Influence of acute exposure to high altitude and hypoxemia on ventricular stimulation thresholds in pacemaker patients

Cardiac stimulation threshold of implanted pacemakers may be influenced by a variety of endogenous and exogenous factors. High altitude provokes hypox emia, which may change stimulation thresholds, besides causing other import ant physiological changes, The aim of our study was to investigate the infl uence of high altitude on ventricular stimulation thresholds in pacemaker p atients. Thirteen patients (10 men; aged 65.5 +/- 4.8 years) with implanted single chamber pacemakers (nine with Pacesetter Regency SR + with the Auto capture 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 perfo rmed every 500 m to measure stimulation threshold at each step. After a sta y 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 de termined. 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 pO(2) 10.7 +/- 1.1 vs 5,5 +/- 0.3 kPa), pC O(2) (5.3 +/-. 0.3 vs 4.7 +/- 0.4 kPa), oxygen saturation measured by arter ial 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 an d the strength-duration curve remained unchanged in all patients throughout the study. In conclusion, exposure to an altitude of 4,000 m above sea lev el with resultant hypobaric hypoxemia has no impact on ventricular stimulat ion thresholds. Therefore, in regard to the safety of pacing, pacemaker pat ients can safely be exposed to this altitude.