A possible mechanism for electrocardiographically silent changes in cardiac repolarization

Despite the widespread use of electrocardiogram (ECG), changes in cardiac activity resulting from ischemia or altered recovery characteristics sometimes remain electrocardiographically "silent" or are first detectable by techniques that measure ventricular contractility, such as ultrasound or blood pressure. Especially local changes in repolarization can go undetected when ECG electrodes do not lie close to the area of the heart affected. Experiments were performed on an isolated, perfused canine heart suspended in a realistically shaped, instrumented, electrolytic torso tank with the goal of determining some mechanisms for these ambiguities. By recording simultaneously both epicardial and torso tank surface potentials, complete descriptions were obtained of the electrical response to interventions such as coronary occlusions and alterations in pacing site and frequency. One hypothesis was that some interventions produce highly variable ECG responses primarily because of differences in their location within the heart. To test this, the effect was measured of repeating the same intervention as the heart's location and orientation in the tank were varied. A numerical forward solution was also used to investigate variation of torso tank potentials with heart location. The resulting changes in tank surface potentials illustrate how, for example, precordial ST-segment shifts following occlusion change from elevation to depression to become almost undetectable as the heart rotates in the tank. The results suggest that some events are electrocardiographically silent because of the complex geometric relationship of the heart, torso, and site of the lesion, as well as the spatial sampling and analysis techniques used in detection.