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Self-Reporting of Symptoms During Orthostatic Stress: Relationship to Mean Cerebral Blood Flow Velocity.   B.L. Burton1, K.D. Cohen2, C.A. Rickards3, and 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

    Previous studies have demonstrated that resistance during inspiration applied with an Impedance Threshold Device (ITD) increases mean middle cerebral artery blood flow velocity (Vm) in supine subjects. Due to the incidence of orthostatic hypotension and intolerance experienced by astronauts returning from space, we were interested in the potential application of an ITD to reduce orthostatic symptoms and defend Vm during an orthostatic stress. Twenty three healthy subjects (13 males and 10 females) participated in the study after giving voluntary consent. We measured Vm (transcranial Doppler) and subject perceived rating (SPR) of orthostatic stress during two separate ITD conditions: 1) breathing through an active ITD set with an impedance of -6 cm H2O; and 2) breathing through a sham (control) ITD without impedance. We used a squat-stand test (SST) protocol consisting of moving to a standing posture after a 4-min period in a deep knee squat for the orthostatic stress. Vm (transcranial Doppler) was measured during the final 15 sec of squatting and the initial 10 sec of standing. Immediately following the SST, each subject was asked to respond to a survey in which they scored 11 symptoms, each based on a 5-point severity scale of no symptoms, mild, moderate, severe, and faint. Compared with the sham ITD, the active ITD produced lower SPR (11.9 ± 0.2 vs. 12.5 ± 0.3, p = 0.04). However, the reduction in Vm (%∆) during the transient phase of standing from the squat posture was not distinguishable (p = 0.27) between the sham (-13.9 ± 1.6 %) and active (-15.2 ± 1.8 %) ITD. The results of this investigation suggest that orthostatic symptoms cannot be completely explained by reductions in cerebral blood flow. Our data support the hypothesis that the application of an ITD could provide a therapeutic intervention to relieve orthostatic symptoms following spaceflight.

(Supported by NASA Spaceflight and Life Sciences Training Program)