Testing

Essential ECG Signal Requirements* - recommendations for the standardization and interpretation of the electrocardiogram (Circulation)

1. Low frequency filtering -a cut-off of 0.5 Hz was widely used in traditional analog filtering, which reduced baseline drift due to the generally lower frequency of respiratory movement. However, this can lead to marked phase distortions in areas where frequency content and wave amplitude changes abruptly, particularly at the ST segment (after the QRS complex), leading to possible artefactual ST-segment deviation. Lowing the frequency cutoff to as low as 0.05 Hz as per the 1975 guidelines (and has since remained the current recommendation) will sequester the distortions of the repolarization waves but at the same time increase the level of baseline drift. -digital filtering allows for methods of increasing the low frequency cutoff (to avoid respiratory noise interference) without the introduction of phase distortion, such as using a flat step response filter that results in zero phase shift. The low frequency cutoff for linear digital filters with zero phase distortion is relaxed to 0.67 Hz (or below; this frequency corresponds to a heart rate of 40 bpm, well within the lower range of the vaste majority of patients).

2. High frequency filtering -High frequency is needed to accurately define the most rapidly changing parts of the ECG signal, namely the QRS complex, where detecting R amplitude and Q waves is essential diagnostic information. -Data being sampled at 500 per second with a high frequency digital filter cutoff of 150 Hz is required to reduce amplitude error measurements ~ 1% in adults. Greater bandwidth of up to 250 Hz may be required for infants. Suboptimal frequencies such as 40 Hz that are used to reduce noise can potentially miss critical diagnostic information

Steps to ensuring safety of ECGs – based on Design and development of a validation framework for ECG device stabilisation; n.b. this list is informative, not official

1. Insulate users from voltage fluctuations or electrical shocks
2. ECG must detect signals of the heart only, and reject signals from other sources
3. Defibrillation/voltage surge protection
4. Preservation of ECG signal without interference from noise
5. Protection against environmental emissions
6. Able to detect all (7) types of arrhythmias
7. Able to detect pacemaker pulses (by oversampling)
8. Able to reject tall T wave signals beyond a limit to prevent heart beat duplication

Essential ECG Testing – based on IEC 60601-2-25

Essential performance requirements are as follows:

1. Defibrillation Protection: -Within 5 s after exposure to the defibrillation voltage of 5kV, the ECG shall resume normal operation in the previous operating mode, without loss of any operator settings or stored data, and shall continue to perform its intended function -Two defibrillation tests need to be done, Common mode and Differential mode

2. Essential Performance and Accuracy of ECG machine *only needed if automated measurements are provided by the machine - refer to Annex AA.3 on pg.52 for guidelines to input ECG data to ECG

   a) Requirements for amplitude measurements – refer to Table GG.1 on pg.63 -Exclude the two biggest differences in the amplitude measurements. The difference for each remaining amplitude measurement shall not deviate from the reference value by more than ±25 μV for reference values ≤500 μV, or by more than 5 % or ±40 μV (whichever is greater) for reference values >500 μV. b) Requirements for absolute interval measurements – refer to Table GG.1 on pg 63 c) Requirements for biological interval measurements – refer to Table GG.2 on pg. 64

3. Filters (including line frequency interference filters) -Filters for line frequency interference suppression shall not introduce on the ECG report more than 50 μV peak-to-valley distortion of the signal in any lead when tested with the test ECG ANE20000 (on pg. 63 of Table GG.1).

4. Electrostatic Discharge -The machine shall resume normal operation in the previous operation mode without loss of operator settings or stored data within 10 s of a discharge (temporary degradation is allowable for up to 10 s).

5. Electrical fast transients and bursts -When exposed to electrical fast transients and bursts (via the power cord supply), the machine shall resume normal operation in the previous operation mode without loss of operator settings or stored data within 10 s. The machine shall comply with the requirements of amplitude measurements when the signal CAL20110 of Table GG.1 is applied.

6. Conducted Disturbances -When exposed to a conducted radio frequency voltage, via the power cord supply, the machine shall continue to perform its intended function. The machine shall comply with the requirements of amplitude measurements when the signal CAL20110 of Table GG.1 is applied. The difference for each amplitude measurement shall not deviate from the reference value by more than ± 50 μV for reference values ≤ 500 μV, or by more than 5% or ±100 μV (whichever is greater) for reference values > 500 μV.

7. Electrosurgery interference -When the machine is used together with high frequency surgical equipment, it shall return to previous operating mode within 10 s after exposure to the field produced by the high frequency surgical equipment, without loss of any stored data.

Calibration Testing - based on IEC 60601-2-25

Electrocardiograms currently on the market fall into two categories: traditional analogue ECGs capable of printing ECG reports on paper or displaying them on a screen, and more modern digital ECGs with integrated signal processing, capable of making measurements and interpretative statements on the ECG report. A major goal of the European CTS-ECG project was to develop signals that can be fed into both older analogue and newer digital ECGs, and therefore be used for both signal reproduction verification on paper-strip as well as for computer programs which have been designed to analyse and interpret ECG reports.

As a result a set of ECG-like signals with well-defined amplitude-time characteristics were created. The signals are generated in digital form and can be used to analyse both:

• the hardware characteristics of analogue systems in terms of amplifier linearity, gain factors, weighting factors for lead networks, low and high frequency response, and signal reproduction on printed reports;
• the software performance, for example in terms of waveform detection, recognition of fiducial points, measurement of ECG parameters, etc.

PhysioNet

One of the major drawbacks of the CSE study is that their database is only available on CD-ROM at high cost (ie closed-source). In an effort to improve transparency, many freely available databases have been provided within PhysioNet, an open source research resource for complex physiological signals including the ECG. The website contains a large cache of clinically validated ECG signals, and also houses open-source software programs that can be used to view, process, and analysis these complex signals. In keeping with Glia’s ideology of making medical grade equipment that is easily accessible, using PhysioNet may be the ideal toolkit to test and validate a prototype ECG machine.

For a list of available ECG databases on PhysioNet, follow this link: https://physionet.org/physiobank/database/#ecg