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Competing Interests: Funding Support and Author Disclosures Dr Pilgrim has received grants to the institution from the Swiss National Science Foundation, the Swiss Heart Foundation, the Swiss Polar Institute, Biotronik, Boston Scientific, Edwards Lifesciences, Medtronic, and ATSens; and has received consultancy/speaker fees from Boston Scientific, Biotronik, Edwards Lifesciences, Abbott, Biosensors, Medtronic, and HighLife. Dr Reichlin has received grants from the Swiss National Science Foundation, the Swiss Heart Foundation, the sitem insel support fund, Biotronik, Boston Scientific, and Medtronic, all for work outside the submitted study; has received speaker/consulting honoraria or travel support from Abbott/SJM, Biosense-Webster, Biotronik, Boston Scientific, and Medtronic; and has received support for his institution’s fellowship program from Abbott/SJM, Biosense-Webster, Biotronik, Boston-Scientific, and Medtronic. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.

Importance: Arterial hypoxemia, electrolyte imbalances, and periodic breathing increase the vulnerability to cardiac arrhythmia at altitude., Objective: To explore the incidence of tachyarrhythmias and bradyarrhythmias in healthy individuals at high altitudes., Design, Setting, and Participants: This prospective cohort study involved healthy individuals at altitude (8849 m) on Mount Everest, Nepal. Recruitment occurred from January 25 to May 9, 2023, and data analysis took place from June to July 2023., Exposure: All study participants underwent 12-lead electrocardiogram, transthoracic echocardiography, and exercise stress testing before and ambulatory rhythm recording both before and during the expedition., Main Outcome: The incidence of a composite of supraventricular (>30 seconds) and ventricular (>3 beats) tachyarrhythmia and bradyarrhythmia (sinoatrial arrest, second- or third-degree atrioventricular block)., Results: Of the 41 individuals recruited, 100% were male, and the mean (SD) age was 33.6 (8.9) years. On baseline investigations, there were no signs of exertional ischemia, wall motion abnormality, or cardiac arrhythmia in any of the participants. Among 34 individuals reaching basecamp at 5300 m, 32 participants climbed to 7900 m or higher, and 14 reached the summit of Mount Everest. A total of 45 primary end point-relevant events were recorded in 13 individuals (38.2%). Forty-three bradyarrhythmic events were documented in 13 individuals (38.2%) and 2 ventricular tachycardias in 2 individuals (5.9%). Nine arrhythmias (20%) in 5 participants occurred when climbers were using supplemental bottled oxygen, whereas 36 events (80%) in 11 participants occurred at lower altitudes when no supplemental bottled oxygen was used. The proportion of individuals with arrhythmia remained stable across levels of increasing altitude, while event rates per 24 hours numerically increased between 5300 m (0.16 per 24 hours) and 7300 m (0.37 per 24 hours) before decreasing again at higher altitudes, where supplemental oxygen was used. None of the study participants reported dizziness or syncope., Conclusion and Relevance: In this study, more than 1 in 3 healthy individuals experienced cardiac arrhythmia during the climb of Mount Everest, thereby confirming the association between exposure to high altitude and incidence of cardiac arrhythmia. Future studies should explore the potential implications of these rhythm disturbances.