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Influence of high-altitude on the heart rate and rhythm response to apnea

  • Author / Creator
    Berthelsen, Lindsey Frances
  • BACKGROUND: Concurrent excitation of the sympathetic and parasympathetic systems occurs at high-altitude (chronic hypoxia), via the carotid chemoreflex. We have demonstrated this autonomic conflict manifests as cardiac arrhythmias during voluntary apnea. We sought to determine the duration of hypoxic exposure at high altitude necessary to unmask cardiac arrhythmias during voluntary apnea. We hypothesized that cardiac arrhythmias during apnea would be increased after 24 hours of high-altitude exposure compared to apnea at low altitude. METHODS: Measurements of steady state chemoreflex drive (SS-CD), continuous electrocardiogram (ECG; lead II) and SpO2 (pulse oximetry) were collected in 22 participants (8 females) at low altitude (1045m) and over 8 consecutive days at high-altitude (3800m). Resting SS-CD was quantified as ventilation (L/min) over stimulus index (PETCO2/SpO2). Following resting baseline participants performed an end-expiratory apnea to volitional breakpoint. ECG rate and rhythm were evaluated at baseline and during apnea. The nadir heart rate was used to quantify bradycardia and abnormal ECG rhythm was classified based on origin (e.g., sinus node) and rhythm (e.g., arrest). Differences in ventilation, SS-CD, SpO2, and the heart rate response to apnea on each day at high altitude (Day 1-8) were compared to that at low altitude (Day 0) using a repeated-measures ANOVA design, with a Holm-Sidak post-hoc analysis where main effect of time was significant. RESULTS: Baseline SpO2 was lower for all days at high-altitude compared to low altitude (p<0.01). On Day 4 and beyond, SS-CD was elevated versus low altitude (Day 4, p=0.02; Day 5, p=0.004; Day 6, p=0.003; Day 7, p=0.005; Day 8, p=0.02), indicating ventilatory acclimatization. At high-altitude (all days), baseline resting heart rate was higher compared to low altitude (p<0.01). A main effect of time (p<0.001) was identified for the change in heart rate during apnea. At low altitude, the average apnea induced decrease in heart rate was -9±15bpm, whereas the average decrease in heart rate at high altitude (all days) was -24±16bpm. Bradycardia became more pronounced with acclimatization, with the greatest drop in heart rate (-31±16bpm) occurring on Day 5. At low altitude 14% (3/22) of participants developed arrhythmias during apnea. At high-altitude, cardiac arrhythmias (e.g., sinus pause and arrest, conduction block) during apnea became most prevalent (>50%) following Day 5. Changes in saturation during apnea and apnea duration were not associated with the magnitude of bradycardia during apnea (saturation, r= -0.007 p=0.92; apnea duration, r=0.005 p=0.94). Interestingly, the magnitude of bradycardia was correlated with the incidence (percentage) of arrhythmia per day (r=0.8; p=0.004), suggesting a similar underlying mechanism. CONCLUSION AND SIGNIFICANCE: Our data demonstrate that chronic hypoxia is associated with progressive increase in vagal tone throughout acclimatization to altitude, as indicated by augmented bradycardia during apnea and progressively incidence of arrhythmia. Additionally, the increased incidence of arrhythmia following Day 5 at high-altitude suggests that chemoreflex sensitization may play a role in this phenomenon. This study provides insight into cardiac electrophysiology under conditions of heightened stress and has implications as a future functional model for exploring the effects of heightened chemoreflex stress on autonomic control of heart.

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  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.