Left axis deviation is a condition in which the electrical axis of the heart's ventricular depolarization is abnormally positioned between negative 30 and negative 90, which suggests an underlying anatomical or physiological condition is affecting the electrical conduction system of the heart. Common causes of left axis deviation include an old or recent myocardial infarction, paced rhythms, emphysema, hyperkalemia, and right-sided accessory pathways, such as Wolff-Parkinson-White syndrome, among other causes, according to ECG Library.
Due to the position of the heart in the chest, electricity normally travels down and to the left during ventricular depolarization, and this creates the ordinary electrical axis of the heart, which is positioned between negative 30 and 90 degrees if it is imagined that the apex of the heart lies at 90 degrees. Ventricular depolarization is represented on an electrical tracing by QRS complexes. Any deviation of the axis from normal can be determined by analyzing the QRS complexes of leads I and aVF on an ECG. The QRS is normally positive in both leads, but the QRS is positive in lead I and negative in lead aVF if the axis deviates to the left, according to one method of interpretation outlined by Family Practice Notebook. Left axis deviation may be a normal variant among healthy individuals, but it may also result from mechanical shift within the thorax.