Inspiratory resistance increases hemodynamic oscillations and tolerance to induced central hypovolemia in humans

Abstract

We have shown that inspiratory resistance induced by breathing through an impedance threshold device (ITD) reduces intrathoracic pressure and increases stroke volume (SV) in supine normovolemic humans. We hypothesized that breathing through an ITD would increase tolerance to progressive central hypovolemia induced by exposure to lower body negative pressure (LBNP), and that increased tolerance would be associated with increased SV. Eight volunteers (5 men, 3 women) were instrumented to record ECG and beat-by-beat arterial pressure and SV (Finometer®). LBNP tolerance was assessed while subjects breathed against either 0 (sham) or −7 cmH2O inspiratory resistance (active ITD). Experiments were performed on separate days. Data were analyzed in both time and frequency domains. The active ITD increased LBNP tolerance time from 2,005 ± 104 s to 2,254 ± 137 s (p = 0.006); this effect was associated with increased systolic pressure and peripheral resistance (p = 0.01), but no alteration in SV. ITD breathing increased spectral power at the low frequency range [(LF; 0.04 – 0.15 Hz)] for both systolic pressure and SV (p ≤ 0.05). These data show that breathing against inspiratory resistance increases tolerance to progressive central hypovolemia by better maintaining arterial pressure and peripheral resistance. Increased SV and systolic pressure oscillations with the ITD suggest that both improvements in venous return and enhanced engagement of the sympathetic baroreflex contribute to this increased tolerance.