Frequency response characteristics of ambulatory ECG monitoring systems and their implications for ST segment analysis.

Because of the increasing interest in using ambulatory ECG monitoring equipment to study cardiac ischemia, we examined the frequency response characteristics and the reproducibility of a standard ST segment shift for equipment from eight manufacturers. Output/input amplitude versus frequency response curves, derived from input sine waves, were compared with American Heart Association (AHA) standards. Low- and high-frequency cutoffs (-3dB) were examined as well as flat response within the pass band. A new test based on Fourier analysis provided evaluation of phase shift versus frequency response and a more detailed analysis of the amplitude versus frequency response. By using a modified ECG simulator, flat-line ST segment depression from 0 to 10 mm (in 1 mm increments) was input to two or three recording devices for each manufacturer. The outputs from the scanners were evaluated for the amount of ST depression and the quality of the waveform reproduction. No system met all AHA standards for frequency response. Each system also had varying amounts of phase shift which could potentially distort the ECG signal. Although some manufacturers' equipment gave faithful reproductions of the simulated ST segment depression, others did not. We conclude that additional investigation is necessary to determine the clinical implications of our findings and to establish better standards for low- and high-frequency cutoffs, flat response within pass band, and phase shift for ambulatory ECG monitoring equipment.