ST segment distortions by high-side frequency filtration in direct-writing ECG recorders.

ST segment depression in 12-lead resting and single-lead or multi-lead exercise ECGs is a useful screening criterion for detection of coronary heart disease (CHD). The recommended minimum frequency specification for direct-writing ECG recorders is 0.05-100 Hz (-3 dB). ST segment distortion by unintention low- and high-side frequency filtration could possibly weaken this screening capability. Consequently, one low- and two high-side potential filtration causes of such distortions were investigated in resting and exercise recordings of normal and both J-junction and ST-depressed ECGs. Of 4,914 filtered ST segments, 365 ST category changes were observed. Of these changes, approximately 15% was produced by filtration of the 0.05-0.1 Hz (amplifier function) and 45-100 Hz (60-Hz "noise" filtering) frequency bands. The remaining 85% was attributable to filtration of the 23-45 Hz (stylus overpressure) band. An optional remedy is discussed.