ECG Electro Cardio Gram
Here is very simple ECG Electro Cardio Gram made with Arduino UNO , Bluetooth HC06 adapter and an AD8232 sensor.
Arduino UNO uses analog (PinC.1) for input. Immediately after the analog signal measurement, it sends the value information via the serial port (Bluetooth module HC06) to the GUI-O application (CH-sweep chart object) on the smartphone.
That's enough speed for a heartbeat.
You can turn the sound ON or OFF with the "sound" button.
Result is on Youtube video - screenvideo from my phone.