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EFFECTS OF INSPIRATORY IMPEDANCE ON HEMO-DYNAMIC RESPONSES TO ORTHOSTATIC HYPOTENSION.  J.M. Crissey¹, V.A. Convertino², D.A. Ratliff³, A. Idris⁴, K.G. Lurie⁵, ¹Trinity University, San Antonio; ²U.S. Army Institute of Surgical Research, San Antonio; ³Bionetics Corporation, KSC; ⁴UT Southwestern, Dallas; ⁵Advanced Circulatory Systems, Minnesota.

   Background:  Recent studies in our laboratory demonstrated that inspiration through an inspiratory impedance threshold device (ITD) increased blood pressure and cardiac output in supine human subjects.  Objective:  We tested the effectiveness of the ITD as a countermeasure against an orthostatic challenge in human subjects.  Methods:  Eighteen healthy, normotensive volunteers (9 males, 9 females), ages 20-56, completed two-counterbalanced 12-minute protocols with and without and ITD set to open at 7 cm H₂O pressure.  The tests were separated by 30 minutes.  Non-invasive measurements of heart rate (HR), stroke volume (SV), and mean arterial pressure (MAP) were performed throughout the duration of a squat-stand stress test (SST).  Results:  Reductions in MAP (-41 ± 3 mmHg) and elevations in HR (20 ± 4 bpm) with spontaneous breathing on the ITD during the transient phase of the SST were similar to those measured while breathing on a sham (control) device.  However, alterations in SV (-1 ± 6 %) and CO (+24 ± 7 %) with the ITD were greater (P < 0.025) than those measured without peripheral vascular resistance was reduced (+44 ± 4 %) more (P = 0.038) with ITD compared to without ITD (-14 ± 18 %).  Conclusions:  Use of an ITD with 7 cm H2O opening pressure defends the vasoconstrictive reserve for blood pressure regulation challenge.  Our results demonstrate the potential application of an ITD as an effective countermeasure against post-spaceflight orthostatic intolerance observed in astronauts.  (Supported by NASA’s 2003 Spaceflight and Life Sciences Training Program)