Why is potential difference in ECG considered a vector? In the electrocardiogram, there are leads placed on the limbs and an "electrical vector" is calculated as the difference in potentials measured I the two leads. The direction is given by the line joining the position (or representation of) of the two leads. 
I don't understand what an electrical vector is because we were taught in school that neither potential (difference) nor current is a vector. Only electric field is. Are we measuring the field? I am a medical student and have just high school understanding of physics. Could someone pls explain what it means? 
What I know - that ECG is a representation of the propagating  wave front of electric conduction in the heart.
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 A: The "vector" referred to in cardiology names the direction of the axis of maximum depolarization (in the biological sense), aka hypopolarization of the heart and the strength of that depolarization. The direction is meant to be a physical direction of this axis relative to the patient's body. However, I get the impression(see footnote [1]) that really this direction is only roughly related to this physical direction and is really a compact description, as a three component linear superposition, of the six lead measurements gathered in the Hexaxial Reference System used in cardiology. Bottom line: the vector information is experimentally proven to be clinically useful in helping people with sick hearts get better and that's all that matters in medicine.
Although many of the comments and Alessandro's Answer make reasonable guesses to do with phasors and anisotropic materials, the notion of a vector in cardiology has nothing to do with any electrical notion of nonscalar quantities we meet in physics.

Footnote
[1] I only know about this stuff because the claim to fame of one of the people involved in a company I have worked for was the invention of the most sensitive methods available for measuring action potential.
A: In an anisotropic material (like the human body) the Ohm's law is written as
$$E_i=\rho_{ij}J_j$$
Where $\rho_{ij}$ is the resistivity tensor, $J_j$ the current density and $E_i$ the electric field.
The voltage between two points is defined as
$$ \Delta V=-\int_{\ell}\mathbf E\cdot d\mathbf l $$
where $\ell$ is the line connecting the two points.
As you correctly note $\Delta V$ is a scalar, while $\mathbf E$ is a vector but when you make a voltage measurement between two points of an extended material (and not between two points of an idealized 1-dimensional wire) the result of your measurement will depend not only on the distance between these points, but also on the exact position. This is because different points are connected by different paths that are characterized by different resistance (because $\rho_{ij}$ is a tensor and not a scalar).
So, my guess is that even if you are measuring a scalar quantity during an ECG, the result is heavily dependent on the vector quantities involved and that justifies the improper way of speaking about eletrical vectors.
