Determining the heart’s electrical axis is a foundational step in interpreting an electrocardiogram (ECG). This axis represents the average direction of the electrical current flow, specifically the ventricular depolarization wave. Analyzing this vector provides immediate insight into the heart’s physical orientation and can alert a reader to potential underlying cardiac or pulmonary conditions.
The electrical axis is determined by analyzing the QRS complex across the six limb leads of the ECG. This complex electrical activity is summarized into a single vector, measured in degrees within the frontal plane. Since the left ventricle is the largest and most muscular chamber, the normal electrical axis is directed downward and slightly to the left, typically ranging from -30 degrees to +90 degrees in adults.
The hexaxial reference system classifies axis deviation into four quadrants. Left Axis Deviation (LAD) is between -30 and -90 degrees, and Right Axis Deviation (RAD) is between +90 and +180 degrees. If the axis falls between -90 and -180 degrees, it is classified as Extreme Axis Deviation, sometimes called “Northwest Axis.”
Defining the Electrical Axis and Normal Parameters
The electrical axis represents the net vector of ventricular depolarization in the frontal plane, where the six limb leads are positioned. This vector points in the direction of the electrical current flow, with its length proportional to the voltage produced. The massive muscle mass of the left ventricle dictates that the overall electrical activity pulls the average vector down toward the left, establishing the normal range.
The normal range of -30 to +90 degrees ensures the vector points downward and toward the left side of the body. When the electrical current moves toward a lead’s positive electrode, the ECG records a positive deflection (an upward spike). Conversely, movement away from the positive electrode results in a negative deflection (a downward spike).
The Quick Method Using Leads I and aVF
The most practical technique for determining the electrical axis is the quadrant method, which relies solely on examining Leads I and aVF. These two leads are oriented perpendicular to one another, dividing the frontal plane into four 90-degree sections. Lead I runs horizontally (0 degrees), and Lead aVF runs vertically toward the feet (+90 degrees).
The first step involves looking at the net deflection of the QRS complex in Lead I. If Lead I is positive, the axis is oriented toward the left side of the patient (upper or lower left quadrant). If Lead I is negative, the axis is oriented toward the right (upper or lower right quadrant).
The second step repeats this analysis for Lead aVF, which determines the vertical placement of the axis. A positive QRS complex in Lead aVF means the vector is directed downward (lower half). A negative QRS complex means the vector is directed upward (upper half).
Combining these observations quickly establishes the quadrant:
- If both Lead I and Lead aVF are positive (up-up), the axis is normal (0 to +90 degrees).
- If Lead I is positive and Lead aVF is negative (up-down), this indicates Left Axis Deviation (0 to -90 degrees).
- A negative Lead I and positive Lead aVF (down-up) points to Right Axis Deviation (+90 to \(\pm\)180 degrees).
- If both are negative (down-down), it signals Extreme Axis Deviation (-90 to \(\pm\)180 degrees).
Precise Measurement using the Isoelectric Lead
While the quadrant method provides rapid categorization, the isoelectric lead method offers a more accurate estimation of the axis in specific degrees. This method utilizes all six limb leads and the hexaxial reference system to pinpoint the vector’s exact direction. The core principle is that the electrical axis will be perpendicular to the lead that registers the smallest electrical activity.
The first action is to identify the isoelectric lead, also known as the equiphasic lead, among the six frontal leads. This is the lead where the net deflection of the QRS complex is closest to zero (the sum of positive and negative deflections is equal). When the electrical vector travels perpendicular to a lead, the forces cancel out, resulting in a minimal tracing.
The electrical axis must lie 90 degrees away from the isoelectric lead’s orientation. For example, if Lead I (0 degrees) is the most isoelectric lead, the axis must be directed toward either +90 or -90 degrees. To resolve this 180-degree ambiguity, one must examine the QRS complex of the lead that is 90 degrees away.
If the perpendicular lead shows a predominantly positive QRS complex, the axis points toward that lead’s positive pole. Conversely, if the perpendicular lead shows a negative QRS complex, the axis points away from that lead’s positive pole. Using the quadrant determined by the quick method often helps to quickly narrow the choice between the two 90-degree possibilities.
Clinical Causes of Axis Deviation
An abnormal electrical axis suggests the balance of electrical forces has shifted, often due to changes in muscle mass or conduction pathways. Left Axis Deviation (LAD) is frequently caused by Left Ventricular Hypertrophy, which increases muscle mass and electrical activity on the left side. Certain conduction abnormalities, such as Left Anterior Fascicular Block, also result in LAD.
Right Axis Deviation (RAD) often results from increased electrical forces on the right side, most commonly due to Right Ventricular Hypertrophy. This hypertrophy can occur secondary to conditions like severe pulmonary hypertension or acute pulmonary embolism, which strain the right ventricle. Conduction defects like Left Posterior Fascicular Block also lead to RAD.