Introduction to Basic Components

Figure 4-12 shows the basic components of the ECG complex. Here are some basic definitions. A wave is a deflection from the baseline that represents some cardiac event. For instance, the P wave represents atrial depolarization. A segment is a specific portion of the complex as it is represented on the ECG. For example, the segment between the end of the P wave and the beginning of the Q wave is known as the PR segment. An interval is the distance, measured as time, occurring between two cardiac events. The time interval between the beginning of the P wave and the beginning of the QRS complex is known as the PR interval. Note that there is a PR interval, as well as a PR segment. In addition to the waves shown in Figure 4-12, there are a few others not mentioned below, such as the R’ (R prime) wave and the U wave, which we will talk about individually. There are also other intervals that we will cover, such as the R-R interval and the P-P interval. Making sure that you understand the definitions of the basic terms will help prevent confusion. In Figure 4-12, we have labeled the waves and segments with colored letters and the intervals with black letters for easier identification.

Figure 4-12 Basic components of the ECG complex.

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Wave Nomenclature

A wave represents an electrical event in the heart, such as atrial depolarization, atrial repolarization, ventricular depolarization, ventricular repolarization, or transmission through the His bundles, and so on. Waves can be single, isolated, positive, or negative deflections; biphasic deflections with both positive and negative components; or combinations that have multiple positive and negative components. Waves are deflections from the baseline. What is the baseline? It is a line from one TP segment to the next.

Let’s look at what that means in Figure 4-13. Note that the QRS complex is a combination of two or more waves. To be completely correct, these waves should be named according to size, location, and direction of deflection. Tall or deep waves in the QRS complex are given capital letters Q, R, S, R’. Small waves are given small letters q, r, s, r’. This is why the example in Figure 4-13 is called a qRs wave. This standard is unfortunately not followed as rigorously as you might expect. Many authors simply use all capital letters. In this book, we will follow the standard nomenclature with capital and small letters when demonstrating this point.

Figure 4-13 A qRs complex.

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R’ and S’ Waves

Just to make matters more interesting, let’s look at some problems with the QRS waves. Changes occurring in the QRS complex can lead to bizarre complexes, and their waves are named differently if they change directions and cross the baseline. Such a wave is called an X’ (X prime) wave, in which X is not an actual wave, but rather a term that can stand for either an R or S wave. R’ and S’ (R prime and S prime) refer to extra waves within the QRS complex. By definition, the first negative wave that we reach after the P wave is called the Q wave. The first positive deflection after the P is the R wave. Here is where it gets tricky: An S wave is the first negative component after an R wave. If we now get another upward component, we start with R’. The next negative component is S’.

A positive wave occurring after the S’ would then be an R” wave (read as R double prime), and so on. Figure 4-14 shows some examples.

Figure 4-14 R’ and S’ complexes (NOTE: The top row does not technically contain S waves. The term S wave only applies to negative components, or components that fall below the baseline. However, it is common for people to refer to any dip in a notched R wave as an S wave, regardless of whether or not it falls below the baseline. By following this logic further, most authors and clinicians refer to the second peak as the R’ wave. Although this nomenclature is technically incorrect, it is so common that people accept this as the norm.)

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