The electrical axis of the heart represents the average direction of the electrical current during ventricular depolarization. Determining this axis is important in ECG interpretation because it reflects the overall spread of electrical activity through the ventricles. In a healthy heart, the electrical impulse travels predominantly downward and to the left, a result of the larger muscle mass of the left ventricle. Deviations from this normal pathway can signal underlying issues such as changes in heart muscle size or defects in the heart’s electrical conduction system.
Understanding the Hexaxial Reference System
The frontal plane electrical axis is determined using the six limb leads, which are geometrically arranged around the heart to form the Hexaxial Reference System. This system visualizes the electrical field in the frontal plane as a 360-degree circle centered on the heart. The six leads include the three standard bipolar limb leads (I, II, and III) and the three augmented unipolar limb leads (aVR, aVL, and aVF).
Each lead acts as a distinct viewpoint, or vector, with a positive and a negative pole. Lead I is positioned horizontally at 0 degrees, and Lead aVF is oriented vertically at +90 degrees. The remaining leads are equally spaced every 30 degrees: Lead II is at +60 degrees, Lead III is at +120 degrees, Lead aVL is at -30 degrees, and Lead aVR is at -150 degrees.
The Hexaxial System relates the direction of the heart’s electrical activity to the deflections seen on the ECG tracing. A positive QRS deflection in a lead indicates that the electrical vector is moving toward that lead’s positive pole. Conversely, a negative deflection means the vector is moving away from the positive pole.
The Quick Quadrant Estimation Method
The Quick Quadrant Method uses only two leads, Lead I and Lead aVF, for rapid axis estimation. These leads are perpendicular and divide the Hexaxial System into four 90-degree quadrants. The net QRS deflection in each lead is assessed by subtracting the total negative amplitude (Q and S waves) from the total positive amplitude (R wave) to determine if the complex is predominantly positive or negative.
If the QRS complex is positive in both Lead I and Lead aVF, the axis falls into the Normal Axis quadrant (0 to +90 degrees). A positive Lead I and a negative Lead aVF place the axis in the Left Axis Deviation (LAD) quadrant (0 to -90 degrees). Since the normal range extends to -30 degrees, this quadrant requires further scrutiny to confirm true pathological LAD.
A negative QRS in Lead I and a positive QRS in Lead aVF signifies Right Axis Deviation (RAD) between +90 and +180 degrees. If both Lead I and Lead aVF show a net negative QRS complex, the axis is in the Extreme Axis Deviation (EAD) or “Northwest” quadrant, spanning -90 to -180 degrees.
Precise Determination Using the Isoelectric Lead
The Isoelectric Lead method offers a more precise estimation of the electrical axis. This technique relies on the principle that if the heart’s electrical vector is perpendicular to a specific lead, the QRS complex in that lead will be equiphasic or isoelectric. This occurs because the positive and negative deflections are equal, causing the net electrical activity to cancel out when viewed from that angle.
To use this method, identify the limb lead with the most isoelectric QRS complex, where the total height of the R wave equals the total depth of the Q and S waves. Once identified, the true electrical axis must lie perpendicular to that lead on the Hexaxial System. For example, if Lead aVL (at -30 degrees) is the most isoelectric, the axis must be perpendicular to it, falling at either +60 degrees (Lead II) or -120 degrees.
The final step is to examine the perpendicular lead (Lead II in the example) to determine if its QRS complex is predominantly positive or negative. A positive QRS indicates the axis is pointing toward that lead’s positive pole (+60 degrees). A negative QRS means the axis is pointing away (-120 degrees), refining the initial quadrant estimation.
Clinical Interpretation of Axis Deviation
The normal electrical axis range is between -30 degrees and +90 degrees. Any measurement outside this range is classified as an axis deviation, indicating changes in the heart’s structure or conduction. Pathological Left Axis Deviation (LAD) is diagnosed when the axis is more negative than -30 degrees, suggesting increased electrical forces directed superiorly and to the left.
Common causes of LAD include left anterior fascicular block, a defect in the conduction system, or an inferior wall myocardial infarction. Non-cardiac factors like obesity or a high-riding diaphragm can also mechanically shift the heart, contributing to LAD. Right Axis Deviation (RAD) is defined as an axis greater than +90 degrees, meaning electrical forces are directed toward the right and inferiorly.
RAD is frequently caused by right ventricular hypertrophy, where increased muscle mass pulls the electrical vector to the right. Conditions leading to right ventricular strain, such as acute pulmonary embolism or chronic obstructive pulmonary disease (COPD), are common etiologies. Determining the axis deviation provides immediate diagnostic clues, prompting a search for specific underlying cardiac or pulmonary diseases.