How Medical Applications Capture Small Signals from an ECG?

In medicine, the electrocardiogram (ECG) is very often used to monitor small electrical changes in the skin of a patient that are related to the activity of the human heart. This non-invasive and straightforward measure facilitates the identification of different heart diseases. The medical industry develops dedicated equipment that contributes to the diagnosis. The design and verification of these devices require precise oscilloscopes.

T & M solution

An ECG device typically has 12 leads connected to the patient's chest, arms, and legs. Voltage is measured between separate connection points. This article describes the example of signal conductor 1, which corresponds to the voltage between the arms.
The figure on the left shows this signal (driver 1), which is repeated according to the heart rate. It begins with a P wave that initiates the cycle, continues with the QRS interval, and ends with the T wave. For the test engineer, the high-noise environment, the single-ended connection of the drivers, and the low amplitude of the signal represents a significant challenge. Typical levels are below one mV and the heart rate signal rate is between 40 and 220 beats per minute (bpm).
In a typical measurement configuration, an amplifier is essential to amplify the signal, but has the disadvantage of additional noise, specific channel delay, and offset error. Conversely, the averaging of the waveform remains unavoidable for the reduction of noise. Indeed, the signal is not repetitive and essential information would be lost. The diagram shows the example of a pulse sequence displaying non-periodic disturbances, which provides medical personnel with valuable indications of specific diseases.
The typical ECG signal amplitude is one mV or less, depending on the measurement point, with a bandwidth of <10 kHz. The small-amplitude makes it challenging to capture and analyze these signals directly from oscilloscopes.
However, the best oscilloscope for hobbyist has the essential functions for linear and in-depth signal analysis.

Application

The screenshot shows an ECG signal captured by the R & S ® RTE in a standard measurement setup. The noise level of the message is high; the P and T waves are barely visible.
To improve the acquired signal, the step of the vertical scale is fixed at 500 μV / div, without zooming limits or bandwidth limits. This function is unique in this category of oscilloscopes. HD mode is used with a bandwidth limit of 10 kHz.
The resolution is significantly increased, as does the trigger sensitivity, which is necessary to obtain a clear and stable acquisition of signals for later analysis.

Using the mask test to facilitate the detection of medical indications

This stable signal makes it possible to perform additional tests. For example, a mask test facilitates the detection of various medical signs contained in a distorted signal. Different masks can be applied to test specific indications. The screenshot shows the ECG signal and the associated mask test from a healthy patient. The white area surrounding the signal trace corresponds to the allowable space; the colored areas (top and bottom) are the top and bottom masks.
Note the behavior shown as 1 frame / s and 1 acquisition/sec in the performance icon; it equates to a heart rate of 60 bpm. In this configuration, the R & S ® RTE triggers acquire and process every single pulse, although the acquisition time occupies 80% of the 1 s period, leaving just 200 ms for processing. This example shows the outstanding performance of the R & S ® RTE.

Comparison with a modern competitor high-resolution oscilloscope

In an identical measurement configuration, the R & S ® RTE was replaced by a competing 10-bit high-resolution oscilloscope. The screenshot below shows the measurement result when the Higher mode is enabled, using the lowest vertical resolution, 1 mV/div.
It turns out that the signal amplitude is lower than the trigger sensitivity, so the oscilloscope can not trigger on the detection of the signal. To visualize this problem on a screenshot, we increase the persistence of the display by a few seconds, and we represent the trigger threshold as a light blue dashed line. The previous curves are in the form of shadows (1).
The filter applied, implemented in the post-processing of the digital signals, reveals an insufficient bandwidth: the short pulses resulting from the measurement configuration are thus no longer filtered (2). Marker 3 indicates the noise and the limited vertical scale of 1 mV/div. As a result, the P wave of the signal is barely visible and is embedded in the noise.

Conclusion

The R & S ® RTE is the best choice for high fidelity analysis of weak signals, such as those of an ECG. It is well suited to medical applications. The front module provides excellent low noise values and, at 500 μV, the smallest vertical scale in its class. The scan requires no additional circuitry and the HD mode increases the vertical resolution, improves the trigger sensitivity and reduces the noise power in the band. This allows the user to capture important signal details for analysis. In this respect, the R & S ®RTE is better than the competitive high-resolution oscilloscopes.