What is the primary lead in ECG that travels down and to the left?

Lead II is considered the primary lead in an electrocardiogram (ECG) that travels down and to the left. This is due to its positioning and orientation. Lead II is formed between the right arm (negative pole) and the left leg (positive pole). As the electrical impulses of the heart conduct through the atria and ventricles, they travel towards the left and downward, aligning with the vector of Lead II. This lead captures a strong representation of the heart's electrical activity, particularly the depolarization of the ventricles, which is critical for assessing heart function.

Leads I, III, and aVR have different orientations; for instance, Lead I is oriented horizontally across the chest from the right arm to the left arm, while Lead III connects the left arm to the left leg, creating an upward axis compared to Lead II. Lead aVR captures electrical activity directed towards the right arm, which does not reflect the general downward and leftward orientation characteristic of the principal vector of heart depolarization. Thus, Lead II is the only choice among the options provided that accurately represents the primary lead orientation towards the heart's electrical conduction from the top right to the bottom left.