High altitude trekking has been associated with hypoxemia, moderate tachycardia and resting electrocardiographic changes. We performed ambulatory electocardiographic monitoring (24 h) on nine men and two women (age range 24 to 64 years) at 4700 m to study a varied population for cardiac rate, rhythm, and possible ischemic effects during physically demanding trekking. Six participants, randomly chosen, were taking pentoxifylline. Sinus tachycardia was sustained particularly in the five older trekkers (mean age 51 years). Their maximum heart rates during trekking averaged 91% of the individual predicted maximum heart rates or 154 beats per minute; their highest mean one hour rates averaged 78% of predicted maximum heart rates or 132 beats per minute. During sleep at 4700 m, the differences between the nadir and the mean rate were significantly (p = .02) greater than at baseline altitude (1524 m). Ambulatory electrocardiographic abnormalities were minimal and no significant drug effects were observed.
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