A quick and painless test that measures the electrical activity of the heart, an electrocardiogram provides your physician with two types of useful information, one being the time it takes for an electrical impulse to pass through the heart. By measuring the impulse intervals on the ECG, your physician may determine if the electrical activity of your heart is fast or slow, normal or irregular. The second type of diagnostic data, gleaned by measuring the amount of electrical activity passing through the heart muscle, enables your physician to determine if parts of your heart are too large or overworked.