yong-Jie
authors.initials: YJ
authors.lastname: Yao
authors.forename: Chang-Bin
authors.initials: CB
authors.lastname: Yang
authors.forename: Shi-Zhong
authors.initials: SZ
authors.lastname: Jiang
authors.forename: Chang-Lin
authors.initials: CL
authors.lastname: Jiang
authors.forename: Wen-Bin
authors.initials: WB
authors.lastname: Liang
mesh_terms: D000328:Adult; D001510:Bed Rest; D001794:Blood Pressure; D002560:Cerebrovascular Circulation; D018475:Head-Down Tilt / Q000009:adverse effects*; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D017585:Ultrasonography, Doppler, Transcranial; D014893:Weightlessness