Casper, I couldn't find my
ECG on the site but I found the file on my computer, so I'm posting it here. If you look, you'll see that the QRS complex and it's frequency are completely unchanged before and after the negative spikes, which says to me that while it DOES have to do with electrochemical discharges and muscle contractions, it's nothing to do with the heart.
Now having said that, I think Jodie also gave some very valuable input. Excitable heart cells in different parts of the heart can cause
PVCs that fall at different times in the cardiac cycle or cause the
PVC to propagate differently through the heart, which can affect how that
PVC is felt (or make your boob shake, as in Jodie's case
).