Comparing del Nido and St. Thomas II cardioplegia in a rat ischemia–reperfusion model: Histopathology, mitochondria, and TEM analysis

Myocardial ischemia-reperfusion (IR) injury remains a major challenge in cardiac surgery, and comparative histological and ultrastructural data on cardioplegia solutions are limited. This study compared the myocardial protective effects of St. Thomas II and del Nido cardioplegia in a controlled rat IR model, focusing on inflammation, mast cell dynamics, and subcellular preservation. Twenty-four Wistar Albino rats were randomized to Control, St. Thomas II, or del Nido groups. After 90 minutes of ischemia and 30 minutes of passive reperfusion, myocardial tissue was analyzed by hematoxylin-eosin, toluidine blue, and transmission electron microscopy. Outcomes included mast cell counts, leukocyte infiltration, karyolysis, and ultrastructural measures (Flameng score, crista density, basement membrane thickness). Both cardioplegia groups preserved myocardial morphology and attenuated inflammatory changes versus control. Light microscopy revealed a consistent mast cell density and reduced karyolysis in hearts treated with cardioplegia, with no significant differences observed between St. Thomas II and del Nido solutions. Conversely, transmission electron microscopy (TEM), the primary endpoint of this study, demonstrated enhanced mitochondrial and endothelial preservation in the del Nido group, as evidenced by significantly lower Flameng scores and increased crista density compared to both St. Thomas II and control groups (p < 0.05). In conclusion, both solutions reduced early IR-related injury, but del Nido provided a significant ultrastructural advantage on TEM despite similar routine light-microscopic findings.