• central sleep apnoea;
  • high altitude;
  • loop gain;
  • partial arterial carbon dioxide concentration;
  • sleep


Background and objective:  Loop gain is an engineering term that predicts the stability of a feedback control system, such as the control of breathing. Based on earlier studies at lower altitudes, it was hypothesized that acclimatization to high altitude would lead to a reduction in loop gain and thus central sleep apnoea (CSA) severity.

Methods:  This study used exposure to very high altitude to induce CSA in healthy subjects to investigate the effect of partial acclimatization on loop gain and CSA severity. Measurements were made on 12 subjects (age 30 ± 10 years, body mass index 22.8 ± 1.9, eight males, four females) at an altitude of 5050 m over a 2-week period upon initial arrival (days 2–4) and following partial acclimatization (days 12–14). Sleep was studied by full polysomnography, and resting arterial blood gases were measured. Loop gain was measured by the ‘duty cycle’ method (duration of hyperpnoea/cycle length).

Results:  Partial acclimatization to high-altitude exposure was associated with both an increase in loop gain (duty cycle fell from 0.60 ± 0.05 to 0.55 ± 0.06 (P = 0.03)) and severity of CSA (apnoea-hypopnoea index increased from 76.8 ± 48.8 to 115.9 ± 20.2 (P = 0.01)), while partial arterial carbon dioxide concentration fell from 29 ± 3 to 26 ± 2 (P = 0.01).

Conclusions:  Contrary to the results at lower altitudes, at high-altitude loop gain and severity of CSA increased.