QRS Complex

Depolarization of the right and left ventricles is indicated on the ECG by what is known as the QRS complex.

The normal QRS complex begins with a negative deflection, which is called the Q wave. This depicts depolarization of the interventricular septum. If the heart muscle is scarred, for example following a myocardial infarction, pathological Q waves are seen on the ECG. But more on that later.

In the normal ECG, the first positive deflection after the P wave is called the R wave. This depicts the depolarization of the main bulk of the ventricular muscle.

In the normal ECG, the R wave is followed by a negative wave, which is called the S wave. If a second positive amplitude occurs after the R, it is referred to as R "dash".

In short: If the QRS is negative at the beginning, it starts with a Q. Regardless of whether a Q is present or not - the first positive amplitude is called R. If a negative amplitude follows, that is called the S wave.

Physiologically, the QRS complex lasts less than 100 ms. This means that the muscle of both ventricles is fully depolarized within a maximum of 100 ms. If the electrical conductivity of the ventricles is disrupted by some type of pathology, delays in the propagation of depolarization, or even a complete blockage of electrical conduction, can occur.

If a blockage occurs, depolarization of the downstream myocardial segments is delayed, as a detour must be made to overcome the obstruction. This means that ventricular depolarization takes longer than usual. When such a delay or blockage of ventricular depolarization occurs, a broadening of the QRS complex is seen on the ECG.

When the duration, or "width", of the QRS complex is 110 ms, we refer to incomplete bundle branch block. When the width is 120 ms or more, we refer to complete bundle branch block. You will learn more about these conditions in a later chapter.

Let's go back to the very start and talk about the different waves of the QRS complex

We talked about this before: A Q wave is any negative deflection that comes after a P wave and before an R wave.

If the R wave is preceded by a positive deflection, there is no Q wave in this lead. If you only find a negative wave, instead of a complete QRS complex, then this is called a QS complex. It is normal to find a QS complex in lead V1.

What does the presence of a Q wave actually tell us?

The Q wave is negative, meaning that depolarization is spreading in the opposite direction to that of the respective lead. Remember: if there is a negative wave, deviation from the lead’s axis must be more than 90 degrees.

In which myocardial segment does depolarization normally progress  in the opposite direction to the main vector?

Depolarization in the left ventricle proceeds from the AV node through the Bundle of His and into the Tawara branches. Shortly after passing the AV node, small lateral branches diverge down in the direction of the interventricular septum.

These small side branches also run to the right, and even to the back, in the direction of the heart base.

At this point, the direction in which the depolarization is spreading is at an angle of more than 90° from the heart’s electrical axis. This means that a negative deflection will be observed in all ECG leads that point in the direction of the heart’s electrical axis. These physiological Q waves represent the depolarization of the interventricular septum. Physiological septal Q waves are usually found in the limb leads, and in v5 and v6.

The occurrence of a Q wave in v1 to v3 is always pathological. No septal Q is to be expected here, since the principal vector of these leads is directed towards the septum.

In patients with severe hypertrophy of the interventricular septum, these physiological Q waves can be very pronounced, due to the increased myocardial mass.

In patients with left bundle branch block, conduction on the left side is blocked, and so the septum is depolarized instead from the right. In this situation, no septal Q waves are observed on the ECG.

Pathological Q waves are the most important ECG sign of a previous myocardial infarction. To understand this, you must first understand that dead myocardial tissue is no longer electrically active. The vector of the affected lead therefore runs in the opposite direction to normal. In acute myocardial infarction, it may take a period of hours to days until pathological Q waves develop.

At this point, it is important to understand that ECG leads should never be considered in isolation. Instead, we have to look for patterns in leads that correspond to specific myocardial areas. We’ll look at this in detail later, in the chapter on the coronary vessels and their corresponding leads.

Let’s re-cap once more on the leads that correspond to the walls of the left ventricle:

The anterior wall is examined using leads v1 to v6.

The inferior wall is examined using leads II, III, and aVF.

The lateral wall is examined using leads I and aVL.The free wall of the right ventricle is examined using atypical right heart leads V3R and V4R.

To be classified as pathological, a Q wave must be present in at least two leads from the same myocardial pattern group.

The presence of a Q wave in lead 1 and in lead 2 is unlikely to indicate any underlying pathology. However, the presence of Q waves in leads 2 and 3, is higly suspicious for the presence of coronary artery disease. Following a severe transmural myocardial infarction, there will be pathological Q waves

-          in 2, 3, and aVF if the inferior wall was affected

-          in 1 and aVL if the lateral wall was affected

-          and in v1 to v6 if the anterior wall was affected.

Now, what is a pathological Q wave?

When analyzing ECGs, you must be able to differentiate physiological Q waves from pathological Q waves. To avoid confusing yourself and your patient, you must know the criteria used to define a pathological Q wave.

Any Q occurring in v1 to v3 is pathological. However, a QS complex may occur in v1.

A Q is pathological if it lasts for longer or equal to 30 ms AND is larger or equal to 0.1 millivolts AND occurs in at least two related leads.

Once you have detected pathological Q-waves, the following differential diagnoses must be considered:

Myocardial infarction or pronounced septum hypertrophy, for example, in hypertrophic cardiomyopathy. Apart from a myocardial infarction, any other type of myocardial fibrosis may lead to a lack of electrical conduction and thus to Q waves.

An extreme rotation of the electric heart axis with Q waves can be found in certain bundle branch blocks or in excitations that arise from the ventricles. Remember that rhythms with a ventricular origin always have a broad QRS complex.

Last, but not least: you cannot believe every ECG. Sometimes electrodes are positioned incorrectly, which can cause considerable variations in the ECG.

The R wave represents depolarization within the ventricles. Since the left ventricle has much more mass than the right ventricle, the left ventricle has the major influence on the R wave vector. The R wave will be highest in those leads that run along the electric heart axis. More on this you find in the chapter on the "normal heart axis".

The end of ventricular depolarization is represented by the S wave. After the apex is fully excited, the basal right ventricular myocardium is excited. At this point, depolarization spreads upwards and to the right again, meaning away from the electrical heart axis.

For this reason, at the end of the QRS complex, depending on the lead, a negative wave occurs, namely the S wave.

Normally, the R and S waves are pointed and slender. Together with the Q, a QRS width of approximately 80 milliseconds results. In the chest wall leads, the R wave continuously increases from v1 to v6, while the S wave becomes smaller and is not seen in leads v5 and v6. We call this R progression. The R/S changeover point denotes the first lead in which R is greater than S. This is usually the case in leads v3 or v4.

If there is inadequate R progression, we refer to delayed R wave progression. If R is absent, we refer to R wave loss. If S remains in leads v5 and v6, we refer to S wave persistence.

The progression and persistence of the R and S waves must always be considered. The possible differential diagnoses are discussed in detail in the upcoming chapters.