The sleep electrocardiogram at extreme altitudes (Operation Everest II)

To evaluate the effect of sleep at extreme altitudes upon heart rate and rhythm, continuous sleep monitoring was performed in 8 normal young men during a 40-day simulated ascent of Mt. Everest in a hypobaric chamber. Recordings were made for 1 hour before sleep, during sleep and for 1 hour after awakening in all subjects at 760 torr (sea level), in 7 subjects at 390 torr (5,490 m), in 6 at 347 torr (6,100 m) and in 4 at 282 torr (7,620 m). The following results were obtained: periods of sinus bradycardia occurred during sleep in all subjects at 3 altitudes with a mean heart rate of 41 +/- 0.5 beats/min compared to a rate of 44 +/- 2 beats/min at sea level; cycling of the heart rate, presumably due to periodic breathing, occurred in 14 of 17 studies at altitude but not at sea level (cycles consisted of bradycardia [40 beats/min] for 13 seconds and tachycardia [120 beats/min for 5 seconds]; and arrhythmias were observed in all subjects during sleep and consisted of transient bradycardia (heart rates as low as 20 beats/min), sinus pauses frequently associated with escape rhythms and occasional blocked P waves. No arrhythmias were observed at sea level. Simultaneous records of respiration and the electrocardiogram at 12,500 feet (3,810 m) in 5 other normal subjects revealed tachycardia occurring during hyperpnea and bradycardia occurring during apnea. Data indicate that during sleep in normal young subjects at high altitude, cycling of the heart rate with periodic breathing is common, as are bradyarrhythmias. The mechanism of these arrhythmias has yet to be defined.