An electrocardiogram (EKG) records the electrical activity of the heart, translating the heart’s depolarization and repolarization into wave patterns. The electrical axis represents the average direction of the electrical current as it flows through the ventricles. Axis deviation occurs when this mean electrical direction shifts significantly away from its normal path. This EKG finding is not a disease itself, but rather a diagnostic signal that the heart may be experiencing structural changes or electrical conduction problems. Interpreting this shift provides a non-invasive clue that helps healthcare providers narrow down possible underlying cardiac or pulmonary conditions.
Understanding the Heart’s Electrical Axis
The electrical axis is determined by calculating the sum of all electrical vectors representing the spread of depolarization across the ventricles. Since the left ventricle is significantly thicker and more muscular than the right, the electrical current naturally pulls toward the left side of the chest. This directional preference establishes the normal electrical axis, which is typically directed downward and slightly to the left. The normal range for this axis in adults is between -30 and +90 degrees on the frontal plane of the EKG. This measurement uses a conceptual system based on the limb leads, which view the heart’s electrical activity from different angles. When the electrical impulse travels outside of this standard range, it suggests that the balance of forces within the heart muscle has been altered.
Identifying Left and Right Axis Deviation
Axis deviation is categorized into two types based on the direction of the electrical shift: left or right. Left Axis Deviation (LAD) occurs when the mean QRS electrical axis shifts more negatively than -30 degrees, typically falling between -30 and -90 degrees. This shift indicates the primary electrical force is pulled upward and to the patient’s left. Right Axis Deviation (RAD) is defined by an axis that shifts more positively than +90 degrees, often extending toward +180 degrees. A quick method to identify these deviations involves examining the QRS complex in two frontal plane leads: Lead I (0 degrees) and Lead aVF (+90 degrees). In LAD, the QRS complex in Lead I will be predominantly positive, while the QRS complex in Lead aVF will be predominantly negative. Conversely, in RAD, the QRS complex in Lead I will be predominantly negative, and Lead aVF will be predominantly positive.
Medical Conditions Associated with Axis Deviation
The most common reason for an electrical axis shift is a change in the physical mass or the conduction timing of the ventricular muscle.
Causes of Left Axis Deviation (LAD)
A frequent cause of a leftward shift is Left Ventricular Hypertrophy (LVH), which is an enlargement of the left ventricle muscle mass. This increased muscle requires more electrical energy to depolarize, pulling the mean electrical vector further to the left and superiorly. LVH often results from chronic conditions like uncontrolled hypertension or aortic valve stenosis, where the left ventricle pumps against increased resistance. Another cause of LAD is a delay in the heart’s conduction system, specifically a Left Anterior Fascicular Block (LAFB). This blockage forces the electrical impulse to detour, causing the current to travel in a superior and leftward direction. An inferior myocardial infarction (heart attack) can also cause LAD by creating a patch of electrically inactive scar tissue.
Causes of Right Axis Deviation (RAD)
Right Axis Deviation is most frequently linked to conditions causing Right Ventricular Hypertrophy (RVH), such as severe lung disease or pulmonary hypertension. High blood pressure in the lung arteries forces the right ventricle to work harder, leading to muscle thickening and enlargement. This increased mass generates a stronger electrical force that pulls the mean axis toward the right and inferiorly. Acute conditions, such as a large pulmonary embolism (a blood clot in the lung artery), can cause sudden, severe strain on the right ventricle, leading to an abrupt shift toward RAD. Additionally, a Left Posterior Fascicular Block (LPFB) can cause RAD by delaying the electrical signal to the lower portion of the left ventricle, shifting the balance of electrical forces to the right.
Clinical Significance and Follow-Up
The detection of axis deviation on an EKG provides evidence that the heart or lungs may be under stress. It prompts the healthcare provider to identify the root cause, as a marked deviation is often associated with structural heart disease. The clinical significance is tied directly to the underlying pathology, which can range from chronic, manageable conditions to acute events. When axis deviation is found, the next step is typically a comprehensive clinical assessment, including a detailed history and physical examination. The most common follow-up test is an echocardiogram, which uses ultrasound to visualize the heart’s structure and function, confirming or ruling out ventricular hypertrophy and assessing heart valve function.