Effects of intermittent versus chronic hypoxia on myocardial ischemic tolerance

Hypoxia, or low supply of oxygen with respect to need, represents a critical risk factor that augments severity and impairs outcome of myocardial ischemia. However, systemic hypoxia is sometimes a cardioprotective rather than risk factor. Three different models of hypoxia can be distinguished, each with its own specific effects on myocardial tolerance to ischemia: chronic hypoxia, chronic hypoxia with aeration and intermittent hypoxia. Severe chronic hypoxia invariably leads to depressed myocardial tolerance to ischemia, but moderate chronic hypoxia may be considered as cardioprotective. Chronic hypoxia with aeration is almost always protective. The degree of protection afforded by intermittent hypoxia depends on many factors including time, severity and duration of the hypoxic challenge. Although the precise molecular background underlying such differential responses still needs to be worked out, some signaling pathways are being discussed, whose recruitment by one or more of the three models of hypoxia appears prominent in conferring protection against ischemia-reperfusion injury.