In this clip we show you how to construct the electrical axis of the heart and why constructing it with Einthoven I and aVF leads to the vector ECG which shows you the electrical axis over time. The cool thing about the vector ECG is that you can see precisely how the excitation of the heart looks like starting from the P wave, over to the QRS complex and then the T wave. It's very easy to see any “detours” of the excitation which creates bulgy vector curves.