Effect of an Inspiratory Impedance Threshold Device on Cerebral Blood Flow During Orthostatic Stress. P. Khatri1, K.D. Cohen2, C.A. Rickards3, V.A. Convertino3

1NASA Spaceflight and Life Sciences Training Program, KSC, FL; 2The Bionetics Corporation, KSC, FL;  3US Army Institute of Surgical Research, Fort Sam Houston, TX

   Astronauts returning to Earth often exhibit orthostatic hypotension, a condition that reduces their ability to tolerate the upright stance due to low blood pressure and cerebral perfusion.  Our objective was to determine the mechanism by which resistive breathing through an inspiratory Impedance Threshold Device (ITD) ameliorates orthostatic symptoms.  We hypothesized that the ITD ameloriates a decrease in cerebral blood flow during orthostatic stress.  Thirteen male and ten female subjects, who gave voluntary consent, performed a squat-stand test (SST) with two forms of intervention: 1) an active ITD with an inspiratory impedance of -7cm H2O and 2) a sham ITD with no resistance (control).  Blood flow velocity within the middle cerebral artery was measured using a transcranial Doppler and central hemodynamics with a Portapres.  In the 13 subjects who demonstrated significant reductions in SV during the transient phase of the SST, the ITD defended stroke volume (-19.2±2.3 % vs. -11.7±1.6%; p=0.01) but failed to attenuate the reduction in Vm upon standing (-12±2 % for sham vs. -15±2% active ITD).  These data suggest that cerebral blood flow autoregulation is independent of central hemodyamics (i.e. SV). 

(Supported by the NASA Spaceflight and Life Sciences Training Program)