This cohort study examines data from climbers at Mount Everest to explore the incidence of tachyarrhythmias and bradyarrhythmias in healthy individuals at high altitudes.
Key Points
Question
Does the hypoxic hypobaric environment of a high altitude increase the risk of cardiac arrhythmia?
Findings
In this cohort study, more than 1 in 3 participants experienced bradyarrhythmia or tachyarrhythmia during the ascent from basecamp to the summit of Mount Everest. The proportion of individuals with arrhythmia remained stable across levels of increasing altitude, while the majority of arrhythmias occurred when no supplemental bottled oxygen was used.
Meaning
The potential implications of the observed rhythm disturbances at high altitudes need to be explored in future studies.
Abstract
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.
Introduction
Arterial hypoxemia, electrolyte imbalances, and periodic breathing increase the susceptibility to cardiac arrhythmia at high altitudes.1 However, available evidence on the occurrence of rhythm disturbances in the hypoxic hypobaric environment of high altitude is scarce. In the present study, we aimed to systematically explore the incidence of tachyarrhythmia or bradyarrhythmia in healthy individuals climbing Mount Everest.
Methods
Setting and Design
The SUMMIT study was a prospective cohort study involving healthy individuals who were climbing Mount Everest. Eligible climbers underwent a baseline assessment within 12 weeks before the expedition, including 12-lead electrocardiogram, transthoracic echocardiography, exercise stress testing, and ambulatory rhythm recording using a wearable continuous electrocardiogram patch (ATP-C130; ATsens) (eFigure 1 in Supplement 1). Ambulatory rhythm recording was repeated during the ascent from basecamp (5300 m) to the summit of Mount Everest (8849 m) so that every participant served as their own control.
The study was approved by the Nepal Health Research Council and the Cantonal Ethics Committee in Bern, Switzerland, and all participants provided written informed consent. The study was registered with ClinicalTrials.gov (NCT05676398). Detailed methods are described in the eMethods in Supplement 1.
Primary End Point and Definitions
The primary end point was the incidence of a composite of supraventricular (>30 seconds) and ventricular (>3 beats) tachyarrhythmia and bradyarrhythmia (sinoatrial arrest, second- or third-degree atrioventricular block) (eTable 1 in Supplement 1). Rhythm disturbances were considered as individual events when they occurred at least 5 minutes apart. The primary end point was independently adjudicated by 2 electrophysiologists blinded to the participant and the altitude of the recording. Discrepancies were resolved by joint review and mutual consensus. Secondary end points were death, high-altitude pulmonary edema, high-altitude cerebral edema, and acute mountain sickness.
Data Collection and Statistical Analysis
Data were collected using the electronic data capture web application REDCap.2 Categorical variables are presented as frequencies and percentages, and continuous variables as mean (SD). Based on an expected event rate of 56%, a precision of 80%, and a dropout rate of 20%, we estimated a sample size of 30 participants for this study. All statistical analyses were performed using R version 4.3.1 (R Foundation for Statistical Computing).
Results
Between January 25 and May 9, 2023, a total of 41 individuals (mean [SD] age, 33.6 [8.9] years, 100% male) were recruited. Baseline characteristics are summarized in Table 1. On baseline investigations, there were no signs of exertional ischemia or wall motion abnormality in any of the participants. Ambulatory rhythm recording at baseline was completed in 35 individuals for a mean (SD) duration of 4.3 (2.3) days and revealed no supraventricular or ventricular arrhythmia in any of the study participants.
Table 1. Baseline Characteristics.
| Characteristic | No. (%) |
|---|---|
| Age, mean (SD), y | 33.6 (8.9) |
| Male sex | 41 (100) |
| Ethnic group | |
| Nepali Sherpa | 33 (80.5) |
| Nepali, non-Sherpa | 6 (14.6) |
| European | 2 (4.0) |
| History of cardiac disease | 0 |
| History of palpitations | 0 |
| No. of 8000-m peaks summitted | |
| 1-5 | 20 (48.8) |
| 6-10 | 9 (22.0) |
| >10 | 10 (24.4) |
| Unknown | 2 (4.9) |
| Physical examination, mean (SD) | |
| Height, cm | 161.3 (7.2) |
| Weight, kg | 69.4 (7.6) |
| Body mass indexa | 26.7 (2.8) |
| Systolic BP, mm Hg | 122.8 (9.5) |
| Diastolic BP, mm Hg | 78.8 (9.5) |
| Oxygen saturation at 1400 m, % | 96.5 (1.4) |
| Oxygen saturation at 5300 m, % | 83.9 (4) |
| Electrocardiography (n = 36) | |
| Heart rate, mean (SD), /min | 72.2 (10.9) |
| Sinus rhythm | 35 (97.2)b |
| First-degree AV block | 1 (2.8) |
| Second-degree AV block | 0 |
| Third-degree AV block | 0 |
| Left bundle-branch block | 0 |
| Right bundle-branch block | 0 |
| Early repolarization pattern | 6 (16.7) |
| PQ interval, mean (SD), ms | 161 (17) |
| QRS duration, mean (SD), ms | 89 (5) |
| QTc time, mean (SD), ms | 402 (19) |
| Transthoracic echocardiography (n = 36), mean (SD) | |
| Left ventricular ejection fraction, % | 60 (1) |
| Left ventricular end-diastolic diameter, mm | 43.2 (4.1) |
| Left ventricular end-systolic diameter, mm | 29.3 (3.2) |
| Left ventricular mass index, g/m2 | 74.3 (16.4) |
| Tricuspid annular plane systolic excursion, mm | 21.7 (1.9) |
| RV/RA gradient, mm Hg | 16.9 (3.3) |
| Systolic pulmonary artery pressure, mm Hg | 23.4 (3.6) |
| Valvular heart disease, No. (%) | 0 |
| Exercise stress testing ( n = 34), mean (SD) | |
| Peak exercise load, MET | 10 (1.7) |
| Pressure rate productc | 22 815 (6057) |
| Clinical symptoms suspicious of ischemia, No. | 0 |
| Maximum heart rate, /min | 165 (9) |
| Maximum systolic BP, mm Hg | 148 (13) |
| ST-segment depression or T-wave inversion, No. | 0 |
| Cardiac arrhythmia, No. | 0 |
| Conduction defects, No. | 0 |
| Ambulatory rhythm recording, mean (SD)d | |
| No. of days of rhythm recording | 4.3 (2.3) |
| Maximum heart rate, /min | 139 (15) |
| Minimum heart rate, /min | 53 (8) |
| Tachyarrhythmia, No. | 0 |
| Bradyarrhythmia, No. | 0 |
Abbreviations: AV, atrioventricular; BP, blood pressure; MET, metabolic equivalent of task; RV/RA, right ventricular/right atrial.
Calculated as weight in kilograms divided by height in meters squared.
One patient had ectopic atrial rhythm.
Calculated as systolic BP × maximum heart rate.
Rhythm recordings were performed in Kathmandu (1400 m) or Bern (540 m).
Three participants did not reach Everest basecamp and did not undergo expedition rhythm recording, 1 participant did not use the wearable continuous electrocardiogram patch for the climb, 1 participant lost the detached wearable continuous electrocardiogram patch during descent, and 2 wearable continuous electrocardiogram patches were empty on return. Among the remaining 34 individuals with rhythm recordings during the expedition, 32 reached an altitude of 7900 m or higher, 23 climbed above 8500 m, and 14 reached the summit of Mount Everest. One participant climbed to 7300 m (camp 3), and 1 participant did not climb above basecamp at 5300 m. Supplemental bottled oxygen was used during exercise by 32 individuals (94.1%) starting from an altitude of 6500 m (n = 2, 5.9%), 7300 m (n = 28, 82.4%), and 7900 m (n = 2, 5.9%). Two individuals used acetazolamide to prevent acute mountain sickness.
The mean (SD) duration of rhythm recording during the climb from basecamp at 5300 m to the summit and back was 7.6 (4.3) days. Resting heart rates were stable across altitude levels. A total of 45 primary end point–relevant events were recorded in 13 individuals (38.2%). Recorded arrhythmias are summarized in Table 2. An illustration of the occurrence of arrhythmias as a function of altitude and use of supplemental bottled oxygen is in the Figure.
Table 2. Arrhythmias at Altitude.
| Type of arrhythmia | Everest basecamp, 5300 m (n = 34)a | Base to camp 2 (n = 33)b | Camp 2 to camp 3 (n = 33)c | Camp 3 to camp 4 (n = 32)d | Camp 4 to summit (n = 28)e | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| No. of events | No. of participants | No. of events | No. of participants | No. of events | No. of participants | No. of events | No. of participants | No. of events | No. of participants | |
| Composite of tachyarrhythmia and bradyarrhythmia | 13 | 6 | 9 | 6 | 13 | 5 | 8 | 5 | 2 | 2 |
| Tachyarrhythmia | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 0 |
| Supraventricular | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Ventricular | 0 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 0 |
| Bradyarrhythmia | 13 | 6 | 9 | 6 | 12 | 5 | 7 | 5 | 2 | 2 |
| Sinoatrial block | 6 | 4 | 1 | 1 | 2 | 2 | 1 | 1 | 1 | 1 |
| Atrioventricular block | 7 | 3 | 8 | 5 | 10 | 4 | 6 | 4 | 1 | 1 |
| Second-degree | ||||||||||
| Mobitz type 1 | 5 | 2 | 4 | 3 | 5 | 2 | 5 | 3 | 1 | 1 |
| Mobitz type 2 | 2 | 2 | 2 | 2 | 5 | 3 | 1 | 1 | 0 | 0 |
| Third-degree | 0 | 0 | 2 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
Supplemental oxygen: 0 participants.
Altitude, 5300-6500 m; supplemental oxygen: 0 participants.
Altitude, 6500-7300 m; supplemental oxygen: 2 participants.
Altitude, 7300-7900 m; supplemental oxygen: 30 participants.
Altitude, 7900-8849 m; supplemental oxygen: all participants (n = 28).
Figure. Occurrence of Arrhythmias as a Function of Altitude and Use of Supplemental Bottled Oxygen.
Each column represents 1 study participant and indicates the maximum altitude climbed without oxygen (beige) and with the use of supplemental oxygen (light blue). Arrhythmias are indicated by blue (bradyarrhythmia) or red (tachyarrhythmia) bars. The position of the bar relative to the y-axis indicates the altitude at which the arrhythmia occurred. EBC indicates Everest basecamp.
Forty-three bradyarrhythmic events were documented in 13 individuals (38.2%) (eFigure 2 in Supplement 1). Seven individuals had a total of 23 bradycardias at rest, and 10 individuals had 19 bradycardias during exercise (eFigure 3 in Supplement 1). P-P slowing preceded bradyarrhythmia in 18 of 43 events (41.9%) (eFigure 4 in Supplement 1). One individual had 2 episodes of third-degree atrioventricular block at rest in camp 2 (6500 m).
Two tachyarrhythmic events were recorded in 2 individuals (5.9%). There was a correlation between altitude level and maximum QTc interval duration (R2 = 0.37) (eFigure 5 in Supplement 1). One climber had a nonsustained ventricular tachycardia with 9 consecutive ventricular beats at a rate of 240/min during descent from camp 4 (7900 m). One climber had a slow monomorphic sustained ventricular tachycardia with a rate of 137/min during the ascent from camp 2 (6500 m) to camp 3 (7300 m). Examples of recorded arrhythmias are shown in eFigure 6 in Supplement 1.
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 increasingly used (Figure and Table 2). 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. Individuals detected to have cardiac arrhythmia were comparable with those who did not with regard to baseline characteristics (eTable 2 in Supplement 1). None of the participants reported dizziness, palpitations, or chest pain. One participant experienced symptoms consistent with acute mountain sickness; none of the participants experienced high-altitude pulmonary edema or high-altitude cerebral edema (eTable 3 in Supplement 1).
Discussion
The SUMMIT study systematically explored the incidence of cardiac arrhythmia in healthy young men climbing Mount Everest and confirmed the association between exposure to high altitude and the occurrence of cardiac arrhythmia.1,3 Distinct patterns of arrhythmia can be differentiated at high altitude, for which a combination of several pathophysiological mechanisms may be accountable. A decrease in atmospheric pressure at high altitude results in hypobaric hypoxia and arterial hypoxemia. In the Caudwell Xtreme Everest research expedition, arterial blood samples obtained from 4 climbers at an altitude of 8400 m revealed an oxygen saturation of 54% (range, 34%-70%).4 Autonomous nervous system mechanisms are thought to play a key role in the development of bradyarrhythmia in the hypobaric hypoxic environment. Periodic breathing, which is frequently observed in healthy mountaineers at high altitude during sleep, is thought to exacerbate antagonistic autonomic responses to hypoxemia and promote a conflict between the parasympathetic pathway and the sympathetic driver mediated by the peripheral chemoreflex.5 In an experimental model of healthy individuals, voluntary apnea at an altitude of 5050 m unmasked the cardiac autonomic conflict and was shown to induce bradyarrhythmia.6 Hypoxia-related periodic breathing in healthy individuals at high altitude shares mechanistic similarities with sleep apnea syndrome, which has been strongly associated with the incidence of bradyarrhythmia.7 Moreover, the physiologic response to altitude features mechanistic parallels with the diving reflex involved in the occurrence of bradyarrhythmia in breath-hold divers.8
Tachyarrhythmias were less frequently documented in our study and may be related to electrolyte imbalances due to hyperventilation. Premature ventricular complexes and nonsustained ventricular tachycardias have been reported under strenuous exercise at high altitudes in previous studies.9,10,11 Respiratory alkalosis is associated with hypokalemia and hypocalcemia. Early afterdepolarizations in hypokalemia induce premature ventricular complexes, which precipitate ventricular tachycardia.12 At the same time, hypocalcemia prolongs the QT-interval and predisposes to ventricular arrhythmia.13 Dehydration may further exacerbate the pro-arrhythmic milieu by augmenting existing electrolyte imbalances.
The proportion of individuals with arrhythmia was stable across altitude levels. Supplemental bottled oxygen may have blunted the effect of hypoxia during exercise at extreme altitudes. Most rhythm disturbances were recorded at an altitude less than 7300 m at which a majority of climbers did not use supplemental bottled oxygen, whereas numerically fewer rhythm disturbances occurred on supplemental bottled oxygen, despite higher altitude.
Limitations
The current analysis must be understood in light of a number of constraints. First, the use of supplemental bottled oxygen during exercise may have mitigated the risk of tachyarrhythmia, whereas more than half of all bradyarrhythmias occurred during periods of rest when no supplemental oxygen was used by a majority of participants. In addition, all study participants underwent acclimatization over several weeks before the climb; the risk of rhythm disturbances may be even higher during acclimatization to high altitude and in people less conditioned to high altitude. Second, the study included only men and climbers with previous exposure to extreme altitude, which may have introduced selection bias. The latter may have favored individuals who tolerated altitude well in the past. Third, the sample size of the SUMMIT study was modest, and we found no association of cardiac arrhythmia with clinical events.
Conclusions
The results of the SUMMIT study showed a substantial incidence of cardiac arrhythmia at extreme altitude. The potential implications of the observed rhythm disturbances need to be explored in future studies.
eMethods
eTable 1. Definitions of bradyarrhythmia and tachyarrhythmia
eTable 2. Characteristics of the cohort
eTable 3. Safety end points and symptoms
eFigure 1. Device specifications of the wearable continuous electrocardiogram patch.
eFigure 2. Bar graph showing the incidence of the primary end point during the climb from basecamp to the summit of Mount Everest and back and incidence of arrhythmias
eFigure 3. Incidence of arrhythmias during periods of rest or activity
eFigure 4. Spaghetti plot illustrating P-P intervals preceding bradyarrhythmia and respective P-P interval durations in milliseconds
eFigure 5. Mean QTc max duration per altitude level
eFigure 6. Examples of recorded arrhythmias in third-degree atrioventricular block and ventricular tachycardia
Data sharing statement
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Associated Data
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Supplementary Materials
eMethods
eTable 1. Definitions of bradyarrhythmia and tachyarrhythmia
eTable 2. Characteristics of the cohort
eTable 3. Safety end points and symptoms
eFigure 1. Device specifications of the wearable continuous electrocardiogram patch.
eFigure 2. Bar graph showing the incidence of the primary end point during the climb from basecamp to the summit of Mount Everest and back and incidence of arrhythmias
eFigure 3. Incidence of arrhythmias during periods of rest or activity
eFigure 4. Spaghetti plot illustrating P-P intervals preceding bradyarrhythmia and respective P-P interval durations in milliseconds
eFigure 5. Mean QTc max duration per altitude level
eFigure 6. Examples of recorded arrhythmias in third-degree atrioventricular block and ventricular tachycardia
Data sharing statement
