What's going on here?

I was correcting EKG's when I hit upon this one. From the room number (not shown), I knew this was a post-open-heart surgery case (the word "Tape" refers to the fact that the patient had tape over part of the chest). These patients often have temporary pacemakers, but that didn't seem to explain things adequately. Note that the tracings, from top to bottom, are simultaneous.

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If you look at the rhythm strip near the bottom of the tracing, by matching the complexes in "V1" to those above and below it, you can figure out which is the QRS. Also, you can figure out that the rounded, positive deflection in that lead is a T-wave (not a P-wave, as it might first appear). That leaves you with the spike. Tempting to call it a pacer spike, but why is one spike in the rhythm strip early, and why don't we see spikes in any other leads? Yes, why is it that only V1 is so bizarre?

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I figured it out as far as described above, and then got up and went to the recovery room for the rest of the answer. Turns out the patient, a 50 year old man, had a tachycardia that was hard to diagnose. So an EKG machine was hooked up and V1 was connected to a temporary atrial wire, thus obtaining an "atrial electrogram". What you are seeing in V1 is an EKG recorded from inside the atrium, which is why the "p-wave" looks like a sharp spike, larger than the QRS-T complex! This was thus a supraventricular (in fact sinus) tachycardia. The early spike, which made me think that we weren't just dealing with an atrial pacemaker, was in fact an atrial premature contraction.

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