The electrocardiogram at extreme altitude: experience on Mt. Everest.

The American Medical Research Expedition to Mt. Everest provided a unique opportunity to record 12-lead resting ECGs in one of the largest groups studied to date at extreme altitude (19 men, aged 25 to 52 years). Twelve of the 19 subjects had four recordings breathing ambient air: May, 1981, at sea level; September at base camp (5400 meters); October at camp 2 (6300 meters); and January through May, 1982, after descent. Five subjects had no recording at camp 2 and two of them had no postdescent record. In the 12 subjects in whom all four recordings were obtained, data were analyzed by means of a two-way analysis of variance. Resting heart rate increased from 57 +/- 11 (SD) to 70 +/- 12 bpm at base camp and to 80 +/- 11 bpm at camp 2 (p less than 0.001). P wave amplitude in standard lead II increased from 0.09 +/- 0.06 to 0.13 +/- 0.045 mv at camp 2 (p less than 0.05); QTc decreased from 424 +/- 72 to 318 +/- 48 msec (p less than 0.001). Mean frontal plane QRS axis increased from +64 +/- 18 degrees to +78 +/- 20 degrees at base camp (p less than 0.001) and to +85 +/- 28 degrees at camp 2 (p less than 0.001). At extreme altitude, three subjects exhibited right bundle branch conduction disturbances and three others showed changes consistent with right ventricular hypertrophy. Seven developed flattened T waves and four developed T wave inversions. One developed premature ventricular beats and one developed premature atrial beats.(ABSTRACT TRUNCATED AT 250 WORDS)