The polymorphic and contradictory aspects of intermittent hypoxia

Abstract

Intermittent hypoxia (IH) has been extensively studied during the last decade, primarily as a surrogate model of sleep apnea. However, IH is a much more pervasive phenomenon in human disease, is viewed as a potential therapeutic approach, and has also been used in other disciplines, such as in competitive sports. In this context, adverse outcomes involving cardiovascular, cognitive, metabolic, and cancer problems have emerged in obstructive sleep apnea-based studies, whereas beneficial effects of IH have also been identified. Those a priori contradictory findings may not be as contradictory as initially thought. Indeed, the opposite outcomes triggered by IH can be explained by the specific characteristics of the large diversity of IH patterns applied in each study. The balance between benefits and injury appears to primarily depend on the ability of the organism to respond and activate adaptive mechanisms to IH. In this context, the adaptive or maladaptive responses can be generally predicted by the frequency, severity, and duration of IH. However, the presence of underlying conditions such as hypertension or obesity, as well as age, sex, or genotypic variance, may be important factors tilting the balance between an appropriate homeostatic response and decompensation. Here, the two possible facets of IH as derived from human and experimental animal settings will be reviewed.

Sleep apnoea and the brain: a complex relationship

“…Major depression, on the other hand, is associated with exaggerated REM sleep qualities and deficiency in monoamine activity . In OSA, repetitive occlusions of the upper airway lead to intermittent hypoxia and recurrent hypoxemia, typically characterized by short cycles of hypoxia and reoxygenation 28 . However, the patterns vary greatly across patients, and, depending on the idiosyncratic characteristics of each individual, the end results might be either adaptive or maladaptive 28 .…”

“…Their overall net result depends on the stage of this dynamic process, effects on other body systems, cognitive reserve and the idiosyncratic susceptibility of each particular patient (for in depth reviews please refer to 2,15,17,[27][28][29][30][31] ). This is important as different therapeutical approaches might benefit different stages and conversely might…”

“…Sleep Apnoea and the Brain: A Complex Relationship aggravate damage in some patients 2,17,28 . To date, the contribution of fragmented sleep to cognitive and emotional performance of OSA patients has been largely overlooked 2 .…”

Fitness and therapeutic potential of intermittent hypoxia training: a matter of dose

“…In this context, the adaptive or maladaptive responses can be generally predicted by the frequency, severity, and duration of intermittent hypoxia [11]. However, the presence of underlying conditions such as hypertension or obesity, as well as age, sex, or genotypic varian ce, may be important factors tilting the ba lan ce between an appropriate homeostatic response and decompensation.…”

“…The biological responses to intermittent hypoxia may be adaptive or maladaptive, depending on the severity of the hypoxemia, its frequency of occurrence, its duration, and, importantly, the "pattern" and timing of each of the HbO 2 desaturation/ resaturation cycles [7,11]. Many types of protocol with different numbers of hypoxia episodes, severity, and total exposure duration have been used by investigators, and these combinations may have resulted in various physiological responses.…”