The viable myocardium is the myocardium with reversible myocardial injury and contractile reserve. In most previous studies, viable myocardium was assessed by establishing a myocardial ischemia-reperfusion model, often overlooking the presence of viable myocardium after coronary ischemia. After reperfusion, there is no reliable basis for the recovery of some myocardial function. This study aimed to represent myocardial viability and evaluate the reliable and sensitive multilayer strain and stress echocardiography indicators of myocardial contractile function and viability after ischemia and LDDSE. The results suggest that layer-specific strain of STI combined with LDDSE can accurately evaluate the changes in myocardial systolic function and viability in rabbits with different ischemic durations. These could provide insights into the selection of an appropriate STI indicators for future reperfusion experiments. These results are supported by Ben DA et al.[20].
Myocardial viability is different according to ischemic time[20, 21], but there are few studies on myocardial viability and myocardial function reserve after different ischemic times. Studies showed that ischemia could lead to coronary microvascular dysfunction, and the degree of damage is affected by the duration of ischemia[22, 23]. The ischemic myocardium has more lactic acid production, which increases with the increase of ischemia time. In this study, the number of viable myocardium segments decreased with increasing blocking time. The GSLsys-endo, GSLsys-mid, GSLsys-epi, and blocking segments of SLsys-endo, SLsys-mid, and SLsys-epi in the 120-min group with fewer viable myocardium segments were significantly lower than in the 60-min and 90-min groups, suggesting that different blocking times may have different effects on the mechanical function of the viable myocardium.
In recent years, with the deepening of research on the mechanism of cardiovascular disease, some studies have found that oxidative stress is closely related to the occurrence and development of various cardiovascular diseases, which may be one of the common mechanisms of cardiovascular disease and play a certain role in the course of atherosclerosis[24, 25]. SOD is one of the oxygen free radical scavengers in the body, reducing the damage of oxygen free radicals on cells[26]. Its activity can be used to assess the degree of oxygen free radical scavenging ability. MDA is a product of lipid peroxidation, and its amount can reflect the degree of oxidative stress in the body[27]. In this study, with the increase of coronary artery occlusion time, the activity of SOD decreased, and the activity of MDA increased, indicating that the ability of cardiomyocytes to scavenge oxygen free radicals decreased and the oxidative stress response increased with the increase of ischemia time, also reflecting the degree of ischemia and hypoxia in cells[28].
In addition, studies showed that active inflammatory reaction in atherosclerotic plaques might be another important risk factor for the increasing severity of coronary heart disease[29]. TNF-α is an important inflammatory cytokine and plays an important role in inducing local adhesion, aggregation, activation, and damage of inflammatory cells. In this study, with the increase of coronary artery occlusion time, the TNF-α content of rabbits in each group increased significantly. This phenomenon might be caused by an increase in reactive oxygen species due to the action of inflammatory cytokines after acute myocardial infarction. In turn, the increase in reactive oxygen species increases myocardial damage, which leads to a vicious circle that further aggravates myocardial damage over time.
Szymczyk et al. [30]considered that the absolute value of the longitudinal strain has a higher prediction probability for the corresponding contraction reserve of the viable myocardium, and the larger the absolute value of the longitudinal strain may correspond to a better reserve function. In this study, the absolute value of GSLsys in each layer of the left ventricle and SLsys-endo, SLsys-mid, and SLsys-epi of the viable myocardium after LDDSE were increased in all groups. However, in the infarcted segments, the absolute values of SLsys-mid and SLsys-epi in 60-min group and SLsys-epi in 90-min group were higher than after ligation. And there were no significant differences in SLsys-endo of 60-min group, SLsys-endo and SLsys-mid of 90-min group and all of 120-min group in infarcted segments after LDDSE. These indicated that it was transmural myocardial infarction in the 120-min group and subendocardial myocardial infarction in the 60-min group. And this was also confirmed by histopathological findings. The GSLsys and the affected segmental SLsys in the 120-min group were significantly lower than in the other two groups after ligation and LDDSE. The probability of ventricular fibrillation in rabbits increased significantly during the experiment, and mortality increased consequently. Therefore, after comprehensive consideration, we may prefer to use the 90-min coronary artery occlusion model as the next acute infarction reperfusion animal model. So the multilayer strain and stress echocardiography may represent myocardial viability in different layers and MI model types in rabbits. The results could help in the selection of an appropriate animal model for future reperfusion experiments.
In the present study, the viability and functional reserve of myocardium after different ischemic durations by multilayer strain and stress echocardiography, which can reflect the changes of whole and local myocardial function and myocardial cell damage before and after coronary artery occlusion. This technique is non-invasive, objective, and not affected by the operator, which is more suitable for clinical use[18]. The changes of SLsys-endo under the endometrium were the most pronounced and consistent with pathological findings. So SLsys-endo could refer as the indicator to early detect the subendocardial myocardial ischemia and infarction.
This study has limitations. First, the number of animals included in this study was limited. The results of this study should be confirmed by a larger sample size study. STI stratified strain requires a high frame rate and clear display of the ventricular endocardium, myocardium, and epicardium, which requires high image quality, but some images were unclear due to the rapid heart rate of rabbits. Rabbit anatomy and coronary angiography are similar to humans, but there are still differences.