First of all, go to this site and see how a normal EKG is created by a "normal" heartbeat. This is a GREAT animation by the way. http://en.wikipedia.org/wiki/Image:ECG_principle_slow.gif
The spike that is shown on this animation is part of the QT interval (the electrical signal of the ventricular contraction). When you are experiencing a-fib the electrical signals in the atria are firing erratically causing the ekg to read bunches of that first lump you see on the animation. There are spikes, but since the electrical impulse that sets off the ventricular contraction are not being sent "normally" the readout does not catch a normal QT interval.
Compare the "normal" heart rhythm animation above to the description of an ekg found here http://en.wikipedia.org/wiki/Image:SinusRhythmLabels.svg and you can see why the electrical impulses wouldn't look the same.
I don't know if this will help you at all. If not, I can try again. Let me know.
Thanks for the links, very helpful and help me understand how HR monitors like the popular Polar exercise watch with chest strap detector/transmitter. I can see, as I understood the case to be, that such a monitor does in fact "see" electrical signals from the heart and it is the ventricle beat, the big spike, that it counts. Not sure I understand the animation ... does it show a normal electrical pattern, or are some of the signals shown the AFib part?
The animation is showing the normal electrical pattern of the heart. The SA node fires off a chemical/electrical signal that stimulates the rest of the cells in the atria to contract and this electrical signal stimulates the ventricular node to initiate the "responding" contraction. So, in afib, all you would see on the electrical record would be a "quivering" line as the atria continue to flutter, with the occasional ventricular spike mixed in. A regular "QT" interval would not be recorded because the normal electrical functioning of the heart is not occurring at that point.
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