Frequency response characteristics of ambulatory ECG monitoring systems and their influence on ST-segment performance

ST-segment analysis in ambulatory ECG monitoring has been accepted only for FM recording systems. The purpose of this study was to investigate the accuracy of the direct-recording long-term ECG systems CardioData Mk 4, Marquette 8000/T, Reynolds Pathfinder II and III, and DMI Eclipse in comparison to the standard ECG. Measurement of the amplitude and phase response showed that none of the systems fulfilled the AHA recommendations. The amplitude response was limited in bandwidth and not flat. Some systems showed a boost at 0.5 to 1 Hz, and most had a phase shift of -30° around 0.3 Hz. The Pathfinder II showed the phase shift at 1.3 Hz. All systems were tested using a standard ECG signal with a predetermined horizontal ST-segment depression from 0 to 0.5 mV and an arbitrary waveform generator. The extent of ST depression was correctly reproduced with CardioData, Marquette, and Eclipse systems. Overestimation was significant with Pathfinder II and moderate with Pathfinder III