The Sleep Electrocardiogram at Extreme Altitudes (Operation Everest II) Mark Malconian, DO, Herbert Hultgren, MD, Masao Nitta, MD, James Anholm, MD, Charles Houston, MD, and Howard Fails

To evaluate the effect of sleep at extreme altitudes upon heart rate and rhythm, continuous steep monitoring was performed in 8 normal young men during a 4O-day sknuiated ascent of Mt. Everest in a hypobaric chamber. Recordings were made for 1 hour before steep, during steep and for 1 hour after awakening in ail subjects at.760 torr (sea level), in 7 subjects at 390 totv (5,46O m), in 6 at 347 torr (6,100 m) and in 4 at 282 torr (7,620 m). The foilowing results were obtained: periods of sinus bradycardia occurred during sleep in ail subjects at 3 attftudes with a mean heart rate of 41 f 0.5 beats/min compared to a rate of 44 f 2 beats/min at sea level; cyding of the heart rate, presumably due to periodic breathing, occurred in 14 of 17 studies at altitude but not at sea level (cydes consisted of bradycardii [4O beats/min] for 13 seconds and tachycardia [12O beats/min for 5 seconds]); and arrhythmias were observed in ail subjects during steep and consisted of transient bradycardia (heart rates as low as 20 beats/min), sinus pauses frequently associated with escape rhythms and occasional Mocked P waves. No arrhythmias were observed at sea level. Simultaneous records of respiration and the etectrocardiogram 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 s&ep 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 deftned. (Am J Cardioi 1990;65:1014-1020)

From the United States Army Research Institute of Environmental Medicine, Natick, Massachusetts, The Arctic Institute of North America, Washington, DC, and the Palo Alto Veterans Administration Medical Center, Palo Alto, California. Manuscript received March 9, 1989; revised manuscript received and accepted December 19, 1989. Address for reprints: Herbert N. Hultgren, MD, Palo Alto Veterans Administration Medical Center, 3801 Miranda Avenue, Palo Alto, California 94304.







uring sleep at high altitude, many normal subjects exhibit periodic breathing.1,2 While many observations of this phenomenon have been published, little attention has been directed toward the associated changes in heart rate and rhythm. Cummings and Lysgaard3 recorded their own cardiac rhythm and noted a pronounced cyclic sinus arrhythmia and bradycardia during sleep at 16,500 feet (5,033 m). During the American Medical Research Expedition to Mt. Everest, electrocardiograms recorded during sleep at 21,000 feet (6,300 m) in 8 subjects revealed bradycardia, sinus arrhythmia and variations in heart rate with periodic breathing.4 A recent hypobaric chamber study, “Operation Everest II,” provided an opportunity to study human physiologic responses to simulated altitudes up to 29,028 feet (8,848 m). During the study healthy male subjects were exposed to a gradual reduction in atmospheric pressure over a 40-day period using an ascent profile typical of an actual Everest climb. Resting electrocardiograms were recorded at 5 simulated altitudes including 29,028 feet. Continuous ambulatory monitoring was performed in normal subjects during sleep at sea level and at 3 altitudes. Similar studies under these controlled conditions have never been reported before.



Two separate studies were performed. Study A was carried out in the hypobaric chamber and study B was conducted at the Barcroft Laboratory of the White Mountain Research Station in California. The studies will be considered separately. Study A: Eight healthy male subjects aged 21 to 31 years began the study after giving informed consent. After several days of sea level studies, the subjects entered a large hypobaric chamber at the United States Army Institute of Environmental Medicine in Natick, Massachusetts, where they remained for 40 days while the chamber was gradually decompressed to a pressure of 240 torr, roughly equivalent to the summit of Mt. Everest. While in the chamber they had ad libitum access to a nutritionally balanced diet and were able to exercise daily on a cycle ergometer, a treadmill or a climbing ergometer when not involved in testing. Two subjects were removed from the study after acute hypoxic episodes, 1 at 380 torr and the other at 280 torr. A more detailed account of the test subjects and conditions of exposure has been published.5 Ambulatory monitoring was performed using a 2channel Del Mar Avionics electrocardiorecorder (model 445B). Chest electrodes were used with 2 leads record-

TABLE I Summary of Mean Heart Rates While Awake During Sleep (S) at Sea Level and 3 Simulated Altitudes 760 Torr (Sea Level)

390 Torr (5,490 m)

347 Torr (6,100 m)


TABLE Altitudes

II Incidence

282 Torr










105 76

81.7 64.2 42


High rates and low rates were rates that perslsted lowest transtent heart rate wth duration

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 durin...
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