The prevalence of obstructive sleep apnea (OSA) is high in the Veteran population and this disorder is linked to numerous cardiovascular, neurocognitive and metabolic abnormalities. Thus, OSA is a major health concern in the Veteran population. Treatment of OSA in many cases does not lead to significant improvements in outcome measures. This inadequacy may be a consequence of reduced treatment adherence with continuous positive airway pressure (CPAP) or because the effect of CPAP on outcome measures is small or absent in some patients despite adequate adherence. Consequently, innovative therapies that directly impact co-morbidities linked to OSA or that increase CPAP adherence could lead to improved outcome measures. In the recent funding cycle, the investigators established that repeated daily exposure to mild intermittent hypoxia (MIH) coupled with CPAP modifies autonomic nervous system activity and dramatically decreases blood pressure compared to CPAP treatment alone. Because MIH was coupled with CPAP, the independent effect of MIH on blood pressure was not established. Moreover, it was not established if these outcomes were sustained for a prolonged time period (i.e. weeks to months).

Although the investigators obtained some indirect evidence that modifications in autonomic nervous system activity were coupled to the reduction in blood pressure, the investigators did not establish if modifications in microvascular function were evident. Microvascular dysfunction together with sympatho-vagal imbalance may have consequences not only for peripheral vascular resistance and blood pressure but also for muscle perfusion and metabolism, thereby limiting exercise performance and increasing fatigability in patients with OSA. Thus, reductions in blood pressure and improvement in microvascular function following treatment with MIH might serve to improve exercise capacity and reverse performance fatigue in individuals with OSA.

Besides its potential effect on autonomic and cardiovascular function, the investigators and others previously established that acute exposure to MIH initiates sustained increases in upper airway muscle activity in humans. This sustained increase is a form of respiratory plasticity known as long-term facilitation. However, in the absence of CPAP the investigators have shown that acute MIH immediately prior to or during sleep leads to increases in apnea severity. This might occur because the manifestation of long-term facilitation is absent in the presence of hypocapnia. Hypocapnia can be induced during sleep by the initiation of another form of plasticity known as progressive augmentation. However, it is possible that the combination of daily exposure to MIH administered many hours before the sleep period may mitigate the effects of progressive augmentation leading to increased upper airway stability.

Independent of this possibility, the investigators showed in the previous funding cycle that increased upper airway stability following treatment with MIH was coupled to a reduction in therapeutic CPAP and improved adherence. However, improved adherence to CPAP might also be linked to an increase in the arousal threshold to both respiratory and non-respiratory stimuli. All the uncertainties outlined above will be addressed in the present proposal.