The QRS complex is a set of waves that represents ventricular depolarization.
Ventricular depolarization occurs in a rather complicated sequence (i.e. septal → free wall → basal wall depolarization). So, the appearance of the QRS complex tends to be more complicated than that of a p wave.
The QRS complex can be thought of as a composite of several different waves. The appearance of a "normal" QRS complex also changes depending on what lead you look at. Even within a particular lead, there may be many variants of a normal QRS complex depending on individual anatomy and variations in lead placements. This leads to a lot of diversity in what a "normal ECG" looks like in different individuals at any given time.
Since there are several possible morphologies of a QRS complex, there is a standardized way of approaching how to name them. This involves using a combination of "q", "r", and "s" waves to describe how a QRS complex looks in any given lead.
In this section, we will discuss the convention used for naming QRS complexes (based on the appearance of their component waves), as well as the physiology that explains the shape of the QRS complex in select leads: lead II, and leads V1-V6.
Naming a QRS Complex
The q wave refers to a very specific wave: it is a negative deflection that precedes the r wave. It must occur at the very beginning of the QRS complex, with no other deflections occurring before it. If the first deflection of the QRS complex is positive, that QRS complex does not have a q wave.
The r wave refers to any positive deflection in the QRS complex.
The s wave refers to any negative deflection that follows an r wave.
A second upward deflection after the first r wave is called r’ (pronounced “r prime”).
If a deflection does not cross the isoelectric baseline, it does not count as a separate wave, and is instead referred to as a notch.
An uppercase letter is used to identify relatively large waves, and a lowercase letter is used to identify relatively small waves.
A single negative wave, with the absence of a positive deflection, has a peculiar naming convention. Because there's no R wave to reference, the single negative wave is referred to as a "QS" wave, though it does not cross the isoelectric line. However, this is more commonly just referred to as a "Q" wave.
The following are a list of examples of different types of QRS complexes and how they are named:
We have a few more special cases below.
Understanding the QRS complex in Lead II
The standard QRS complex is often depicted as the following qRs wave:
This typically represents the standard QRS wave in lead II, a very important lead in cardiac monitoring.
The three different parts of the qRs complex in lead II correspond to different parts of ventricular depolarization:
q wave: a small wave representative of septal depolarization
R wave: a large wave representative of ventricular free wall depolarization
s wave: a small wave representative of basal depolarization
Q Wave
Examine the animation above and notice how the LV depolarizes. You can see all the individual currents in red, and the cumulative effect of all these individual currents is represented as the central, white, rotating arrow. This cumulative effect (analogous to the net cardiac dipole) is what the ECG actually measures. The magnitude of the net cardiac dipole and angle that it makes with lead II determine the amplitude and polarity of the ECG deflection respectively.
Recall that the septum depolarizes from left-to-right because the left bundle starts branching earlier than the right bundle. Therefore, the q wave is negative in lead II because septal depolarization occurs opposite to the direction of the lead. Moreover, since the septum has a relatively lower mass, the amplitude of the q wave is usually smaller.
Normally the q wave is <1 mm wide, <2 mm deep, and <1/4 the height of the R wave.
R wave
The R wave is positive in lead II because free wall and apical depolarization occurs towards the bottom left, along lead II. Because LV mass > RV mass, the net cardiac dipole is tilted towards the left ventricle, and the effect of the rightward depolarization of the RV is effectively hidden in the ECG tracing.
S Wave
The s wave is negative because depolarization of the basal walls of the LV occurs away from lead II, due to the curvature of the ventricles.
The QRS Complex in the Precordial Leads
The appearance of the QRS complex changes throughout the precordial leads.
V1 is the only rightward-point precordial lead, and V6 is the most leftward-pointing precordial lead.
Generally speaking, currents going rightward will appear positive in V1 whereas currents going leftward will appear negative in V1. The opposite tends to happen in V6.
The QRS in Lead V1
Notice how the QRS complex in V1 takes on either a rS or rSr’ appearance as a normal variant. Keep in mind that rightward currents are positive in V1.
The first r wave is due to left-to-right septal depolarization.
The following S wave is due to leftward apical and LV free wall activation.
Depending on variations in heart anatomy, orientation of the heart in the chest, and precordial lead placement, basal depolarization may either appear negative or positive in V1. If it is negative, basal depolarization blends in with the end of the S wave. However, if it is positive, you may see a small r’ wave at the end of the QRS complex.
R Wave Progression
The QRS complexes will change as we progress through the precordial leads, with the R wave becoming more prominent and the S wave becoming less prominent. This is known as normal R wave progression. The peak height of the R wave typically occurs around leads V4-V5.
The QRS in Lead V6
In V6, the QRS complex typically takes on a qRs complex, but qR is a normal variant.
The left-to-right septal depolarization causes an initial small q wave.
The leftward apical and LV free wall activation causes a tall R wave.
Basal depolarization typically points rightward and produces a small s wave at the end of the QRS. However, due to normal variants in anatomy and precordial lead placement, basal depolarization may be leftward-oriented and thus just contribute to the R wave without producing a separate s wave.
The QRS Width
The QRS is generally narrow, measuring <120 ms in width. This is because the Purkinje fibres are able to conduct so quickly that all the endocardium gets activated almost simultaneously. In the normal heart, most of the time it takes to depolarize the ventricles comes from the time it takes the depolarization wave to move from endocardium to epicardium.
Instead, if the His-Purkinje system were dysfunctional, or if the depolarization propagated through the myocardium only and NOT the Purkinje fibres (causing slow and sequential activation), it would take much longer for the signals to traverse through the myocardium.
The peak activation time (PAT) is the time from the start of the QRS complex to the peak of the R wave. It represents the amount of time required to achieve the maximal depolarization in that lead’s direction. Ventricles activated through the Purkinje fibres will have short PATs, whereas ventricles activated through the myocardium will have long PATs.
Other terms used interchangeable with PAT:
Ventricular activation time (VAT)
R-wave peak time (RWPT)
Time to Intrinsicoid deflection, where the intrinsicoid deflection is the downward deflection following the peak of the R wave
Normally, PAT is <45 ms.
Summary of the Normal QRS
QRS complexes are normally <120 ms wide with a peak activation time <45 ms.
In Lead II, the complex is qRs.
In Lead V1, the complex is rS or rSr’.
In Lead V6, the complex is qRs or qR.
The normal R wave progression is shown below. Normally, the peak R wave height occurs around V4-V5.