Incomplete ECG expression of acute true posterior myocardial infarction, owing to an antecedent anterior infarction

We present a 75-year-old man with an inferior and true posterior (TP) myocardial infarction (MI), who showed disproportionately deeper ST-segment depression (↓ ST) in the precordial electrocardiogram (ECG) leads than the ST-segment elevation (↑ ST) noted in leads 2, 3, and aVF. This suggested that the precordial ↓ ST was not reciprocal to the inferior ↑ ST, but it was indicative of TP ischemic injury. However, the precordial ↓ ST, in subsequent serial ECGs, was not followed up by R waves of increasing amplitude, or tall T waves in the V₁ and V₂ leads, as would be expected had this been a case of evolving TP MI. Nevertheless, TP MI was confirmed by echocardiography and dipyridamole/thallium SPECT myocardial perfusion scintigraphy. These last 2 modalities also revealed evidence of an anterior (A) MI, which had indeed occurred 11 years previously, and was confirmed by a history of hospitalization, serial ECGs, and enzymatic evidence of necrosis. This incomplete ECG expression of TP MI in our patient was felt to be owing to the previous large A MI, which had long ago deprived the heart from requisite healthy myocardium opposite the recent TP MI, for generation of tall R waves or R/S ratio greater than 1 in the V₁ and V₂ leads. This case is presented as an example of electrical cancellation affecting the QRS complexes.