Recent studies in our laboratory demonstrated that spontaneous breathing through an inspiratory impedance threshold device (ITD) increased heart rate (HR), stroke volume (SV), cardiac output (Q), and mean arterial blood pressure (MAP) in supine human subjects. In this study, we tested the effectiveness of an ITD as a countermeasure against development of orthostatic hypotension, provoked using a squat-to-stand test (SST). Using a prospective, randomized blinded protocol, 18 healthy, normotensive volunteers (9 males, 9 females) completed two-counterbalanced 6-min SST protocols with and without (sham) an ITD set to open at 0.7 kPa (7-cm H2O) pressure. HR, SV, Q, total peripheral resistance (TPR), and MAP were assessed noninvasively with infrared finger photoplethysmography. Symptoms were recorded on a 5-point scale (1 = normal; 5=faint) of subject perceived rating (SPR). The reduction in TPR produced by SST (-35±5 %) was not affected by the ITD. Reduction in MAP with ITD during the transient phase of the SST (-3.6±0.5 kPa or -27±4 mmHg) was less (P =0.03) than that measured while breathing through a sham device (-4.8±0.4 kPa or -36±3 mmHg) despite similar (P <0.926) elevations in HR of 15±2 bpm. SV (+2±4 %) and Q (+22±5 %) with the ITD were higher (P <0.04) than SV (-8±4 %) and Q (+10±6 %) without the ITD. SPR was 1.4±0.1 with ITD compared to 2.0±0.2 with the sham device (P <0.04). This reduction in orthostatic symptoms with application of an ITD during the SST was associated with higher MAP, SV and Q. Our results demonstrate the potential application of an ITD as a countermeasure against orthostatic hypotension.
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Acknowledgments The authors thank the subjects for their cheerful cooperation; Sandy Reed, Barry Slack and Robert Cummings for their engineering and technical assistance during the experiments; Greg Hall for computer programming of LabView data acquisition; Ivonne Garcia for her assistance with medical monitoring of the subjects during the experiments; Colie G. Warren, Kevin Eisenhower and Joseph Sithole for assistance with data collection; and the participation of the 2003 NASA Space Life Sciences Training Program for their assistance in data collection and analysis. This research was supported by Cooperative Research and Development Agreements between the US Army Institute of Surgical Research (USAISR) and the National Aeronautics and Space Administration (CRDA No. DAMD17-01-0112) and between the USAISR and Advanced Circulatory Systems, Inc. (CRDA No. DAMD17-02-0160), Department of Defense Small Business Innovative Research #W81XWH-04-C-0022 grant award to Advanced Circulatory Systems, Inc., Eden Prairie, Minnesota, and by funding from the US Army Combat Casualty Care Research Program. The views expressed herein are the private views of the authors and are not to be construed as representing those of the National Aeronautics and Space Administration, Department of Defense, or Department of the Army. Disclaimer: K. Lurie is a coinventor of the impedance threshold device and founded Advanced Circulatory Systems, Inc. to develop the device.
- Blood pressure
- Cardiac output
- Heart rate
- Peripheral vascular resistance
- Stroke volume