ECG : The Axis

The cardiac axis refers to the overall direction of the heart's electrical depolarization during ventricular contraction, as indicated by the QRS complex on an electrocardiogram (ECG). Assessing the cardiac axis can provide important insights into the heart's orientation and function, and help identify underlying conditions.

Various methods exist for estimating the QRS axis, including the Quadrant Method, Isoelectric Lead Analysis, and others. Here, we focus on the Three-Lead Analysis (Leads I, II, and aVF) for assessing the cardiac axis. Understanding and accurately assessing the cardiac axis is essential for diagnosing various cardiac conditions and guiding clinical management.

Steps in Assessment of Cardiac Axis Using Three-Lead Analysis:

1. Determine the QRS Axis:

   • Identify the QRS complexes in leads I, II, and aVF.
   • Determine whether the QRS complex in these leads is predominantly positive or negative.

2. Identify Normal Axis:

   • If the QRS complexes are predominantly positive in leads I and II, the axis is considered normal, typically between -30° and +90°.

3. Assess for Left Axis Deviation (LAD):

   • If the QRS complex is predominantly positive in lead I and negative in lead II and aVF, suspect left axis deviation.
   • Common causes include left ventricular hypertrophy, left anterior fascicular block, inferior myocardial infarction, left bundle...