Inspiratory resistance delays the reporting of symptoms with central hypovolemia: Association with cerebral blood flow

We tested the hypothesis that breathing through an inspiratory threshold device (ITD) during progressive central hypovolemia would protect cerebral perfusion and attenuate the reporting of presyncopal symptoms. Eight human subjects were exposed to lower-body negative pressure (LBNP) until the presence of symptoms while breathing through either an active ITD (-7 cmH₂O impedance) or a sham ITD (0 cmH₂O). Cerebral blood flow velocity (CBFV) was measured continuously via transcranial Doppler and analyzed in both time and frequency domains. Subjects were asked to report any subjective presyncopal symptoms (e.g., dizziness, nausea) at the conclusion of each LBNP exposure. Symptoms were coincident with physiological evidence of cardiovascular collapse (e.g., hypotension, bradycardia). Breathing on the active ITD increased LBNP tolerance time (mean ± SE) from 2,014 ± 106 s to 2,259 ± 138 s (P = 0.006). We compared CBFV responses at the time of symptoms during the sham ITD trial with those at the same absolute time during the active ITD trial (when there were no symptoms). While there was no difference in mean CBFV at these time points (sham, 44 ± 4 cm/s vs. active, 47 ± 4; P = 0.587), total oscillations (sum of high- and low-frequency spectral power) of CBFV were higher (P = 0.004) with the active ITD (45.6 ± 10.2 cm/s²) than the sham ITD (22.1 ± 5.4 cm/s²). We conclude that greater oscillations around the same absolute level of mean CBFV are induced by inspiratory resistance and may contribute to the delay in symptoms and cardiovascular collapse that accompany progressive central hypovolemia.