QRS Complex

QRS Complex

QRS complex represents the depolarization of the ventricles. Initial negative deflection is named as the Q wave and a negative deflection after a positive deflection the S wave. First positive deflection is called R wave.

If the first deflection is positive, the ensuing negative deflection is called S wave and not Q wave. If there are two positive deflections, then the second one is named R’. Similarly, an additional negative deflection after R’ is called an S’ wave. Duration of the QRS complex in an adult is up to 110 milliseconds. If it is more than that, it is called as a wide QRS.

Waves of low amplitude are denoted by lower case letters and larger amplitude waves by upper case letters. e.g. rSr’ means a small r wave followed by a deep S wave and a secondary r’ wave. If there is only a single deep negative deflection, it is also called a QS complex.

Normal precordial QRS pattern

Normally V1 shows a small r followed by an S wave. Q waves except a QS complex in V1 is considered abnormal and could represent an anterior myocardial infarction or an abnormal activation sequence as in left bundle branch block or ventricular inversion. In V6, the usual pattern is qRs.

The rS pattern in right precordial leads changes to a qRs pattern in V3 or V4. This is known as the transition zone. In simpler terms, transition is from a dominant S wave to a dominant R wave. When transition occurs earlier as in V2, it is called early transition of QRS pattern and if it occurs only in V5 or V6, it is called late or delayed QRS transition. Late transition with rS pattern extending up to V6 is seen in emphysema, where the heart is more vertical and pushed down so that the QRS pattern similar to V1 are recorded in all precordial leads.