The electrical axis of the heart, often referred to as the QRS axis, indicates the general direction of the heart’s electrical activity during ventricular depolarization in the frontal plane. Determining this axis is a fundamental step in interpreting an electrocardiogram (EKG) because it provides a baseline assessment of the heart’s electrical health. An axis measurement that deviates significantly from the typical range can suggest underlying changes in the heart’s size, position, or electrical conduction pathways.
Understanding the Hexaxial Reference System
The Hexaxial Reference System is the coordinate system used to map the heart’s electrical activity in the frontal plane. This system arranges the six limb leads—three bipolar (Leads I, II, III) and three augmented unipolar (Leads aVR, aVL, aVF)—around a central point. The resulting diagram is a circle divided into 30-degree segments, with the leads positioned to represent their electrical viewpoint of the heart.
Each lead has a positive and negative pole, defining its angle within the system. Lead I is positioned horizontally at 0 degrees, and Lead aVF is positioned straight down at +90 degrees. These two leads form perpendicular lines that divide the circle into four 90-degree quadrants.
The system allows the mean QRS axis to be assigned a precise angle ranging from -180 degrees to +180 degrees. For example, Lead aVL is at -30 degrees, and Lead II is at +60 degrees. Understanding these fixed angles is necessary because subsequent calculation methods rely on these specific lead positions.
The Quick Quadrant Method
The Quadrant Method offers a rapid, two-step assessment to determine the general location of the QRS axis without calculating a specific degree. This technique relies solely on observing the net deflection of the QRS complex in Lead I (0 degrees) and Lead aVF (+90 degrees). Net deflection is determined by comparing the total height of the positive deflections (R wave) to the total depth of the negative deflections (Q and S waves).
If the QRS complex is net positive in both Lead I and Lead aVF, the axis falls between 0 and +90 degrees, which is the normal range for most adults. When Lead I is positive but Lead aVF is negative, the axis shifts into the upper-left quadrant, indicating Left Axis Deviation (LAD). Conversely, a negative Lead I paired with a positive Lead aVF signifies Right Axis Deviation (RAD) in the lower-right quadrant.
The final quadrant is defined by a negative deflection in both Lead I and Lead aVF, known as Extreme Axis Deviation or “no-man’s land.” This quick method serves as an excellent screening tool, identifying whether the axis is normal or significantly deviated, which can then prompt a more precise calculation.
The Precise Isoelectric Method
For a more exact determination of the cardiac axis, the Precise Isoelectric Method is used, building upon the framework of the Hexaxial System. This method utilizes the principle that the heart’s overall electrical vector must run perpendicular to the lead that registers an isoelectric QRS complex. An isoelectric complex occurs when the net positive deflection perfectly balances the net negative deflection.
The first step involves scanning the six limb leads to identify the one with the most isoelectric QRS complex. If multiple leads are close, the one with the smallest overall amplitude is chosen. Once the isoelectric lead is found, the axis is known to be perpendicular (90 degrees away) to that lead’s angle on the Hexaxial System. For example, if Lead III (+120 degrees) is the most isoelectric, the axis must be either +30 degrees or -150 degrees.
The final step requires looking at the QRS complex of the lead that is perpendicular to the isoelectric one. If the perpendicular lead shows a net positive deflection, the axis points toward that lead’s positive pole. If the deflection is negative, the axis points toward the opposite pole, narrowing the two possibilities down to a single, specific degree.
Clinical Interpretation of Axis Deviation
The normal adult QRS axis falls within the range of -30 degrees to +90 degrees, indicating electrical activity is directed downward and toward the left. A deviation outside this normal zone is not a diagnosis itself but indicates that the heart’s electrical pathway or physical structure has changed, requiring further investigation. The most common cause of an axis shift is an increase in the muscle mass of one of the ventricles, known as hypertrophy.
Left Axis Deviation (LAD), defined as an axis more negative than -30 degrees, is associated with conditions that increase electrical forces in the left ventricle. Common causes include Left Ventricular Hypertrophy or a disruption in the conduction system, such as a Left Anterior Fascicular Block.
Right Axis Deviation (RAD), an axis greater than +90 degrees, occurs when electrical forces shift toward the right side of the heart. RAD can be caused by Right Ventricular Hypertrophy, often resulting from chronic lung diseases like COPD.
Other non-cardiac factors can also temporarily shift the axis, including improper electrode placement, significant obesity, or pregnancy, which physically alters the heart’s position. Interpreting axis deviation requires considering the patient’s entire clinical picture, as the EKG finding is a sign, not the final answer.