Recently I've been having trouble understanding how(and even what) the EKG measures and have not been able to find a satisfactory response. The health science forums haven't been much help and I thought since EKG is practically pure physics I might as well ask from the experts themselves. Keep in mind I have only basic rusty high school physics.
QUESTION: Anyone who has seen an EKG will know that it commences with a P wave and once the P wave ends an isoelectric line follows(PR segment). Now everywhere I search I find the majority saying an EKG is a voltmeter/galvanometer (more familiar with a voltmeter but once I start trying to understand galvanometers electromagnetic fields induced by currents and stuff pop up and I just break down). Now my question is if an EKG truly is a voltmeter then why is it that when the atria are fully depolarized while the ventricles are polarized the EKG doesn't pick up this huge potential difference (and hence show an isoelectric line, If anything I'd expect the P wave to be at it's greatest peak then, since there is such a big difference in charges between the 2 areas of the heart).
If you feel like I need to understand some prerequisites to fully understand and you can't be bothered to explain them just share some useful resources or tell me what I gotta know to truly understand. Thanks for your help