Cardiorespiratory hysteresis during incremental high‐altitude ascent–descent quantifies the magnitude of ventilatory acclimatization

Abstract

Maintenance of arterial blood gases is achieved through sophisticated regulation of ventilation, mediated by central and peripheral chemoreflexes. Respiratory chemoreflexes are important during exposure to high altitude owing to the competing influence of hypoxia and hypoxic hyperventilation-mediated hypocapnia on steadystate ventilatory drive. Inter-individual variability exists in ventilatory acclimatization to high altitude, potentially affecting the development of acute mountain sickness (AMS). We aimed to quantify ventilatory acclimatization to high altitude by comparing differential ascent and descent values (i.e. hysteresis) in steady-state cardiorespiratory variables. We hypothesized that: (i) the hysteresis area formed by cardiorespiratory variables during ascent and descent would quantify the magnitude of ventilatory acclimatization; and (ii) larger hysteresis areas would be associated with lower AMS symptom scores during ascent. In 25 healthy, acetazolamide-free trekkers ascending to and descending from 5160 m, cardiorespiratory hysteresis was measured in the partial pressure of end-tidal CO 2 , peripheral oxygen saturation, minute ventilation, chemoreceptor stimulus index (end-tidal CO 2 /peripheral oxygen saturation) and the calculated steady-state chemoreflex drive (SS-CD; minute ventilation/chemoreceptor stimulus index) using portable devices (capnograph, peripheral pulse oximeter and respirometer, respectively). Symptoms of AMS were assessed daily using the Lake Louise questionnaire. We found that: (i) ascent-descent hysteresis was present in all cardiorespiratory variables; (ii) SS-CD is a valid metric for tracking ventilatory acclimatization to high altitude; and (iii) the highest AMS scores during ascent exhibited a significant, moderate and inverse correlation with the magnitude of SS-CD hysteresis (r s = −0.408, P = 0.043). We propose that ascent-descent hysteresis is a

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

“…Although this study took place as a part of a large expedition to HA in May 2018, the experimental question and design were planned a priori in advance. Some overlap in the participant pool and ancillary cardiorespiratory measures exists with Childebayeva et al (2019), Leacy et al (2021) and Lefferts et al (2019a;2019b).…”

Prior oxygenation, but not chemoreflex responsiveness, determines breath‐hold duration during voluntary apnea

“…These two antagonistic factors, where V̇CO 2 was higher but starting CO 2 was lower compared to the normoxic BH, (a) are likely negligible in their contribution to BHD and (b) likely cancel each other out across the breath hold, leaving the effects of prior oxygenation as the key variable driving differences in BHD in our study. Indeed, this complex interplay in generating a new steady state in the face of antagonistic stimuli was in part the initial justification for developing and applying our SS‐CD metric (see Bruce et al., 2018; Leacy et al., 2020; Pfoh et al., 2017).…”

“…Our results suggest that the SS‐CD does not determine BHD when compared in room air and normobaric steady‐state hypoxia. Although we found no correlations between BHD and SS‐CD magnitude in either acute normoxia or hypoxia, there may be merit in assessing the relationship between BHD and SS‐CD during chronic hypoxic conditions (i.e., high altitude), where hypobaric hypoxia decreases voluntary breath hold duration (e.g., Ferris et al, 1946) and ventilatory acclimatization increases SS‐CD (e.g., Bruce et al., 2018; Leacy et al., 2020). However, it is possible that the temporal domain of breath holding (i.e., acute dynamic chemostimulation) and calculating the SS‐CD (i.e., obtained during steady state) may be unrelated.…”

Experimental Physiology special issue: Extreme environmental physiology

“…Acclimatisation a powerful way of ameliorating the negative physiological and pathophysiological responses to altitude. The paper by Leacy and colleagues investigates the extent to which hysteresis in steady-state cardiorespiratory variables can quantify ventilatory acclimatization to high altitude(Leacy et al, 2021), while Holmström and colleagues consider the contribution of splenic contraction early in acclimatization(Holmström et al, 2021).Heat is a ubiquitous stress. Athletes worry about it in preparation for major sporting events, occupational groups have to work in it and protect against it and, with heat waves becoming more frequent due to climate change, the threat to vulnerable groups such as the elderly increases.…”

“…Acclimatisation a powerful way of ameliorating the negative physiological and pathophysiological responses to altitude. The paper by Leacy and colleagues investigates the extent to which hysteresis in steady‐state cardiorespiratory variables can quantify ventilatory acclimatization to high altitude (Leacy et al., 2021), while Holmström and colleagues consider the contribution of splenic contraction early in acclimatization (Holmström et al., 2021).…”