Biomedical Engineering Theory And Practice/Biomedical Instrumentation/Electrocardiography

This chapter should cover the basics of ECG systems from a design point of view; not from a diagnosis point of view.

Physiological Background

Typical ECG signal

An animation showing how the electrical activity of the heart is reflected on the ECG signal

The function of the heart is to contract rhythmically and pump blood to the lungs for oxygenation and then pump this oxygenated blood into the general circulation. This perfect rhythm is continuously maintained and signaled by the spread of electrical signals generated by the heart pacemaker, the sinoatrial (SA) node. Detecting such electrical activity of the heart can help identify many heart disorders. This is the main concept behind using an ECG (Electrocardiogram), tracing the electrical activity of the heart.

By measuring and tracing the potential difference between two points on the outer surface of the body we obtain the simplest ECG chart. Two typical measuring points between the left arm and the right arm. By defining the two points and setting up the conventional positive direction for measuring the voltage, we create what is called a "Lead".

The first phase of the cardiac muscle activation is the stimulation of the right and left atria by an electrical signal generated from the SA node. This phase appears as the P-wave on the ECG chart. The electrical signal, originally generated by the SA node, then spreads through the Atrioventricual (AV) Junction, the bundle of His, and the Purkinje fibers to finally reach and stimulate the ventricles. The spread of the electrical signal through the ventricles causes ventricular contraction. The phase of ventricular contraction appears as the characteristic QRS complex on the ECG chart. Finally, with the relaxation of the two ventricles, a depolarization signal is generated and appears as the T-wave on the ECG chart.

Design of a Basic ECG Signal Acquisition Module

The acquisition of the ECG signal is a rather challenging task, as the case with many biological signals. ECG voltage signal is very low in magnitude (few millivolts) and has relatively low frequency content. The expected bandwidth of the signal typically begins from 0.01 Hz and extends to no more than 150 Hz.

Another challenge in acquiring the ECG signal is the power-line interference that is often order of magnitudes greater than the original ECG signal. All this suggests challenging requirements for the design of the signal acquisition module: It should have minimum loading effect, it should contain an amplification stage to make the signal level appropriate for further use, and it should contain a filtration stage customized to remove the expected noise and power-line interference that often corrupts the ECG.

In this section, we are NOT going to consider the full design of the ECG signal; rather, we shall focus on a minimal ECG acquisition module design that would just work.

A VERY simplified block diagram of an ECG amplifier. The voltage of the right arm (w.r.t right leg) is subtracted from the voltage of the left arm (w.r.t right leg) to get the ECG Lead I signal).

As discussed in the physiological background section, an ECG signal is obtained as the voltage difference between two points on the skin. This suggests the need for some subtraction mechanism. The subtraction could be done using an electronic Difference Amplifier. This amplifier basically subtracts and amplifies the difference between two electrical points. For the subtraction to work correctly, the voltage of both electrical points should be measured with respect to a common electrical reference. This common reference is typically chosen to be the right leg of the patient.

So, a simplified diagram of a simple ECG acquisition module would be as shown in the figure.

Signal Artifacts

Artifacts that corrupt the raw ECG signal have either physiological or non-physiological origin. The most dominant artifact is the power-line interference which appears as an sinusoidal wave of frequency 50 Hz (or 60 Hz in USA). Other artifacts include:

• Movement Artifacts due to patient movement, etc...
• Baseline Wander where the ECG waveform baseline starts to drift up and down in a sinusoidal pattern following the patient breathing
• EMG Interference where muscle contraction signals interfere with the ECG.
• Electrode Contact Noise where the electrodes are not tightly coupled to the patient causing some distortion
• Electrosurgical Unit (ESU) interference where high-frequency signals from the ESU used by surgeons during operation interfere with the ECG

Basic Signal Conditioning for the ECG

This section should discuss theoretical signal processing solutions to the above artifacts

Further Reading

• Programmable Chip EEG
• the OpenEEG Wiki

• The thread "amplifying biomedical signals: 150 uA with 16 bit resolution?" has several op-amp suggestions, and mentions that "a good, low-noise, low-cost, isolated EMG/EEG amplifier is one of the most demanding analog electronics designs."
• How to build your own ECG device
• TI app note "Biophysical Monitoring: Electrocardiogram (ECG) Front End" has a simple circuit: 390 KOhm resistors in-line with each lead -- one end touches patient, the other end directly connected to the instrumentation amp input (or the right-leg drive amplifier output, which has no further protection). The inst. amp has 2 protection diodes on each input, directly to +power and -power. Also, 39 pF capacitor from each input to analog GND, and 200 pF between the 2 inputs. The TI publication "Information for Medical Applications" (2Q 2004) reprints that circuit, but leaves out the caps and the diodes.
• Some people use 420 Hz sampling rate, 10 bits/sample.
• "High-Resolution QRS Detection Algorithm for Sparsely Sampled ECG Recordings" by Timo Bragge et al. 2004 recommends: "the sampling frequency of the ECG should be at least 500 Hz"
• "Low-Power, Low-Voltage IC Choices for ECG System Requirements" by Jon Firth and Paul Errico says "The multiplexed architecture, based on an old assumption that the converter is by far the most-expensive front-end component, is prevalent in today's electrophysiological measurement systems. However, with the proliferation of sigma-delta converter architectures, converter-per-channel is now a power- and cost-competitive alternative". It also gives a typical schematic for both architectures and suggests some parts.
• Is there a Medical Electronics Forum?
• Make: Homemade Electrocardiograph ([1] recommends skin lotion or shampoo as a low-cost electrode gel)

References

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