The QRS, which represents ventricular depolarization, is somewhat more complex than the P wave, but the same basic ECG rules apply to both.
Predict what the QRS will look like in the different leads, you must first know the direction of ventricular depolarization. Although the direction of atrial depolarization can be represented by a single arrow, the spread of ventricular depolarization consists of two major sequential phases:
The first phase is of relatively brief duration (shorter than 0.04 sec) and small amplitude; it results from the spread of stimulus through the ventricular septum. The ventricular septum is the first part of the ventricles to be stimulated. Furthermore, the left side of the septum is stimulated first (by a branch of the left bundle of His); thus the depolarization spreads from the left ventricle to the right across the septum. Phase one of ventricular depolarization, the phase of septal stimulation, can therefore be represented by a small arrow pointing from the left septal wall the right.
The second phase of verntricular depolarization involves the simultaneous stimulation of the main mass of both the left and right ventricles from the inside (endocardium) to the outside (epicardium) of the heart muscle. In the normal heart the left ventricle is electrically predominant. In other words, it electrically overbalances the right ventricle. Therefore an arrow representing phase two of ventricular stimulation will point toward the left ventricle.
Lead V1 shows voltages detected by an electrode placed on the right side of the sternum (fourth intercostal space). Lead V6, a left chest lead, shows voltages detected in left midaxillary line . What will the QRS complex look like in these leads?
The first phase of ventricular stimulation, septal stimulation, will produce a small positive r wave in lead V1, reflecting the left-to-right spread of stimulus through the septum. The arrow representing septal stimulation will point toward lead V1. what will lead V6 show? The left-to-right spread of septal stimulation will produce a small negative deflection (q wave) in lead V6. thus the same electrical event, septal stimulation, will produce a small positive deflection (or r wave) in lead V1 and a small negative deflection (q wave) in a left precordial lead like V6. (This situation is analogous to the one described for the P wave, which is normally positive in lead II but always negative in lead aVR.)
The second phase of ventricular stimulation is represented by an arrow pointing in the direction of the left ventricle. The spread of stimulation to the left during the second phase will result in a negative deflection in the right precordial leads and a positive deflection in the left precordial leads. Lead V1 will therefore show a deep negative (S) wave while lead V6 shows a tall positive (R) wave.
Let us summarize what we have learned about the normal QRS pattern in leads V1 and V6. normally lead V1 will show an rS type of complex. The small initial r wave reprewents the left-to-right spread of septal stimulation. This wave is sometimes referred to as the septal r wave because it reflects septal stimulation. The negative (S) wave reflects the spread of ventricular stimulation forces during phase two, away from the right and toward the dominant left ventricle. Conversely, the same electrical events, septal and ventricular stimulation, viewed from an electrode in the V6 position will produce a qR pattern. The q wave is a septal q wave, reflecting the left-to-right spread of the stimulus through the septum away from lead V6. the positive ( R) wave reflects the leftward spread of ventricular stimulation voltages toward the left ventricle.
Once again, we reemphasize, the same electrical event, whether depolarization of the atria or depolarization of the ventricles, will produce very different-looking waveforms in different leads because the spatial orientation of the leads in different.
We have described the patterns normally seen in leads V1and V6. What happens between these leads? The answer is that as you move across the chest (in the direction of the electrically predominant left ventricle) the R wave tends to become relatively larger and the S wave becomes relatively smaller. This increase in height of the R wave, which usually reaches a maximum around lead V4 or V5, is called normal R wave progression.
At some point, generally around the V3 or V4 position, the R/S ratio becomes 1. This point, where the amplitude of the R wave equals that of the S wave, is called the transition zone. In some normal people the transition may be seen as early as lead V2. This is called early transition. In other cases the transition zone may be delayed to leas V5 and V6, and is called a delayed transition.
Examine the set of normal chest leads. Note the rS complex in lead V1 and qR complex in lead V6. The R wave tends to get gradually larger here as you move toward the left chest leads. The transition zone, where the R wave and S wave are about equal, is in lead V4. In normal chest leads the R wave voltage need not get literally larger as you go from leads V1 to V6. However, the overall trend should show a relative increase. For example, notice that in this example there is not much difference between the complexes in leads V2 and V3, and that the R wave in lead V5 is taller than the R wave in lead V6.
The positive pole of lead aVR is oriented upward and toward the right shoulder. The ventricular stimulation forces are oriented primarily toward the left ventricle. Therefore, lead aVR normally shows a predominantly negative QRS complex. You may see any of the QRS-T complexes shown in lead aVR. In all cases, the QRS is predominantly negative. The T wave in lead aVR is also normally negative.
The QRS patterns in the other five extremity leads are somewhat more complicated. The reason is that there is considerable normal variation in the QRS patterns seen in the extremity leads. For example, some normal people have an ECG that shows one pattern in the extremity leads. In this case, leads I and aVL show qR-type complexes while leads III and aVF show rS-type complexes. In other normal people the extremity leads may show just the reverse picture. Here, leads II, III, and aVF show qR complexes while lead aVL and sometimes lead I show RS complexes.
The extremity leads in normal people can show a variable QRS pattern. Lead aVR normally always shows a predominantly negative QRS complex (Qr, QS, or rS). The QRS patterns in the other extremity leads will vary depending on the “electrical position” (QRS axis ) of the heart, with an electrically vertical axis, leads I and aVL show qR waves. Therefore, there is no single normal ECG pattern; rather, there is a normal variability. Students and clinicians must familiarize themselves with the normal variants we have described both in the chest leads and in the extremity leads.