Holter ECG monitoring is a valuable diagnostic technique in both cardiac arrhythmias and ischemic heart disease, known for over 30 years. Current methods of ST segment analyses require the use of computers. Myocardial ischemia is diagnosed if ST segment deviation from isoelectric axis is at least 1 mm, duration of this change is at least 1 minute, and interval is at least 1 minute. There are four ways to measure ST segment deviation from isoelectric axis: measurement of "area of given surface", mean length of ST segment, measurement of the real time of ST segment duration, and optic technique. Use of appropriate leads system is always necessary.