Vector current and its measurement

[BobP]

An electrocardigram measures the electrical current in the heart by using several "leads". These leads have +ve and -ve ends.
http://www.heartaxis.com/
If you take a look at the interactive FLASH about 1/4 of the way down the page (yellow arrow and green line ppd to each other) and the associated graphs for each lead we can see that graph aVF is biphasic.
As the current (yellow line) is perpendicular to the aVF lead (green line) surely there should be no change on the aVF graph (it should be a flat line).
Why does this not happen? Why is a biphasic graph produced?
Thank you

 

[Vailanor]

.The biphasic graph on the aVF lead is due to the changing electrical currents in the heart. As the heart contracts and relaxes, the electrical current in the aVF lead changes direction, resulting in the biphasic graph pattern. This is because the electrical current is perpendicular to the aVF lead, so as the current changes direction, the aVF lead is able to detect this change and produce a biphasic graph.