Myocardial hibernation. A form of endogenous protection?

The origins and mechanisms of myocardial hibernation are obscure and controversial. Indeed, there is even debate over whether the hibernating heart is 'ischaemic'. In attempting to clarify this last point, it is clear that there is no universally agreed definition of ischaemia. It is proposed that much of the confusion can be resolved distinguishing between biochemical and physiological ischaemia and ascertaining whether the tissue metabolism is in steady state equilibrium or is progressively deteriorating. On the basis of this, the hibernating heart would be designated as physiologically ischaemic but it would not exhibit the characteristics of biochemical ischaemia. This distinction also allows one to argue that hibernation is an adaptive phenomenon in which cardiac metabolism and function are down-regulated to match the available energy supply (perfusion-contraction matching). Such an adaptive response would be expected to avoid the occurrence of tissue injury but it would not be expected to prevent the occurrence of adaptive morphological changes that always occur with prolonged inactivity of muscle. However, the morphological characteristics of hibernation are controversial and, like so many other aspects of the hibernation controversy, the problem may not be resolved until an acceptable animal model of hibernation has been developed.