Myocardial ischemia, such as might occur due to a recent myocardial infarction, tends to cause the affected region of the ventricle to be seriously depolarized. This is because the poor blood flow upsets the normal repolarization of cardiac muscle cells. The electrocardiogram typically signals such a condition by an elevation or depression of the ST segment (depending on the exact location of the ischemia). Another indication of myocardial ischemia is presence of inverted T waves.
Observe the ST depression in the trace below from an aid truck.
Verapamil is a Ca++ channel blocker. Note how it reduces the excessive excitability of the heart.