A comparison of normobaric and hypobaric hypoxia effects on cerebrovascular response pre and post maximal exercise

A lack of consensus remains on whether normobaric hypoxia (NH) and hypobaric hypoxia (HH) may differentially impact physiological factors affecting cerebrovascular regulation, particularly with an additional strenuous exercise component. We sought to compare the acute effects of NH and HH on global cerebral blood flow (gCBF) at an altitude corresponding to 4000 m. In this randomised, single-blind crossover study, eight lowlanders (3 females) completed three identical trials inside a hypobaric chamber: the first in normobaric normoxia, for familiarisation, followed in random order by one in NH and one in HH. In each trial, gCBF was measured at two time points via duplex ultrasound, first after 25 min of rest, and second, directly after a graded exercise test (GXT) to volitional exhaustion. Cardiorespiratory responses and cerebral oxygenation ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>r</mml:mi> <mml:msub><mml:mi>S</mml:mi> <mml:mrow><mml:mi>c</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>${\mathrm{r}}{S_{{\mathrm{c}}{{\mathrm{O}}_{\mathrm{2}}}}}$</mml:annotation></mml:semantics> </mml:math> ) were assessed during all gCBF measurements. At rest, gCBF was higher in HH than in NH (944 ± 230 vs. 883 ± 226 mL min^-1; P = 0.027, respectively), whereas <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>r</mml:mi> <mml:msub><mml:mi>S</mml:mi> <mml:mrow><mml:mi>c</mml:mi> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:mrow> <mml:annotation>${\mathrm{r}}{S_{{\mathrm{c}}{{\mathrm{O}}_{\mathrm{2}}}}}$</mml:annotation></mml:semantics> </mml:math> remained unchanged. Cardiorespiratory parameters did not differ, except for a reduction in the ratio of dead space to tidal volume in HH compared to NH (P = 0.028). Post-GXT, no differential response between the two hypoxic conditions was found. In comparison to NH, at rest gCBF is increased in HH for a given partial pressure of inspired oxygen, a response that is subsequently abolished post maximal cycling exercise. Although subtle, this response indicates that cerebrovascular regulation is affected differently in NH and HH, despite negligible changes in ventilation, and thus, alternative explanations are explored for future investigation.