Hypoxic preconditioning of ischaemic canine myocardium

Objective: The aim was to test whether a brief period of non-ischaemic hypoxia can precondition myocardium. Methods: 60 anaesthetised adult mongrel dogs of either sex underwent 60 min occlusion of the left anterior descending coronary artery, followed by 5 h reperfusion. In treated groups, hearts were either preconditioned with 5 min coronary perfusion with hypoxic blood [O₂ content 9.2(SEM 0.6) ml·litre^-1] or 5 min occlusion followed by a 10 min reperfusion period prior to 60 min occlusion. The effect of these treatments on myocardial infarct size and regional contractile function was assessed. Results: Infarct size, determined by tetrazolium staining, as a percentage of anatomical area at risk was markedly decreased in hypoxia preconditioned hearts, at 7.2(1.8)% v 22.4(4.6)% in controls (p<0.01), but did not differ from ischaemia preconditioned hearts [4.6(1.7)%; p<0.01 v control]. Anatomical area at risk, expressed as a percentage of left ventricular mass, and collateral blood flow to the inner two thirds of the ischaemic wall did not differ among the groups. Regional contractile function was depressed following ischaemic preconditioning but not following hypoxic preconditioning. During reperfusion following 60 min occlusion, marked paradoxical systolic lengthening was evident in ischaemia preconditioned and control hearts but not in hypoxia preconditioned myocardium. Conclusions: Five minutes of hypoxic and ischaemic preconditioning were equipotent in preventing infarction, whereas ischaemic preconditioning caused a greater decrement in postischaemic contractile function.