Meet the challenge of high-pass filter and ST-segment requirements with a DC-coupled digital electrocardiogram amplifier

• Published in: Journal of electrocardiology Vol. 42; no. 6; pp. 574 - 579
• Main Authors: Abächerli, Roger, PhD, Schmid, Hans-Jakob, MSE
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.11.2009; Elsevier Science Ltd
• Subjects: Amplifiers, Electronic; Analog-Digital Conversion; Cardiovascular; Computer-Aided Design; DC coupling; Digital amplifier; Electrocardiogram; Electrocardiography - instrumentation; Equipment Design; Equipment Failure Analysis; High-pass filter; Reproducibility of Results; Sensitivity and Specificity; Signal Processing, Computer-Assisted - instrumentation; High-pass filter; Digital amplifier; Electrocardiogram; DC coupling
• ISSN: 0022-0736; 1532-8430
• DOI: 10.1016/j.jelectrocard.2009.07.012

Abstract Background The high-pass filter (HPF) in an electrocardiogram (ECG) amplifier can distort the ST segment required for ischemia interpretation. Therefore, the current standards and guidelines require −3 dB for monitoring and −0.9 dB for diagnostic purposes at 0.67 Hz. In addition, a minimal reaction to a rectangular pulse of 300 μ V has to be proven. We raise the question of why the design of a DC-coupled digital ECG amplifier is reasonable when today the AC-coupled digital ECG amplifier including a 0.05-Hz HPF works so well, meets all required standards, and is already safe. We make the hypothesis that a digital DC-coupled ECG amplifier can as well meet the requirements and guarantee the same safety levels at the same time provide a higher degree of freedom for future improvements of the ECG signal quality. Methods Firstly, a historical research of the origin of the 0.05-Hz requirement has been made. Secondly, triangular pulses simulating unipolar QRS complexes have been passed through a digital filter to get qualitative results of the HPF response. And finally, to quantitatively describe the filter response, corresponding test requirement signals have been passed through a digital filter to simulate the HPF behavior, therefore understanding the reasons for the required tests. Results The oldest reference found to the 0.05-Hz filter dates from 1937. At that time, DC-coupled analogue ECG amplifiers were used. The simulation of the AC-coupled ECG amplifier with a first-order analogue HPF shows that the rectangular 300- μ V pulse is a phase requirement and more restrictive than the frequency requirements. The phase requirement in fact corresponds to the requirement of a 0.05-Hz first-order analogue HPF (−3 dB) even if −0.9 dB at 0.67 Hz is required. The DC-coupled ECG amplifier (without an analogue HPF and during online and off-line acquisition) fulfils the phase and frequency requirements, just as the digital AC-coupled ECG amplifier does. Conclusions An AC-coupled ECG amplifier based on a first-order analogue HPF must have a maximum cutoff frequency of 0.05 Hz or requires a phase equalizer causing a delay of the acquired ECG. Because the desired delay during online acquisition should be short, the solution is practical but could be improved. Not the frequency cutoff of the HPF but the phase distortion of such a filter should be discussed. The DC-coupled ECG amplifier is as safe as the AC-coupled ECG amplifier; but it provides a higher degree of freedom for future filter designs certainly improving the ECG signal quality, while the safety can be guaranteed. Furthermore, the DC-coupled ECG amplifier allows investigation of the HPF, which is not easily possible when an AC-coupled ECG amplifier including the HPF is to be investigated.