From the Guidelines
To determine the axis on an electrocardiogram (EKG), you should analyze the QRS complex in leads I and aVF, considering the net deflection in these leads to estimate the axis as normal, leftward, rightward, or extreme rightward. The axis is a crucial aspect of EKG interpretation, reflecting the overall direction of electrical depolarization through the ventricles. According to the American Heart Association recommendations 1, the normal QRS axis in adults is considered to be within -30° and 90°, with left-axis deviation being beyond -30° and right-axis deviation being beyond 90°.
Key Points for Axis Determination:
- Analyze leads I and aVF for net QRS complex deflection
- Use additional leads or the hexaxial reference system for precise axis determination
- Consider age and body habitus variations in axis interpretation
- Recognize that axis deviation outside the normal range may indicate cardiac pathology
The determination of the axis is essential for diagnosing various cardiac conditions, including left or right ventricular hypertrophy, bundle branch blocks, or fascicular blocks. It's also important to note that the appearance of certain conduction disturbances, like left bundle branch block (LBBB), can change the mean QRS axis in the frontal plane 1. However, for the purpose of determining the axis, focusing on the QRS complex in leads I and aVF provides a straightforward initial approach.
From the Research
Determining the Axis on an EKG
To determine the axis on an electrocardiogram (EKG), several methods can be employed, including:
- Analyzing the QRS direction and magnitude in limb leads 2
- Using a mnemonic such as "+/- 90, -30, +30, -60, +60, and 0" to estimate the frontal mean QRS axis 2
- Identifying the most isoelectric lead and matching the corresponding pre-determined value to estimate the frontal mean QRS axis 2
- Interpolating between two leads if no single isoelectric lead is identified 2
- Utilizing a new method that divides six limb leads into two groups based on the axis perpendicular to the mean QRS axis (P-ÂQRS) 3
- Checking external leads (aVL or III) to determine the P-ÂQRS and subsequently the ÂQRS 3
Methods for Estimating the QRS Axis
Different studies have proposed various methods for estimating the QRS axis, including:
- A simple method of estimating the axis by assigning pre-determined values to each frontal lead in ECGs 2
- A new method that uses the P-ÂQRS to determine the ÂQRS 3
- A method that calculates the mean QRS axis using the net voltage of QRS complexes in Lead-I and Lead-III 4
- A method that uses the inspection of the QRS voltage in six of the limb leads, although this method has an inherent element of subjectivity 4
Clinical Significance of the QRS Axis
The QRS axis has clinical significance, as an abnormal QRS axis can indicate underlying myocardial disease 5