The QRS

The QRS is naturally the largest complex on the ECG because it corresponds to depolarization of the ventricles, with their larger muscle mass. Therefore, QRS amplitude may normally reach as high as 25 mm or more (five big boxes) in large individuals, or in those with thin chest walls that actually allow the precordial electrodes to be closer to the heart. Amplitudes >25 mm are frequently associated with chamber enlargement (ventricular hypertrophy), as seen in Figure 2.5. Conversely, very low QRS amplitudes are also abnormal and may be seen with diffuse, severe cardiac disease or illnesses such as pericardial effusion and hypothyroidism.

If the conduction system is working properly, the duration of the QRS should be <0.10 s. Durations of 0.10 s or greater indicate a delay in the spread of depolarization through the ventricles, the so-called intraventricular conduction delay, as is seen in BBBs (Figure 2.6). More on that later.

Figure 2.6. Two examples of BBB. Note that the QRS duration is increased to 0.12 s or greater, and that there is deformity of the ST and T waves, with T waves usually inscribed in the opposite direction from the QRS. The characteristic RSR' pattern is seen in V1 or V2 in RBBB, and in V5 or V6 in LBBB.

Figure 2.6. Two examples of BBB. Note that the QRS duration is increased to 0.12 s or greater, and that there is deformity of the ST and T waves, with T waves usually inscribed in the opposite direction from the QRS. The characteristic RSR' pattern is seen in V1 or V2 in RBBB, and in V5 or V6 in LBBB.

ST Segment 9

Figure 2.7. QRS Nomenclature. Note that when the complex has no Q or S wave, it is still permissible to call it a "QRS."

Later, you will see that it becomes very important to be able to describe the various combinations of positive and negative deflections of the QRS very accurately. Therefore, it is worthwhile to spend a little time now reviewing nomenclature of the QRS.

Figure 2.7 shows some of the various possible QRS inscription combinations. The rules are as follows:

1. The first deflection of the complex is called a Q wave if it is negative.

2. The first positive deflection of the complex is called an R wave.

3. A negative deflection coming after an R wave is called an S wave.

4. Positive deflections coming after the first R wave are labeled R'

5. Negative deflections coming after the first S wave are labeled S

Figure 2.7. QRS Nomenclature. Note that when the complex has no Q or S wave, it is still permissible to call it a "QRS."

Was this article helpful?

0 0

Post a comment