Left axis deviation, or LAD, is a commonly noted abnormality on an electrocardiogram (EKG). It can be identified on a graph of heart activity. Where a normal axis falls within a range of -30° to +90° in the right quadrants, a deviation to the left axis would fall anywhere from -30° to -90° in the right upper quadrant of the graph. In general, left axis deviation is not in itself problematic unless it is deviated to a large extent, or unless it is accompanied by other cardiac problems.

There are many different causes for a left axis deviation. For example, it could be what is called a normal variant, in that it shows solely on an electrocardiogram without any other indications of heart disease. Any axis deviation from the normal range, however, is termed an EKG abnormality, and further clinical testing is generally recommended when this type of pattern is shown.

Different types of cardiac disease can also cause left axis deviation, with some types being more serious than others. A left anterior fascicular block — also known as left anterior hemiblock — can cause the deviation, but it is generally not considered serious in the absence of other EKG irregularities. Additionally, it has been known to appear in about 14% of all EKGs that were otherwise normal and were taken of persons without known heart problems.

An inferior myocardial infarction, or heart attack, is a more serious cause of left axis deviation. This tends to be accompanied by other EKG irregularities, however, as well as physical symptoms of chest pain or loss of consciousness. An abnormal left anterior hemiblock has been found to occur in about four percent of myocardial infarctions and has a left axis deviation of between -45° and -60°.

Chronic obstructive pulmonary disease (COPD) and emphysema are two diseases of the lungs that can also cause left axis deviation. The breathing difficulties experienced by those who suffer from either of these medical conditions can lead to the lungs becoming chronically overinflated and shift the heart to the right. The shift skews the EKG reading further toward the left axis than it would normally be, thus causing a left axis deviation. This is also sometimes termed a mechanical deviation.