The heart functions as a pump, propelled by electrical signals that coordinate its contractions. This electrical activity is fundamental to circulating blood throughout the body. Understanding the direction of these electrical impulses provides insight into heart function and health.
What is Heart Axis?
The heart axis represents the average direction of electrical impulses through the ventricles during a heartbeat. It’s like a compass needle, reflecting the net electrical flow. It indicates the electrical activity of the ventricles, the main pumping chambers.
Electrical impulses start in the right atrium and spread through the ventricles. The left ventricle is thicker than the right, so its electrical influence is stronger, causing the average direction to lean left. This consistent direction allows assessment of the heart’s electrical orientation.
How Heart Axis is Measured
Heart axis is determined using an electrocardiogram (ECG or EKG). This non-invasive test places electrodes on the skin to detect the heart’s electrical signals. These electrodes provide different “views,” or leads, of electrical activity.
Each lead records whether the electrical impulse moves towards or away from it, resulting in upward or downward deflections on the ECG. By observing these deflections across several leads, especially leads I and aVF, healthcare providers calculate the heart’s electrical current direction. This establishes the heart’s electrical orientation.
Normal and Abnormal Heart Axis
A normal heart axis points downwards and to the left, typically within -30 to +90 degrees. Most healthy adults have an axis between 0 and +75 degrees. Deviation from this range is called axis deviation.
Left Axis Deviation (LAD) occurs when the axis shifts left, generally between -30 and -90 degrees. Right Axis Deviation (RAD) means the axis points right, usually between +90 and +180 degrees. Extreme axis deviation, sometimes called “Northwest Axis,” indicates electrical activity between -90 and 180 degrees.
What Causes Heart Axis Deviations?
Heart axis deviations are indicators of underlying heart conditions or other health issues. Left Axis Deviation (LAD) can result from factors affecting the left side of the heart. Common causes include left ventricular hypertrophy (enlargement of the left ventricle), often due to high blood pressure or aortic stenosis. Conduction defects, such as a left anterior fascicular block (a delay in electrical signals), also frequently lead to LAD. An inferior myocardial infarction (heart attack affecting the lower heart) can also shift the axis left.
Right Axis Deviation (RAD) suggests an issue affecting the right side of the heart. The most frequent cause is right ventricular hypertrophy (enlargement of the right ventricle), seen in conditions like pulmonary hypertension or chronic lung diseases such as COPD. A pulmonary embolism (sudden blockage in lung arteries) can also cause acute right ventricular strain and lead to RAD.
Certain conduction defects, such as a left posterior fascicular block, are also associated with RAD. Non-cardiac factors like body build, pregnancy, or incorrect ECG electrode placement can also influence the axis. A doctor uses heart axis information, combined with other diagnostic findings, to understand a patient’s heart health.