Confused medical student needs explanation of the electrocardiogram

Question

From what I understand, as the heart depolarizes it turns into a lot of electric dipoles , the resultant of which can (somehow) be measured in the extremities. But how does this actually happen? How is the potential in the extremities changed by the heart? And why do they teach us that voltage is a vector (it's a scalar, isn't it?) Can someone tell me what the cardiac vector really is? Thanks

Answer 1

You are right - potential is a scalar. But a dipole moment is a vector - it has magnitude and direction.

When sodium channels open up, charge flows. Lots of charge moving a little bit causes a change in the dipole moment of the heart. This in turn induces charge to move elsewhere in response (the dielectric properties of tissue cause a propagation of the polarization).

The net result is that you will see the polarization "from a distance" as a change in the local potential. Usually when you do a 12 lead cardiogram (which only has 10 leads...), a lot of the interesting information comes from the difference between the electrodes closest to the heart (V1 through V6). But the electrodes on the extremities don't just establish "ground" - they also give a general view of the overall cardiac vector. There's a really nice introduction available at this link

The polarization due to the heart can be seen as occurring at some angle to the heart axis (changing with time as the currents flow around the heart), and therefore inducing different potentials at the corners of the triangle. The potential on each corner (for example the shoulder) will essentially be the same as the potential at the corresponding extremity (for example the wrist) as there is virtually no current flowing in the arm.

Put differently: polarization is a charge $\times$ distance effect. This results in an induced potential as a function of position. There is little potential difference once you are "far away" since there is no current flowing there.