The high-frequency components in the QRS complex were examined before and during the occlusion of the left anterior descending coronary artery. ECG waveforms from different stages of the ischemia state were averaged using cross-correlation technique and fast Fourier transform algorithm. The average waveforms were the filtered with nonrecursive digital filters (150-250 Hz) to obtain the high-frequency QRS complex. We tested our nonrecursive digital filter for ringing and the trigger jitter during the averaging process. We demonstrated that the high-frequency signal left after filtering is the result of a subtle low-amplitude constituent of the QRS complex and that the morphological change during ischemia (as zone of reduced amplitude) is a result of changes in these components of the original ECG signal.