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ECG with Theremino

With Theremino, with the component AD8232 and the software SignalScope you can easily build a small amateur electrocardiograph. In this post we will describe this equipment and show some examples of the charts that can be obtained.

Electrocardiogram

In general terms, the electrocardiogram is a graphic representation of the electrical activity of the heart. The ECG is separated into two basic intervals, the PR and QT interval, as shown below:

PR Interval

The PR interval is the initial wave generated by an electrical impulse traveling from the right atrium to the left. The right atrium is the first chamber to see an electrical impulse. This electrical impulse causes the chambers to “depolarize”. This forces it to contract and drain de-oxygenated blood from both the Superior and Inferior vena cava into the right ventricle. As the electrical impulse travels across the top of the heart it then triggers the left atrium to contract. The left atrium is responsible for receiving newly oxygenated blood from the lungs into the left ventricle via the left and right pulmonary veins.

QT Interval

The QT Interval is where things get really interesting. The QRS is a complex process that generates the signature “beep” in cardiac monitors. During QRS both ventricles begin to pump. The right ventricle begins to pump de-oxygenated blood into the lungs through the left and right pulmonary arteries. The left ventricle is also beginning to pump freshly oxygenated blood through the aorta and into the rest of the body. After the initial contraction comes the ST segment. The ST segment is fairly quiet electrically as it is the time where the ventricles waiting to be “re-polarized”. Finally the T wave becomes present to actively “re-polarize”, or relax the ventricles. This relaxation phase resets the ventricles to be filled again by the atrium.

AD8232 Component

This component is a small chip specifically designed to measure the electrical activity of the heart. The cardiac electrical activity is tracked in an ECG (electrocardiogram). Electrocardiography is used to monitor the proper functioning of the heart and to diagnose various heart diseases. Of course the equipment that we propose has only a amateur / educational purpose without any intention of replacing professional equipment.

The AD8232 from Analog Devices is a dedicated single lead heart rate monitor front end integrated circuit.  The AD8232 is an integrated signal conditioning block for ECG and other bio-potential measurement applications. It is designed to extract, amplify, and filter small bio-potential signals in the presence of noisy conditions, such as those created by motion or remote electrode placement. This design allows for an ultra-low power analog-to-digital converter (ADC) or an embedded microcontroller to acquire the output signal easily.

The image below shows the scheme of the device. It is provided the use of three electrodes, one on the right arm, one on the left arm, and a third one on the right leg to provide the reference potential.

Setup

The simple and easy to use breakout board for heart rate monitoring from Sparkfun. This board measures electrical activity of heart through the Electrode pads placed on the skin.

The analog output produced by the AD8232 chip is acquired via an analog input of a Theremino Master. The signal, digitized on 16 bits, is then sent to SignalScope software that provides to display the electrical pulse.

ECG Examples

In the detail shown below the main characteristics of an ECG are evident : in particular, the pulses P, QRS and T.

The graphs below show, however, some ECG tracings as they are represented by SignalScope software.

Conclusions

The Theremino HW / SW system, with the chip AD8232, has proved very useful in the acquisition and representation of biological signals such as cardiac bio-potential. With a breakout board, a Theremino Master, the special electrodes and the SignalScope software, in a short time one is able to prepare a fully functional although amateur electrocardiograph.

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