Abstract
Haemoconcentration and ventilatory instability are early physiological responses to hypobaric hypoxia (HH) that are both considered to influence cerebral oxygen delivery and vascular resistance. However, the extent to which these factors contribute to cerebral blood flow (CBF) regulation and interindividual variability during altitude acclimatization remains unclear. Herein we explore findings from two hypobaric chamber studies designed to experimentally isolate these key factors. First, twenty-three healthy lowlanders (12 females) were exposed to two 4-day sojourns, one in normobaric normoxia and one in HH (3500 m), with serial CBF assessment via duplex ultrasound. Notably, reversing the hypoxia-induced haemoconcentration via hypervolaemic haemodilution at the end of the HH sojourn did not impact CBF, suggesting haemoconcentration is not the principal driver of CBF normalisation with acclimatization. Second, the isolated effects of hypoxia induced nocturnal periodic breathing (nPB) on cerebrovascular tone, cardiorespiratory factors and acute mountain sickness were examined in nine healthy male lowlanders over two three-day sojourns in HH (4000 m). These data suggest that nPB has a negligible effect on cerebrovascular pathophysiology, waking ventilation and sleep architecture. Collectively, these findings underline that cerebrovascular acclimatization is multifactorial, highlighting the need for integrative models not only to define normative adaptation, but also to identify maladaptive responses relevant to hypoxia-related pathologies.