A heart attack that does not cause changes known as "Q-waves" on the electrocardiogram (ECG). However, other changes on the ECG are often seen. In addition, chemical markers in the blood indicate that damage has occurred to the heart muscle. In non-Q-wave MI, a clot may block the coronary artery for a period of time, and then break up by itself or collateral circulation may help to restore blood flow. The size of damage is fairly small; therefore, overall function of the heart is usually maintained.