Assessment of Cardiac Arrhythmias at Extreme High Altitude Using an Implantable Cardiac Monitor
REVEAL HA Study (REVEAL High Altitude)
It has been suggested, although still unproven, that exposure at high altitude (HA) is proarrhythmic and could potentially contribute to an increased risk of sudden cardiac death.1,2 However, limited data are available to substantiate this claim, particularly at >5000 m. We hypothesized that extreme HA leads to an increased risk of pathological cardiac tachyarrhythmias, detected using an implantable cardiac monitor (ICM).
Sixteen healthy adult white male British military servicemen underwent continuous ECG monitoring using a Reveal LINQ ICM (Medtronic Ltd) for ≥7 weeks before, during, and >8 weeks after an attempted summit of Mount Dhaulagiri (8167 m). They were required to have a normal 12-lead ECG and transthoracic echocardiogram at recruitment and were excluded if they had a history of cardiac arrhythmia. They underwent written informed consent, and the study was approved by the Ministry of Defense Research and Medical Ethics Committee. The participants flew from the United Kingdom to Kathmandu, Nepal (1400 m, days 1–2), then by road (days 3–4) to 2679 m. Thereafter, they trekked carrying moderate loads to 3720 m (day 5), 4150 m (day 7), and 5140 m, where they remained (days 11–24) for attempts on 6035- and 6800-m peaks. One subject aborted at 4100 m because of severe gastrointestinal symptoms. On day 25, 5 subjects descended, and 10 climbers remained at 4800 m for an attempted summit of Mount Dhaulagiri over days 26 to 51. The subjects were monitored wirelessly before and after departure (Medtronic MyCareLink Monitor) and during trekking using a portable Medtronic Programmer every 2 to 5 days, depending on environmental conditions.
The subjects were 35.1±6.6 (24–48) years of age. Fifteen (93.8%) achieved an altitude of ≥6035 m, 6 to 6800 m, 1 to 7100 m, and 3 to 7550 m. Unfortunately, an attempted summit of Mount Dhaulagiri became impossible because of adverse weather conditions. Spo2 significantly fell at increasing HA from 96.4±1.6% at 1400 m to 93.2±2.8% at 2650 m, 88.8±3.5% at 4100 m, 80.6±5.0% at 5140 m, and 78.1±4.5% at 5340 m (ordinary analysis of covariance P<0.0001). The ICM rhythm-detection settings are shown in Table. Significant rhythm abnormalities were observed in 9 of 16 subjects (56.3%) at HA and only at ≥4100 m. Symptom-related device activation was triggered on 18 occasions in 8/16 subjects at HA and related to extreme breathlessness and palpitations. Subject 5 developed an episode of nocturnal symptomatic rapid atrial fibrillation at 4100 m during the initial ascent phase, which occurred immediately after drinking cold water. It lasted for 282 minutes at a mean ventricular rate 133/minute. Subject 4 developed an episode of supraventricular tachycardia lasting for 30.8 seconds (mean rate 207/minute). It occurred immediately on attempting to lift a 30-kg load at 5200 m and was associated with sudden and transient light-headedness and breathlessness.
Significant pauses (>3 seconds) were identified at HA in 8 of 15 (53.3%) subjects at ≥4800 m only, with none detected in any subjects below this altitude (Fisher’s Exact Test P=0.0008). Overall, 82 pauses (3.0–7.0 seconds) were noted, which were sinus in 80 with evidence of high-grade heart block in 2 cases (mean number10.3±14.1; range 1–41) (Table). The number of pauses increased with altitude gain from 0 at <4800 m to 4.2 at 4800 m and 14.3 at >6000 m (Kruskal-Wallis Test P<0.0001) with 19.3±20.6 pauses at 7550 m (n=3) versus 1.9±4.2 among the rest of the subjects (n=13; χ-square test P=0.007). The number of pauses increased with duration of HA exposure: 6 during the first 17 days (tercile, 15–16 subjects), 29 during days 18 to 34 (10–15 subjects), and 47 (10 subjects) during days 35 to 51. The pauses typically occurred after cyclic periods of heart rate acceleration and then deceleration preceding it.
This study is the first to convincingly demonstrate the proarrhythmic risks of significant HA and, to the author’s knowledge, the first to continuously monitor healthy subjects > 6325 m at terrestrial HA. In the only previous ICM study at HA, 9 subjects were studied using first-generation reveal ICM, which lacked auto-detection capabilities, and only 2 subjects were assessed at 6325 m.1 They observed 1 short-lived episode of atrial flutter at 150/minutes (8.5 minutes) immediately after a severe exertion at 4500 m. The episode of nocturnal atrial fibrillation detected in our study would suggest vagally mediated atrial fibrillation. We would also postulate that the pauses observed in our study were likely physiological and also related to the effects of increased nocturnal vagal tone and sleep-disordered breathing, which are well recognized at HA.3,4 We believe the episode of supraventricular tachycardia relates to the combination of sympathetic activation, hypoxia, and sudden explosive exercise at HA. In addition to the factors outlined earlier, the proarrhythmic effects of HA may be partly explained by other factors, including acclimatization, changes in heart rate variability, sleep deprivation, dehydration, and anxiety.5
In conclusion, HA exposure to ≥4100 m is associated with significant brady- and tachyarrhythmias in healthy adult men supporting a potential proarrhythmic risk. No link was detected between HA and sustained ventricular arrhythmias linked to an increased risk of sudden cardiac death.
Sources of Funding
This study was supported by a project grant from Medtronic to fund the costs of the ICMs.
Circulation is available at http://circ.ahajournals.org.
- © 2017 American Heart Association, Inc.
- Lombardi C,
- Meriggi P,
- Agostoni P,
- Faini A,
- Bilo G,
- Revera M,
- Caldara G,
- Di Rienzo M,
- Castiglioni P,
- Maurizio B,
- Gregorini F,
- Mancia G,
- Parati G