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Cerebral Autoregulation and Orthostatic Stress.   C.T. Lee1, 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.

   Syncope due to orthostatic hypotension is a significant research interest for NASA due to its high prevalence in astronauts returning to a normogravity environment.  Recent studies have demonstrated the efficacy of an inspiratory Impedance Threshold Device (ITD) in reducing the cardiovascular (blood pressure, stroke volume, and cardiac output) and subjective symptoms of orthostatic stress during a squat-stand test (SST).  The purpose of this study was to determine the effect of the ITD on cerebral autoregulation.  We hypothesized that the ITD would cause an increase in mean middle cerebral artery blood flow velocity (Vm) relative to SHAM, and could also correct for impaired cerebral autoregulation in subjects sensitive to the SST.  However, repeated measures two-way ANOVA revealed no distinguishable significant effect of treatment (ITD vs. SHAM; p=0.64), response group (RESPONDERS vs. NON-RESPONDERS; p=0.10), or their interaction (p=0.51) on the autoregulatory profiles of volunteers.  Additionally, the ITD had no effect on either Vm (p=0.27), although ITD application improved subjective scores of orthostatic intolerance (p=0.02).  The relative independence of Vm in spite of systemic changes induced by the ITD may be attributed to intact cerebral autoregulation.  These results demonstrate that the effects of the ITD in ameliorating orthostatic symptoms are systemic, versus cerebral, in nature which are consistent with prior studies indicating that ITD breathing has no effect on cerebral autoregulation.

(Supported by NASA Spaceflight and Life Sciences Training Program)