Assessing the pattern of ST-segment depression during subendocardial ischemia using a computer simulation of the ventricular electrogram.

The EG waveforms are know to depend on the properties of transmembrane action potentials (APs) of atrial and ventricular myocytes, the spread of excitation, and the characteristics of the volume conductor. Transmembrane AP is an electromotive generator that plays a central role, and it is the principal responsible for the potential differences that are recorded as an EG. The EG can be considered as the algebric sum of 2 transmembrane APs, that is, the AP of the underlying endocardial region minus the AP of the underlying epicardial region. METHODS: Using an educational PC software (Microsoft Excel), a normal EG was simulated reproducing planimetrically, point-by-point, normal transmembrane APs recorded from the epicardial and endocardial regions in normal animals. The shape and the voltage of the APs were then modified to closely mimic human APs. To simulate typical subendocardial ischemia, we changed the subendocardial AP according to experimental and clinical observations. RESULTS: The reconstruction of EG by the algebric subtraction (endocardial minus epicardial) APs was possible. The EG, mirroring typical subendocardial ischemia, was simulated without changing the epicardial AP. The EG simulating typical subendocardial ischemia showed a horizontal pattern of ST segment depression. In our model modification of the subendocardial AP combined with "unnatural" changes of the phase 3 of the subendocardial AP produced a downsloping pattern of ST-segment depression. CONCLUSION: The derivated EG waveform obtained with our PC program properly describe the algebric sum of endocardial and epicardial APs. In our opinion, this method represents a useful tool for the study of the AP changes. The simulated ST-depression morphology during subendocardial ischemia appears to be essentially "horizontal" and not downsloping. On the basis of our simplified theoretical model, we propose that ischemia-induced downsloping ST depression should be considered a reciprocal EG change and a manifestation of transmural ischemia in the wall opposite the exploring electrode.