Cardiac stimulation threshold of implanted pacemakers may be influenced by a variety of endogenous and exogenous factors. High altitude provokes hypoxemia, which may change stimulation thresholds, besides causing other important physiological changes. The aim of our study was to investigate the influence of high altitude on ventricular stimulation thresholds in pacemaker patients. Thirteen patients (10 men; aged 65.5 +/- 4.8 years) with implanted single chamber pacemakers (nine with Pacesetter Regency SR+ with the Autocapture feature) were exposed to an altitude of 4,000 m above sea level, as simulated in a hypobaric chamber. Stepwise ascension was performed with a speed of 5 m/s starting at 450 m above sea level. A 5-minute rest was performed every 500 m to measure stimulation threshold at each step. After a stay of 30 minutes at 4,000 m stimulation threshold was measured, followed by a stepwise descent. Pacemaker interrogation and arterial blood gas analysis were performed at 450 and at 4,000 m, and a strength-duration curve was determined. Blood pressure, heart rate, and oxygen saturation were monitored continuously during the study. Ascent to 4,000 m above sea level induced a significant decrease in arterial pO2 (10.7 +/- 1.1 vs 5.5 +/- 0.3 kPa), pCO2 (5.3 +/- 0.3 vs 4.7 +/- 0.4 kPa), oxygen saturation measured by arterial blood gas analysis (95.5% +/- 1.2% vs 79.1% +/- 2.5%), and increase in pH (7.39 +/- 0.02 vs 7.45 +/- 0.04) (P < 0.0001). Stimulation thresholds and the strength-duration curve remained unchanged in all patients throughout the study. In conclusion, exposure to an altitude of 4,000 m above sea level with resultant hypobaric hypoxemia has no impact on ventricular stimulation thresholds. Therefore, in regard to the safety of pacing, pacemaker patients can safely be exposed to this altitude.