The cardiac axis is a measurement used to determine the overall direction of the electrical signal as it spreads through the heart’s lower chambers, the ventricles, during a heartbeat. This electrical signal, which triggers muscular contraction, is summarized as a single vector, known as the mean QRS vector. The axis is easily calculated from a standard electrocardiogram (ECG or EKG) and provides insight into the electrical health and physical structure of the heart.
How the Heart’s Electrical Activity is Measured
The heart’s electrical current, which represents the depolarization of the ventricles, travels in a specific direction. Since the electrical energy flows through the heart muscle, its average direction can be affected by the size of the heart chambers or any damage to the electrical pathways.
The electrocardiogram uses small sensors, known as leads, placed on the limbs and chest to record the heart’s electrical activity. These leads act like “cameras” positioned around the heart, each capturing the electrical vector from a different angle. The limb leads, specifically Lead I, Lead II, and Lead aVF, are the primary leads used to calculate the cardiac axis in the frontal plane.
The axis is calculated by observing whether the electrical signal is moving toward or away from each lead, which is seen as an upward (positive) or downward (negative) deflection on the ECG tracing. For example, Lead I looks horizontally across the heart, and Lead aVF looks vertically from below. By assessing the deflections in these leads, medical professionals determine the average direction of the electrical current as a single angle measured in degrees.
What Constitutes a Normal Cardiac Axis
The cardiac axis is measured in degrees using a standardized coordinate system, where 0 degrees points horizontally to the patient’s left and +90 degrees points straight down. Under normal circumstances, the electrical impulse naturally travels from the upper right of the heart toward the lower left, where the largest muscle mass, the left ventricle, is located.
The normal range for the cardiac axis is between -30 degrees and +90 degrees. This range signifies that the electrical activation follows the expected, healthy path through the ventricles. Because the left ventricle is significantly thicker than the right, its electrical activity dominates the overall vector, pulling the average direction downward and to the left.
Slight variations in axis can be normal and are often related to a person’s physical build or body position. For instance, a taller, thinner person may have a more vertically positioned heart, resulting in an axis closer to +90 degrees. Conversely, a person with obesity or pregnancy may have a more horizontal heart, shifting the axis closer to 0 degrees.
Interpreting Abnormal Axis Shifts
When the electrical current deviates significantly from the normal range, it is termed axis deviation, suggesting an underlying cardiac or pulmonary issue that has physically or electrically altered the heart. These abnormal shifts are categorized into Left Axis Deviation (LAD) and Right Axis Deviation (RAD). An axis deviation is a strong indicator, but it is not a diagnosis on its own and requires a full clinical evaluation to determine the cause.
Left Axis Deviation (LAD)
Left Axis Deviation occurs when the calculated axis is more negative than -30 degrees, meaning the electrical signal is abnormally directed upward and sharply to the left. The most frequent cause of LAD is a problem within the heart’s conduction system, specifically a block in the left anterior fascicle, one of the electrical pathways in the left ventricle.
LAD can also be caused by conditions that increase the electrical influence of the left side of the heart or cause a mechanical shift. Left ventricular hypertrophy, a thickening of the left ventricle muscle often due to long-standing high blood pressure, can cause this shift. Damage to the inferior wall of the heart from a prior heart attack can also change the direction of the electrical flow, leading to LAD.
Right Axis Deviation (RAD)
Right Axis Deviation is defined as an axis greater than +90 degrees, meaning the electrical signal is directed strongly downward and to the right. This shift indicates that the right side of the heart is exerting a stronger electrical force than it should, often due to an increased workload. Since the right ventricle is typically much thinner than the left, its electrical dominance suggests a significant change in its structure.
Right ventricular hypertrophy is a common cause of RAD, often developing when the right side of the heart has to pump against abnormally high pressure in the lungs. Chronic lung diseases like Chronic Obstructive Pulmonary Disease (COPD) can lead to pulmonary hypertension, causing the right ventricle to thicken and dominate the electrical vector. Acute conditions, such as a large pulmonary embolism, can also cause a sudden strain on the right ventricle, resulting in an immediate RAD.
Other causes of RAD include conduction system problems, such as a block in the left posterior fascicle, or congenital heart defects that alter the structure of the ventricles. Although RAD can sometimes be a normal variant in children or very thin adults, its presence in a typical adult often prompts a search for underlying cardiopulmonary pathology.