INTRODUCTION: Long QTc syndrome can predispose patients to fatal ventricular arrhythmias. We studied the effects of a rapid ascent and a multiday stay at high altitude on QTc interval. METHODS: We recorded electrocardiograms (ECGs) on study subjects at sea level and then again after a 1-d ascent to 3800 m. Two sea-level ECGs were recorded at the initial screening visit (SL1) and 14 days later (SL2). Altitude ECGs were recorded at approximately 16 and 86 h after arrival. SpO RESULTS: We enrolled 107 subjects: 61 males and 46 females between the ages of 19 and 54 y, with a median age of 34 y. QTcF (QT interval for varying heart rates using Fridericia's formula) at SL1 was 405±17 ms
at SL2, 404±17 ms
at 16 h, 411±18 ms
and at 86 h, 427±24 ms. After 86 h at altitude, 12 participants met the criteria for prolonged QT syndrome, and 61% of subjects had QTc prolongation of >
20 ms from sea-level values. This was independent of changes in serum electrolyte levels, (i.e., sodium, potassium, and calcium), bicarbonate levels, and SpO CONCLUSION: High altitude exposure can lengthen a healthy adult's QTc interval by more than 20 ms. It remains unknown whether this increase might raise the risk of torsades de pointes in people with long QT intervals at baseline.