An ECG built from discrete components

The brief

Design, build and test an ECG acquisition system from discrete components that can record usable cardiac waveforms.

What I did

A surface ECG signal is a few millivolts and sits under mains interference and movement artefact, so the design focused on the signal chain.

• An instrumentation amplifier front end, chosen for its common-mode rejection, to suppress 50 Hz mains interference.
• Active and passive high-pass and low-pass filters, plus a Twin-T notch. I measured each stage with Bode plots (magnitude and phase) to confirm it behaved as designed.
• Sampling at 600 Hz on an Arduino, with the analysis in MATLAB, including testing against the QTDB clinical database.

I also built a photoplethysmogram (IR LEDs, photodiodes and op-amps, read through the Arduino) to measure pulse optically.

Result

The system recorded clear cardiac waveforms. Detection of first-degree heart block reached 72% sensitivity and 87% specificity. Graded 92%.

For screening, sensitivity is the more important measure, and 72% would need to improve. I identified the main limitations (noise, movement artefact and component tolerances) and compared the prototype against clinical requirements: IEC 60601, CMRR and patient isolation.