Approximately 8 % of the Veteran population in the United States suffers from sleep apnea. Consequences of untreated sleep apnea include increased daytime fatigue, hypertension and stroke. Thus, sleep apnea is a major health concern. One of the primary hallmarks of sleep apnea is exposure to intermittent hypoxia (IH) which occurs as a consequence of central or obstructive apneas. Exposure to IH may lead to neural plasticity (i.e. a change in system performance based on prior experience) of the respiratory and autonomic nervous system. One adaptation that has been shown to manifest itself in animals following exposure to IH is long-term facilitation (LTF) of ventilation and sympathetic nervous system activity (SNSA). This phenomenon is characterized by a gradual increase in respiratory motor activity and SNSA during successive periods of normoxia that separate hypoxic episodes and by activity that persists above baseline levels for up to 90 minutes following exposure to IH. Although LTF of minute ventilation has been well established in animals it has not been observed consistently in healthy humans or in individuals with obstructive sleep apnea. Similarly, although a few studies have shown that exposure to IH leads to increases in SNSA in healthy individuals the magnitude of the response has varied significantly. Findings from animal studies suggest that the manifestation of LTF in humans might in part be dependent on a variety of factors, including prior exposure to IH, arousal state (wake vs. sleep) and gender. Thus, the initial aim of our proposal will establish whether LTF can be induced in healthy humans and individuals with obstructive sleep apnea and whether the magnitude of the response is dependent on those factors mentioned above. Moreover, the initial aim will explore whether the presence of LTF of minute ventilation promotes apnea severity. Animal studies have also indicated that LTF of respiratory and autonomic activity may in part be induced by increases in oxidative stress. Thus, the second objective of our proposal will explore whether administration of an antioxidant cocktail impacts of respiratory and autonomic nervous system plasticity during wakefulness and sleep following IH. Likewise, the second aim will explore whether administration of an antioxidant cocktail mitigates apnea severity following exposure to IH. Establishing whether LTF of minute ventilation exists in individuals with sleep apnea is important since activation of this phenomenon could impact on apnea severity across the night. Similarly, LTF of SNSA activity and possibly long-term depression (LTD) of parasympathetic nervous system activity (PNSA) could ultimately lead to persistent increases in blood pressure and heart rate abnormalities. Furthermore, given that exposure to IH may lead to long-term plasticity of respiratory and autonomic activity that are physiologically detrimental, exploring mechanisms that ultimately lead to treatments that may mitigate or prevent the manifestation of this phenomenon are important.
Narrative The prevalence of obstructive sleep apnea is high in the Veteran population. If not treated promptly, obstructive sleep apnea may result in daytime fatigue which may lead to increased prevalence of accidents while driving or in the workplace. Recent large scale epidemiological studies have shown that the prevalence of excessive daytime sleepiness increases from 15 % in individuals who do not suffer from obstructive sleep apnea to 39 % in individuals with obstructive sleep apnea. Obstructive sleep apnea may also result in the development of hypertension, cerebral and subarachnoid hemorrhage, and ventricular arrhythmias. Results obtained from the Sleep Heart Health Study revealed that subjects with sleep apnea have 42 % greater odds of prevalent coronary vascular disease (i.e. coronary heart disease, stroke and congestive heart failure) compared to individuals with an apnea/hypopnea index of less than 1 event per hour. Thus, a significant amount of empirical evidence suggests that obstructive sleep apnea is a major health concern in the Veteran population. Consequently, determining the mechanisms that may impact on apnea severity and exacerbate autonomic perturbations associated with this disorder is important, as is discovering novel treatments.
