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Filtering ECG's for Diagnostics and Monitoring

Abstract

ECG signal processing techniques and standards differ depending on the purpose of the ECG. Diagnostic ECG’s require preservation of very low frequencies to preserve ST-Segment fidelity. Monitoring ECG’s are more permissive, and allow the use of efficient recursive high pass filters for on-line processing, but these may not preserve ST-Segment fidelity. We note specific applicable medical device standards.

 

Filtering ECG signals will inevitably change some aspects of the raw signal, or we would not have to recourse to filters in the first place. Generally, we want to attenuate noise with as little effect on the signal as possible. Filtering is necessarily obtrusive, but is required for most applications, particularly those involving mobile subjects engaged in walking or running during the collection of the ECG.

 

Some relevant standards

IEC 60601-2-25 Medical electrical equipment - Part 2-25: Particular requirements for the basic safety and essential performance of electrocardiographs


IEC 60601-2-27 Medical electrical equipment - Part 2-27: Particular requirements for the basic safety and essential performance of electrocardiographic monitoring equipment


IEC 60601-2-47 Medical electrical equipment - Part 2-47: Particular requirements for the basic safety and essential performance of ambulatory electrocardiographic systems


ANSI/AAMI EC11-1991/(R)2001: Diagnostic Electrocardiographic Devices, 2ed

 

Application types


Heart Rate/HRV: Bandwidth is unspecified: There are no particular requirements for signal bandwidth preservation in this application, since HR/HRV is derived from QRS complex fiducial points only.


Ambulatory, and Monitoring: 1Hz to 40Hz bandwidth is suitable for heart rate estimation and on-line monitoring displays.


Diagnostic: 0.05Hz to 150Hz frequency band supports ST segment mea- surements. Moreover, diagnostic ECG’s are taken from clinically placed 12-lead ECG’s, so sensor placement may be critical to obtain diagnostic fidelity. 0.05 Hz seems very low, relative to a 1 Hz. heart rate, but the non-linear smearing of the QRS complex by realtime monitoring filters causes clinically significant ST-Segment changes if the recursive high pass filter is set higher than 0.05 Hz.


The ST-Segment contains low frequency components which must be preserved for ST-Segment diagnostics. Monitoring applications such as rhythm strips are not as stringent in their requirements for low frequencies. Thus frequency bandwidth requirements vary with the application.


Buenda-Funetes et al. describe the clinically significant alterations to the ST segments made by by applying various filters at monitoring and diagnostic bandwidths[1].


Watford provides some helpful illustrations of the functioning of the recursive non-linear phase, and non-recursive linear phase filters on the ST-Segment[2].


See the PDF for more details.

 

References

[1] F. Buendîa-Fuentes. High-bandpass filters in electrocardiography: Source of error in the interpretation of the st segment. ISRN Cardiology, 2012(706217):1–10, April 2012.


[2] Christopher Watford. Understanding ECG filtering, March 2014. http://www.ems12lead.com/2014/03/10/understanding-ecg-filtering/.

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