Sudden cardiac death (SCD) represents a major priority for public health-care systems worldwide. Though early myocardial ischemia (EMI: ischemia during the initial 4 hours after interruption of coronary blood flow) is often a probable explanation for cardiac arrest in SCD cases, an established postmortem method for its diagnosis is still lacking. Within this series of investigations, diagnostic tools of EMI were evaluated in order to be applied in forensic (and clinical) pathology cases of SCD. EMI tissue markers were first investigated in an experimental model of myocardial ischemia (left anterior descending coronary artery ligation in rats) by using conventional methods (immunohistochemistry and Western blotting), as well as an advanced gene-expression investigation tool (nCounterÒ technology). Then they were evaluated in postmortem human cases of EMI and myocardial infarction (MI). These studies revealed the great interest of some tissue markers for the diagnosis of EMI in the experimental model, because of their early expression and sensitivity, as it was evidenced for dephosphorylated connexin 43 (Cx43) and JunB (expressed in ischemic rat myocardium as early as 15 and 30 minutes, respectively). On the other hand, when applied to human postmortem cases of EMI and MI these same markers lost their specificity, being expressed not only in cases of EMI and MI, but also in deaths by hanging (global hypoxia). Nonetheless, their tissular distribution in ischemia and global hypoxia revealed distinct patterns, with an expression localized in the subendocardium in ischemic cases and diffuse to all the myocardium in hypoxic cases. In another investigation, as a complementary diagnostic tool, we explored, with encouraging results, the feasibility of multi-phase postmortem CT angiography (MPMCTA) for the diagnosis of MI (visualized as a regional enhancement of the affected myocardium) and for guiding the sampling for histological analyses. In this panel of investigations we also introduce, for the first time in the field of MI diagnosis, an advanced technology (mass spectrometry immunohistochemistry: MS-IHC) for the simultaneous detection and quantification of multiple antibodies/markers (multiplexing) of MI/ischemia (troponin, myoglobin, fibronectin, C5b-9, dephosphorylated Cx43, JunB and VEGF-B) in the same tissue section. An exhaustive review on molecular changes occurring in the myocardium during EMI accomplishes the series of the studies presented in this thesis, highlighing the potential of some markers and providing an impulse for future investigations in the field.