The sleep electrocardiogram at extreme altitudes (Operation Everest II)

Area of Science:

• Cardiology
• Sleep Medicine
• Altitude Physiology

Background:

• Understanding the physiological impact of extreme altitudes on cardiovascular function during sleep is crucial.
• Previous research has indicated potential alterations in heart rate and rhythm at high altitudes, but detailed sleep studies are limited.

Purpose of the Study:

• To investigate the effects of simulated extreme altitude exposure on heart rate and rhythm during sleep.
• To identify specific changes in cardiac function, such as bradycardia, heart rate variability, and arrhythmias, under hypobaric conditions.

Main Methods:

• Continuous sleep monitoring using electrocardiography and respiration recordings in 8 healthy young men over 40 days.
• Simulated ascent to various altitudes (5,490 m, 6,100 m, 7,620 m) in a hypobaric chamber, with baseline recordings at sea level (760 torr).
• Analysis of heart rate patterns, including sinus bradycardia, heart rate cycling, and arrhythmias, in relation to sleep and breathing patterns.

Main Results:

• Sinus bradycardia (mean 41 beats/min) occurred during sleep at altitude, compared to sea level (44 beats/min).
• Heart rate cycling, linked to periodic breathing, was observed in 14 of 17 altitude studies, characterized by alternating bradycardia and tachycardia.
• Arrhythmias, including transient bradycardia (down to 20 beats/min), sinus pauses, and blocked P waves, were prevalent during sleep at altitude but absent at sea level.

Conclusions:

• Sleep at extreme altitudes commonly induces heart rate cycling associated with periodic breathing and bradyarrhythmias in healthy young individuals.
• The observed cardiac events suggest significant autonomic nervous system dysregulation during sleep under hypobaric hypoxia.
• Further research is needed to elucidate the precise mechanisms underlying these altitude-induced sleep-related cardiac arrhythmias.