What is the QRS axis on electrocardiography (ECG)?

Updated: Mar 11, 2019
  • Author: Tarek Ajam, MD, MS; Chief Editor: Terrence X O'Brien, MD, MS, FACC  more...
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Answer

Answer

The QRS axis represents the major vector of ventricular activation, which is the overall direction of electrical activity. The electrical activity in healthy individuals starts at the sinoatrial node and spreads to the atrioventricular node down the Bundle of His, followed by conduction through the left and right bundle branches, and then to the Purkinje fibers to cause ventricular contraction. A positive deflection is when the direction of the overal electrical activity is toward that lead. Therefore, the cardiac axis may provide the overall direction of electrical activity when the ventricles depolarize. The normal cardiac axis is expected to lie between -30º and 90º, which means the overall direction of electrical activity is toward leads I, II, and III.

Electrocardiographic axis. Normal axis is between Electrocardiographic axis. Normal axis is between -30º and 90º; left axis deviation (LAD) is between -30º and - 90º; right axis deviation (RAD) is between 90º and 180º; extreme axis deviation (EAD) (left or right) is between -90º and -180º.

 

The electrical axis can be estimated from the standard frontal leads. There are shortcut methods to determine the axis. For example, if the QRS complex is upright in both leads I and II, then the axis must fall somewhere between -30º and 90º and the axis is normal.

If the complexes are negative in lead I and positive in lead aVF, then the axis is rightward. If the complexes are positive in lead I but negative in lead II, then the axis is leftward. If the complexes are negative in both leads I and aVF, then the axis is extreme.

The causes of left axis deviation include normal variation, left ventricular hypertrophy, left anterior fascicular block, congenital heart disease with primum atrial septal defect or endocardial cushion defect, ventricular ectopic beats, and preexcitation syndromes. [26]

The causes of right axis deviation include normal variation, right ventricular hypertrophy, left posterior fascicular block, ventricular ectopic beats, preexcitation,  and dextrocardia. [26]  


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