An electrocardiogram (EKG) provides a visual representation of the heart’s electrical activity. It records the electrical signals generated by the heart muscle as it contracts and relaxes. The EKG axis refers to the general direction of this electrical current. Understanding the EKG axis is important for interpreting an EKG, offering insights into the heart’s overall electrical health.
The Heart’s Electrical Compass
The heart functions through an electrical system, originating with specialized cells that generate impulses. These impulses spread throughout the heart muscle, orchestrating its coordinated contractions. This electrical flow has a primary direction, or vector, representing the average path of current during ventricular depolarization. The left ventricle, being the largest portion of the heart’s muscle, largely shapes this overall direction. The EKG axis quantifies this collective electrical force.
Decoding EKG Leads
EKG leads act like different camera angles, each providing a unique perspective on the heart’s electrical activity. These leads are strategically placed on the body to capture the electrical signals from various directions. For determining the EKG axis, two specific limb leads are particularly useful: Lead I and Lead aVF. Lead I records electrical activity horizontally across the chest, from the right arm to the left arm. Lead aVF, on the other hand, monitors electrical activity vertically, from the left leg upwards towards the heart.
Current movement relative to these leads dictates the tracing seen on the EKG. If the electrical current flows towards a positive electrode of a lead, it creates an upward (positive) deflection on the EKG tracing. Conversely, if the current flows away from a positive electrode, it results in a downward (negative) deflection. An isoelectric deflection, where the upward and downward movements are roughly equal, indicates that the electrical current is flowing perpendicular to that lead.
Simple Steps to Determine Axis
A straightforward approach to determine the EKG axis involves using the deflections observed in Lead I and Lead aVF, often called the quadrant method. This method divides the electrical plane around the heart into four 90-degree quadrants. First, examine the QRS complex in Lead I to see if the net deflection is positive (mostly above the baseline) or negative (mostly below the baseline). Then, do the same for Lead aVF.
If both Lead I and Lead aVF show a net positive deflection, the EKG axis is normal (0° to +90°), indicating expected downward and slightly leftward electrical flow. If Lead I is positive but Lead aVF is negative, this suggests left axis deviation (0° to -90°). Conversely, a negative Lead I and a positive Lead aVF indicate right axis deviation (+90° to +180°). If both Lead I and Lead aVF are negative, it points to an extreme axis deviation (sometimes called a “northwest axis”), found between -90° and -180°.
What Axis Deviations Indicate
A normal EKG axis, generally ranging from -30° to +90°, suggests the heart’s electrical impulses follow their typical pathway through the ventricles. When the axis deviates, it can provide clues about potential changes within the heart. For example, left axis deviation might suggest conditions that increase the electrical influence of the left side of the heart, such as an enlargement of the left ventricle or certain conduction abnormalities.
Right axis deviation could be associated with conditions affecting the right side of the heart. This might include an enlargement of the right ventricle or issues related to the lungs that place a strain on the right heart chambers. Axis deviations are indicators, not definitive diagnoses.
The Importance of Professional Interpretation
Understanding EKG axis basics is informative, but has limitations. Accurate EKG interpretation, including axis assessment, requires extensive training and experience. Only qualified medical professionals, such as physicians or cardiologists, possess the expertise to properly evaluate EKG findings within the context of a patient’s complete medical history and other clinical information. Self-interpretation or self-diagnosis based solely on EKG axis is misleading and not recommended.