"When sodium and calcium are pumped out of the heart cell, potassium is pumped in. This eventually creates an imbalance in their equilibrium, with many more sodium and potassium ions outside of the heart cell than inside. This creates a “positive” charge outside of the heart cell...body eventually wants to correct this imbalance of sodium, calcium, and potassium. So the opposite occurs. Potassium rushes out while the sodium and calcium rush in...Every time this reversal of flow occurs, it generates a spark of electricity which races through the heart. It is this electrical spark that causes heart cells to contract and the heart to beat."
Why having a fast heart beat is not a good thing: Myrna's heart is in tachycardia due to damage sustained in the heart attack-part of the heart was deprived of oxygen and the muscle was damaged. But now that she's in tachycardia, the heart can't completely fill with blood and get the oxygen it needs:
"Tachycardia- An abnormally fast heartbeat. If the heart beats too fast is does not spend enough time in diastole. Therefore the heart chambers do not fill up enough with blood, so the heart does not pump out an adequate amount of blood for the needs of the cells. In addition, the lack of time in diastole causes the heart muscle itself (myocardium) to suffer since it is in diastole that blood flows from the coronary arteries into the heart muscle."