FIG. 5.

Nocturnal ventilatory phenotypes during sleep at high altitude. Diagram illustrates the mechanism by which nocturnal ventilatory phenotypes during sleep at HA develop. Sleep can be considered a physiological stress for the respiratory system, which aggravates with altitude hypobaric hypoxia. This increased burden during sleep at HA can trigger compensatory mechanisms for the maintenance of nocturnal S_pO₂. All subjects experience nocturnal hypoxemia during sleep, but some experience sharp decreases in the mean nocturnal S_pO₂ with few central apneas (if they have a high arousal threshold), whereas others will maintain the mean nocturnal S_pO₂ but demonstrate high frequencies of central apneas (low arousal threshold). These two phenotypes are characterized by either sleep continuity at expense of hypoventilation and desaturation, or frequent arousals from nocturnal alterations in gas exchange, thereby maintaining ventilation and oxygenation at expense of sleep continuity.