The electrical axis of the heart represents the average direction of the electrical current as it flows through the ventricles during depolarization. Determining this axis is a fundamental step in analyzing an electrocardiogram (ECG). Significant deviation from the normal path provides insight into underlying changes in the heart’s muscle mass or its electrical conduction system.
Defining the Normal Electrical Axis
The electrical axis is measured in degrees within the frontal plane, which is an imaginary flat surface that cuts through the body from front to back. The standard system uses a coordinate plane superimposed on the body, with \(0^{\circ}\) pointing to the patient’s left shoulder and \(+90^{\circ}\) pointing straight down toward the feet.
The accepted range for a normal adult electrical axis is typically between \(-30^{\circ}\) and \(+90^{\circ}\). This wide range reflects the natural downward and leftward spread of ventricular depolarization, which is dominated by the large muscle mass of the left ventricle. When the electrical impulse travels toward a specific ECG lead, it generates a positive, upward deflection, but if it travels away from the lead, it creates a negative, downward deflection. The frontal plane leads, specifically Leads I, II, and aVF, serve as the reference points to pinpoint the final electrical vector.
The Rapid Two-Lead Assessment Method
A quick and reliable way to estimate the electrical axis is by using the Quadrant Method, which focuses exclusively on the net deflection of the QRS complex in two specific frontal plane leads: Lead I and Lead aVF. These two leads are positioned at \(0^{\circ}\) (Lead I) and \(+90^{\circ}\) (Lead aVF), effectively dividing the frontal plane into four \(90^{\circ}\) quadrants.
The initial step is to examine Lead I, which runs horizontally across the chest, to determine if the net QRS complex is predominantly positive or negative. Next, the QRS complex in Lead aVF, which runs vertically toward the feet, is assessed to see if it is positive or negative.
If both Lead I and Lead aVF show a net positive deflection, the axis must fall into the quadrant defined by \(0^{\circ}\) and \(+90^{\circ}\), confirming a normal axis. If the deflections are mixed, the axis falls into one of the three abnormal quadrants, signaling a deviation.
Recognizing Patterns of Axis Deviation
A Left Axis Deviation (LAD) is identified when the net QRS complex is positive in Lead I but negative in Lead aVF. This pattern places the electrical vector between \(0^{\circ}\) and \(-90^{\circ}\), pushing the axis upward and to the left. Clinicians often look at Lead II as well, since a negative QRS complex in Lead II confirms a pathological LAD, meaning the axis is more negative than \(-30^{\circ}\).
Right Axis Deviation (RAD) is indicated by a pattern that is the mirror image of LAD, where the QRS complex is negative in Lead I and positive in Lead aVF. This shifts the electrical activity downward and to the right, placing the vector between \(+90^{\circ}\) and \(+180^{\circ}\).
The most extreme shift is the Extreme Axis Deviation, also known as the Northwest Axis. This occurs when both Lead I and Lead aVF show a net negative QRS complex. This results in the electrical vector pointing into the upper right quadrant, between \(-90^{\circ}\) and \(\pm 180^{\circ}\). This finding almost always signifies a serious underlying pathology.
Clinical Conditions Associated with Deviations
Axis deviations often point toward specific cardiac or pulmonary conditions that alter the balance of electrical forces. Left Axis Deviation is frequently associated with an increase in the electrical influence of the left ventricle, such as in Left Ventricular Hypertrophy (LVH), where the muscle mass is enlarged. It can also be a sign of a conduction block, such as Left Anterior Fascicular Block, which redirects the electrical signal.
Right Axis Deviation commonly suggests a greater electrical influence from the right side of the heart. This is often seen in conditions like Right Ventricular Hypertrophy, which may develop in patients with severe lung diseases such as chronic obstructive pulmonary disease (COPD) or pulmonary hypertension. An acute cause of RAD can be a large pulmonary embolism, which creates a sudden overload of the right ventricle.
The presence of an Extreme Axis Deviation is a strong indicator of significant electrical disruption, sometimes linked to complex congenital heart disease or certain ventricular arrhythmias. Mechanical factors, such as a high diaphragm during pregnancy or obesity, can also physically shift the heart and cause a mild deviation.