Abstract
Intermittent hypoxia (IH) is defined as repeated episodes of hypoxia interspersed with normoxic periods. Since IH is generally associated with obstructive sleep apnoea (OSA) and the related adverse effects, the term interval hypoxic therapy / training (IHT) was introduced to distinguish between OSA and the therapeutic use of IH. For many years Russian physicians used short-term interval hypoxia for clinical purposes (Serebrovskaya, 2002). The training program mostly consists of interval hypoxic-normoxic cycles of short duration (approximately 5 min) performed for 1 hour a day, up to 5 times a week for several weeks. The main rationale for its use is the potential cross-protective effect, which infers that adaptations to one stressor may provide resistance to another. During many years of clinical experience, various positive effects have been described in mostly uncontrolled study settings. Interval hypoxia training for instance has been found to provoke health beneficial ventilatory, metabolic and sympathetic nervous system adaptations (Serebrovskaya, 2002; Serebrovskaya et al., 2008). Recently, randomised controlled trials confirmed the potential positive effects of interval hypoxia. Burtscher et al. found that IHT reduced heart rate, systolic blood pressure, blood lactate, and rate of perceived exertion, and increased arterial oxygen saturation and arterial oxygen content in patients with COPD or CAD (Burtscher et al., 2010). Additionally, IHT increased total hemoglobin mass and improved lung diffusion capacity in COPD patient (Burtscher et al., 2009). Moreover, activation of the HIF pathway may further contribute to the positive health effects (Dale et al., 2014) even though potential detrimental effects (e.g., tumor growth) have to be equally considered (Schito and Semenza, 2016), especially when IHT is applied in patient groups. In summary, literature suggests that repeated and well-dosed hypoxic-normoxic cycles might evoke beneficial adaptations of the haematological, the neurohumoural, the antioxidant, and cardio-respiratory systems, resulting in improved health and exercise tolerance (Burtscher et al., 2010). Mechanisms pertaining to beneficial adaptations for health remain speculative and need to be addressed further in future investigations.