The normal axis is located between - 30 and + 90° (wide variability depending on the patient). The heart is said to be horizontal when the axis is between 0 and -30° and vertical when it is between 60 and 90°; the axis approaches vertical for slim subjects (tall and thin) and horizontal for stocky subjects (short and overweight) or older. A deviation beyond -30° is considered a left axis deviation and beyond 90° as a right axis deviation (or 110° depending on the definitions).
A left axis deviation of the QRS-complex can be observed in patients with left anterior fascicular block (the most common cause in the absence of heart disease), inferior myocardial infarction, left bundle branch block, left ventricular hypertrophy, ventricular pre-excitation, ostium primum atrial septal defect, hyperkalemia or tricuspid atresia.
A right axis deflection of the QRS-complex can be observed physiologically in the newborn, upon reversal of the arm electrodes, in patients with right ventricular hypertrophy, in various chronic lung diseases, pulmonary embolism, left posterior fascicular block, right bundle branch block, anterolateral myocardial infarction (Q wave in lead I), dextrocardia or ostium secundum atrial septal defect.
An extreme-right axis of the QRS-complex (upper-left quadrant, between 180 and 270°) is observed following a reversal of electrodes, hyperkalemia, ventricular tachycardia or ventricular pre-excitation but also in certain patients presenting a combination of two abnormalities: right ventricular hypertrophy (negative lead I) with a left anterior fascicular block (negative aVF), lateral infarction with Q waves in lead I and left anterior fascicular block, or inferior infarction with left posterior fascicular block.