The chest leads look at the heart in a horizontal plane.
V1 represents the rightmost view, and V6 - the leftmost.
The QRS complex represents depolarization of the ventricles which starts with the interventricular
septum.
In normal conduction, depolarization of the septum is initiated from the left bundle going
to the right, TOWARD V1 and AWAY from V6.
This results in a small positive deflection in V1 and a negative deflection in V6.
The signals then move both directions to the two ventricles, but as the left ventricle
is usually much larger, the NET movement is to the left, AWAY from V1, TOWARD V6.
This corresponds to a negative wave in V1 and a positive wave in V6.
Thus, the QRS complex starts as predominantly negative in V1, and ends as predominantly
positive in V6.
Somewhere in between, usually from V3 to V4, it is isoelectric, with equal positive and
negative deflections.
This is known as the transitional zone.
In addition, there is a gradual increase in amplitude of R wave from V1 to V5.
This is known as R wave progression.
The normal transitional zone is between V3 and V4.
When transition happens at or before V2, it is referred to as early transition, rightward
shift, or counter-clockwise rotation.
This is because these ECG patterns would have been generated if the heart had rotated counter-clockwise
around the longitudinal axis.
Reversely, when the transition occurs after V4, it is referred to as late transition,
leftward shift, or clockwise rotation.
These shifts may or may not be signs of heart diseases.
In many cases, these are simply artefacts, resulting from incorrect placement of the
chest electrodes - too low or too high.
In other cases, they are due to normal anatomical variations of the heart’s shape and orientation.
Clockwise rotation is more commonly associated with cardiovascular diseases while counter-clockwise
rotation is more common in healthy individuals.
Some clinical causes of clockwise rotation include: - Physical rotation of the heart
in conditions such as chronic obstructive pulmonary disease - Conduction problems due
to anterior myocardial infarction - Heart chambers dilatation (Dilated cardiomyopathy)
Some clinical causes of counter-clockwise rotation include: - Conduction problems due
to posterior myocardial infarction - Electrical shift to the right in conditions such as right
ventricular hypertrophy When the transitional zone is absent, or is not clear, it is usually
clinical.
In this case it may be helpful to look at R wave progression.
Non-progression or poor progression of R wave - R wave stays low and S wave remains deep
throughout all chest leads.
This is an extreme case of clockwise rotation and is suggestive of extensive anterior myocardial
infarction.
Reverse progression of R wave - tall R wave in V1, tallest in V1 or V2 - is usually seen
in right ventricular hypertrophy.
Increased muscle mass in the right ventricle results in net electrical movement towards
the right chest leads.