Inspiratory impedance effects on hemodynamic responses to orthostasis in normal subjects.

BACKGROUND

Breathing through an impedance threshold device (ITD) might prove effective as a countermeasure against post-spaceflight orthostatic hypotension since it increased blood pressure (BP) and cardiac output in supine human subjects.

OBJECTIVE

We tested the hypothesis that spontaneous breathing through an ITD would attenuate the reduction in stroke volume and BP during orthostasis in human subjects.

METHODS

There were 19 volunteers (10 men, 9 women) who completed two 80 degrees head-up tilt (HUT) protocols with (active) and without (sham control) an ITD set to open at -7 cm H2O pressure. Heart rate (HR), stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP), and total peripheral resistance (TPR) were measured non-invasively during transition from supine to HUT.

RESULTS

HUT caused significant elevation in HR and reductions in SV, CO, TPR, and MAP. Hemodynamic effects of HUT were similar for sham and active ITD. Further analysis revealed a subset (n = 11) of subjects who demonstrated a > 20% decrease in SV during HUT with the sham ITD. In this subset of subjects, the ITD attenuated (p = 0.004) the %deltaSV (-22.5 +/- 3.0%) during HUT compared with the sham ITD (%deltaSV = -37.4 +/- 2.6%). There was no statistical effect of ITD use in the subgroup who demonstrated < 20% reduction in SV (-16.6 +/- 0.4%).

CONCLUSIONS

Use of an ITD may provide significant protection against orthostatic compromise in individuals with greater than 20% reductions in SV, such as astronauts returning from space.