- Mean ventricular (QRS) axis between -45° and -90° in adults with a mean QRS duration <120 ms
- Typically qR complexes in I and aVL, along with rS complexes in II, aVF, and III
- qR pattern in aVL with >45 ms delay from beginning of QRS complex to peak of R wave
- Absence of other causes of left axis deviation (e.g. inferior myocardial infarction, left ventricular hypertrophy)
- Note: The entire left bundle conduction system of the heart is made up of a left anterior and posterior fascicle. The left anterior fascicle supplies fibers to the anterior and lateral walls of the left ventricle.
- Note: The above criteria do not apply to patients with congenital heart disease in whom left axis deviation is present in infancy.