The ST segment of the electrocardiogram will reflect the lack of voltage gradients during the plateau phase and will be isoelectric as it was when the cells were in their resting stage and the transmembrane potential of all cells was in the range of -85 mV. The important thing to remember is this:
IT IS THE VOLTAGE GRADIENTS CREATED AS THE MEMBRANE SEGMENTS SEQUENTIALLY DEPOLARIZE AND THEN SEQUENTIALLY REPOLARIZE THAT ARE RESPONSIBLE FOR THE WAVEFORM RECORDED ON THE BODY SURFACE ELECTROCARIOGRAM.
If there are no voltage gradients, then the wavefrom will show no deflections and it will be flat and isoelectric.