Though ischemia and infarction are similar processes of varying severity, ST segment depression indicates ischemia and ST segment elevation indicates infarction in the leads representing the affected part of myocardium in EKG.
It is the fundamental fact of ECG that it shows deflection in positive direction when the current flows towards the lead from negative to positive in the interstitium of myocardium. In both ischemia and infarction, there is negative charge in interstitial space in the affected part. When there is ischemia, there is no enough ATP in the cell to run Na+-K+ ATPase which is required for repolarization of the myocardium and because Na+ ions are trapped in the myocardial cells during depolarization, the deficit of Na+ in interstitium produces negative charge. In infarction, there is injury and lysis of the cell membrane, leading to release of proteins and other negatively charged ions into the interstitium. The resting membrane potential of about -60 mv inside the myocardium is literally transferred to interstitium.
The ST segment of ECG represents state of myocardial depolarization and at this time, all the interstitium has negative charge as QRS wave represents myocardial depolarization and T wave represents repolarization, as shown in Fig.2. And the TP segment and the PR segment represents resting (repolarized) state as myocardium repolarizes during T wave and next depolarization occurs during QRS wave. During the resting state, all the interstitium has positive charge in normal heart, as shown in Fig.1. During both the depolarized and repolarized states, there is no net current flow towards or away from the lead and thus there is no deflection in ST, TP and PR segments i.e. they remain isoelectric.
As myocardial ischemia progresses from endocardium to epicardium and the leads records the electrical changes in the interstitium from the epicardial side, during ischemia, there is negative interstitium towards the endocardium and positive towards the epicardium in resting state (Fig.4) and negative throughout the wall during depolarized state (Fig.3). As we can see in the illustrations, there is net current flow towards the lead during repolarized state, which results in positive deflection in TP and PR segments and there is no net current flow during repolarized state which means ST segment will show no deflection. These positive deflections of TP and PR segments makes ST segment appear depressed.
When the ischemia progresses from sub endocardium and becomes transmural, the part of the myocardium affected first and most severely infarcts as illustrated as black areas in Fig. 5 and 6. So when there is ischemia involving whole wall thickness, there is some infarction. In this situation whole of the interstitium is negative during depolarized state, but the affected part’s interstitium is negative during repolarized state as well. So during, depolarized state there is no net current flow, but during repolarized state, there is net current flow away from the lead because affected part’s interstitium is negative while it is positive behind it. This leads to net current flow away from the lead during repolarized state leading to actual depression of TP and PR segments. This makes the ST segment appear elevated.
It seems that whenever there is infarction, there has to be accompanied transmural ischemia to have the ECG manifestation of ST elevation. It can be argued that there can be infarction without transmural ischemia, and indeed this can happen and we get non ST segment elevation MI (NSTEMI) when there is no transmural ischemia but there is enzymatic evidence of infarction. Whenever there is NSTEMI, this model predicts that there has to be ST segment depression. There are other things that can be predicted by this model. The degree of ST segment depression represents the severity of ischemia; degree of ST segment elevation represents bulk of myocardium involved. But because the leads of EKG measures the current flow in one plane, the current vectors might be at varying angle in different subjects and thus exact correlations as predicted cannot be applied.
