(I do hope my question is not off-topic)
Currently learning about ECG recorders (Our 7-months old daughter has to have one during her sleep), I wonder whether this is a pure "passive" measuring device (like a multimeter) or not.
I do think I understood that it measures heart activity by comparing potential differences between 2 or more body-connected electrodes.
But I did not understand whether this works purely by getting signals from the heart and processing them or whether the device actively sends some signals through the electrodes and measures what comes back.
So my question is:
Does an ECG recorder send signals trough the electrodes or not?
Best Answer
An ECG recorder works by recording electrical signals produced by the heart, and does not need to provide any active signal to do so.
That said, most commercial ECG systems will actually use a driven ground arrangement, called a "driven right leg" to make the recordings better (feeding back the common mode signal through a reference electrode), and thus do "send signals through the electrodes" -- but those signals have nothing to do with the signals generated by the heart. From an electrical engineering point of view, this minimizes the effect of different impedances between the different electrodes and the skin, among other things.
There's a reasonable review of the method at http://www.ti.com/lit/an/sbaa188/sbaa188.pdf
As pointed out by Zayzoon there may also be a small current for detection of leads falling off.
All these currents are not dangerous, and you can use the device without fear of injury.