[Changes of cerebral blood flow during 21 d head-down tilt bed rest and the effect of lower body negative pressure during the last week].

Objective To investigate the changes of cerebral blood flow(CBF) during 21 d head down tilt (HDT)bed rest and the effect of lower body negative pressure(LBNP) in the last week. Method Twelve healthy male subjects were randomly divided into control and LBNP groups, with 6 in each group. All of them were exposed to -6°HDT for 21 d. The LBNP group received -4.0 kPa LBNP training 1 h/d in the last week of HDT while the control group did not. CBF and cerebral vascular resistance were measured by rheoencephalogram in pre HDT, day 3, 10 and 21 of HDT. Result In control group, left cerebral I quadrant area and inrush velocity decreased significantly during HDT, and left cerebral ΔGy wave crest height decreased significantly, whereas left cerebral resistance index increased significantly on day 3 and 21 of HDT as compared to those of pre HDT.In LBNP group,left cerebral I quadrant area decreased significantly, and left cerebral ΔGy wave crest height and inrush velocity tended to decrease on day 3 and 21 of HDT,whereas left cerebral resistance index increased significantly during HDT as compared to those of pre HDT.There were no significant differences between above indexes in control group and LBNP group. Conclusion It is suggested that 21 d HDT may increase cerebral vascular resistance and decrease CBF, which can not be prevented by LBNP training in the last week of 21 d HDT.