A heart monitor is disclosed. The monitor computes ST segment deviations and stores the results in heart rate based histograms. Periodically, the monitor analyzes the histogram data to determine a normal range of ST deviation for a particular heart rate range. The monitor computes heart rate dependent ischemia detection thresholds based on the upper and lower boundaries of the normal range. A current threshold for a heart rate range is set as a weighted average of the prior threshold and a provisional threshold based on recent data, thereby limiting the amount the actual detection threshold changes from period to period.
