In vivo MRI and ex vivo histological assessment of the cardioprotection induced by ischemic preconditioning, postconditioning and remote conditioning in a closed-chest porcine model of reperfused acute myocardial infarction: importance of microvasculature

Abstract

Background: Cardioprotective value of ischemic post- (IPostC), remote (RIC) conditioning in acute myocardial infarction (AMI) is unclear in clinical trials. To evaluate cardioprotection, most translational animal studies and clinical trials utilize necrotic tissue referred to the area at risk (AAR) by magnetic resonance imaging (MRI). However, determination of AAR by MRI' may not be accurate, since MRI-indices of microvascular damage, i.e., myocardial edema and microvascular obstruction (MVO), may be affected by cardioprotection independently from myocardial necrosis. Therefore, we assessed the effect of IPostC, RIC conditioning and ischemic preconditioning (IPreC; positive control) on myocardial necrosis, edema and MVO in a clinically relevant, closed-chest pig model of AMI.

Methods and results: Acute myocardial infarction was induced by a 90-min balloon occlusion of the left anterior descending coronary artery (LAD) in domestic juvenile female pigs. IPostC (6 × 30 s ischemia/reperfusion after 90-min occlusion) and RIC (4 × 5 min hind limb ischemia/reperfusion during 90-min LAD occlusion) did not reduce myocardial necrosis as assessed by late gadolinium enhancement 3 days after reperfusion and by ex vivo triphenyltetrazolium chloride staining 3 h after reperfusion, however, the positive control, IPreC (3 × 5 min ischemia/reperfusion before 90-min LAD occlusion) did. IPostC and RIC attenuated myocardial edema as measured by cardiac T2-weighted MRI 3 days after reperfusion, however, AAR measured by Evans blue staining was not different among groups, which confirms that myocardial edema is not a measure of AAR, IPostC and IPreC but not RIC decreased MVO.

Conclusion: We conclude that IPostC and RIC interventions may protect the coronary microvasculature even without reducing myocardial necrosis.

Keywords: Area at risk; Ischemia/reperfusion injury; Ischemic postconditioning; Ischemic preconditioning; Myocardial edema; Remote conditioning.

Fig. 1

Experimental protocol. Isch ischemia only, IPreC ischemic preconditioning, IPostC ischemic postconditioning, RIC remote ischemic conditioning, LAD left anterior descendent coronary artery, TTC triphenyl tetrazolium chloride, MRI magnetic resonance imaging

Fig. 2

The effect of IPreC, IPostC and RIC on myocardial necrosis (a; n = 4–8/group), edema (b; n = 4–9/group) and MVO (c; n = 4–8/group) size as evaluated with cardiac MRI. d-e Representative MRI images of myocardial necrosis and edema. Green line: apicardial outline, red line: endocardial outline, red area: myocardial necrosis, dark blue area: MVO, light blue area: myocardial edema. *p < 0.05 vs. Isch. Isch ischemia only, IPreC ischemic preconditioning, IPostC ischemic postconditioning, RIC remote ischemic conditioning, MRI magnetic resonance imaging, LV left ventricle, MVO microvascular obrstruction

Fig. 3

The effect of IPreC, IPostC and RIC on myocardial necrosis (a; n = 5–6/group) and AAR (b; n = 5–6/group) as evaluated with conventional TTC and Evans blue staining. c Representative images of Evans blue/TTC-stained heart sections from three different hearts indicating myocardial necrosis and AAR (Each image was taken from the apical side of the third 1-cm slice. Images are optimized for visualization of TTC staining in the InfarctSize software.). Orange slice outline, green ventricular chamber, purple AAR, yellow myocardial necrosis. *p < 0.05 vs. Isch. Isch ischemia only, IPreC ischemic preconditioning, IPostC ischemic postconditioning, RIC remote ischemic conditioning, AAR area at risk, TTC triphenyl tetrazolium chloride, MRI LV left ventricle