The aVR lead is

About the ECG and heart vector.

The aVR lead is $-\frac{1}{2}{\mathbf{L}}_{\mathbf{I}}-\frac{1}{2}{\mathbf{L}}_{\mathbf{II}}$. Find the aVL and aVF leads toward the left arm and foot. Show that $\text{aVR}+\text{aVL}+\text{aVF}=0$. They go out from the center at $120^{\circ }$ angles.

Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §11.2 Planes and Projections, #45