Steady-state cerebral blood flow regulation at altitude: interaction between oxygen and carbon dioxide

The effects of high altitude ascent on splenic contraction and the diving response during voluntary apnoea

“…During the first few days of exposure to HA, resting HR and vascular tone increase as a consequence of acute hypobaric hypoxia by stimulation of peripheral chemoreceptors, increasing sympathetic nervous system activity (Bärtsch & Gibbs, 2007;Hainsworth, Drinkhill, & Rivera-Chira, 2007). However, in the present investigation, resting HR and MAP were unchanged with ascent, most likely a response due to a relatively slow ascent profile with regular rest days enabling proper acclimatization to occur, as well as the vasodilatory effects of systemic hypoxia (Lafave et al, 2019;Leacy et al, 2018).…”

Bilateral regional extracranial blood flow regulation to hypoxia and unilateral duplex ultrasound measurement error

“…These regional blood flow responses to acute hypoxia are similar to those previously reported from studies employing the typical method of doubling unilateral measurements (Lewis et al., 2014; Morris et al., 2017; Willie et al., 2012). There are as many studies reporting that the increase in blood flow to hypoxia is mediated by vasodilatation in both ICAs and VAs to extreme (<80% ) poikilocapnic hypoxia (Lewis et al., 2014; Morris et al., 2017) or isocapnic hypoxia (Fernandes et al., 2018; Hoiland et al., 2017) as there are reporting no vasodilatation (Lafave et al., 2019; Ogoh et al., 2013; Willie et al., 2012; Willie, Smith, et al., 2014), with others suggesting regionally specific vasodilatation (Kellawan et al., 2017; Subudhi et al., 2014). Notwithstanding the methodological differences of inducing hypoxia that is known to affect the cerebrovascular response, such as the clamping of carbon dioxide (Kellawan et al., 2017; Ogoh et al., 2013; Willie et al., 2012), exposure to high‐altitude hypobaric hypoxia (Hoiland et al., 2017; Lafave et al., 2019; Subudhi et al., 2014; Willie, Smith, et al., 2014) and length of exposure (Lewis et al., 2014), the aforementioned studies are often limited by their sample size and therefore sensitivity to detect small differences where high inter‐individual variability with exposure to acute severe hypoxia is notable (Willie, Smith, et al., 2014).…”

“…In investigations of extracranial blood flow regulation to hypoxia, a unilateral measurement of the right VA is overwhelmingly favoured (Fernandes et al., 2018; Hoiland et al., 2017; Lafave et al., 2019; Lewis et al., 2014; Morris et al., 2017; Ogoh et al., 2013; Willie et al., 2012) compared to the left VA (Subudhi et al., 2014; Willie, Smith, et al., 2014). The rationale often stated for the right side being chosen is to account for the 20–30% smaller blood flow in the right VA compared to the left VA such that absolute calculations of regional and global blood flow are an underestimation (Lewis et al., 2014; Ogoh et al., 2013).…”

“…xia (Hoiland et al, 2017;Lafave et al, 2019;Subudhi et al, 2014;Willie, Smith, et al, 2014) and length of exposure (Lewis et al, 2014), the aforementioned studies are often limited by their sample size and therefore sensitivity to detect small differences where high inter-individual variability with exposure to acute severe hypoxia is notable (Willie, Smith, et al, 2014). Compared to previous literature, the present study was conducted in a relatively large cohort and is strengthened by bilateral measurement of the blood flow response to hypoxia, which provides more certainty that blood flow is similarly regulated in ICAs and VAs in response to acute poikilocapnic hypoxia.…”

“…Regional extracranial blood flow regulation can be measured non‐invasively by duplex ultrasound of the vertebral arteries (VAs) that feed the homeostatic posterior brain regions and the internal carotid arteries (ICAs) that feed the more functional anterior brain regions. In response to hypoxia, absolute increases in blood flow within the ICAs are greater than in the VAs (Lafave et al., 2019). When indexed as a relative response some evidence indicates that the blood flow increase to hypoxia in the VAs is greater than in the ICAs (Lewis et al., 2014; Ogoh et al., 2013; Subudhi et al., 2014; Willie et al., 2012).…”