Electrical Right and Left Axis Deviation

One of the key steps in interpreting an electrocardiogram (ECG) is determining the heart's electrical axis or the sum of all depolarization vectors of the heart. The cardiac vector suggests the direction of the net flow of current through the heart. Determining the electrical axis can provide insight into underlying disease states and help steer the differential diagnosis towards or away from certain diagnoses. Axis determination is also important to correctly identify certain life-threatening arrhythmias that have implications for definitive management strategy.

A vector is illustrated using an arrow, with the arrowhead pointing towards the net direction of the current flow. The length of the arrow is proportional to the voltage. A normal cardiac vector typically runs from the base of the heart to the apex. Cardiac depolarization occurs from the endocardium to the epicardium; however, some current may also travel through the walls of the cardiac chambers to the epicardium. The sum of all vectors/potentials that run from the base to the apex at an instant is also called an instantaneous mean vector. Vector analysis is the basic concept of analyzing ECG. Adequate knowledge and application of vector analysis are important for all clinicians. Axis and vector are used interchangeably throughout this topic.