Chapter 4 Understanding the Normal ECG
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The previous chapters reviewed the cycle of atrial and ventricular depolarization and repolarization detected by the ECG as well as the 12-lead system used to record this electrical activity. This chapter describes the P-QRS-T patterns seen normally in each of the 12 leads. Fortunately, you do not have to memorize 12 or more separate patterns. Rather, if you understand a few basic ECG principles and the sequence of atrial and ventricular depolarization, you can predict the normal ECG patterns in each lead.
As the sample ECG in Figure 3-2 showed, the patterns in various leads can appear to be different, and even opposite of each other. For example, in some, the P waves are positive (upward); in others they are negative (downward). In some leads the QRS complexes are represented by an rS wave; in other leads they are represented by RS or qR waves. Finally, the T waves are positive in some leads and negative in others.
Two related and key questions, therefore, are: What determines this variety in the appearance of ECG complexes in the different leads, and how does the same cycle of cardiac electrical activity produce such different patterns in these leads?
Three Basic “Laws” of Electrocardiography
To answer these questions, you need to understand three basic ECG “laws” (Fig. 4-1):
1. A positive (upward) deflection appears in any lead if the wave of depolarization spreads toward the positive pole of that lead. Thus, if the path of atrial stimulation is directed downward and to the patient’s left, toward the positive pole of lead II, a positive (upward) P wave is seen in lead II (Figs. 4-2 and 4-3). Similarly, if the ventricular stimulation path is directed to the left, a positive deflection (R wave) is seen in lead I (see Fig. 4-1A).
2. A negative (downward) deflection appears in any lead if the wave of depolarization spreads toward the negative pole of that lead (or away from the positive pole). Thus, if the atrial stimulation path spreads downward and to the left, a negative P wave is seen in lead aVR (see Figs. 4-2 and 4-3). If the ventricular stimulation path is directed entirely away from the positive pole of any lead, a negative QRS complex (QS deflection) is seen (see Fig. 4-1B).
3. If the mean depolarization path is directed at right angles (perpendicular) to any lead, a small biphasic deflection (consisting of positive and negative deflections of equal size) is usually seen. If the atrial stimulation path spreads at right angles to any lead, a biphasic P wave is seen in that lead. If the ventricular stimulation path spreads at right angles to any lead, the QRS complex is biphasic (see Fig. 4-1C). A biphasic QRS complex may consist of either an RS pattern or a QR pattern.
Figure 4-1 A, A positive complex is seen in any lead if the wave of depolarization spreads toward the positive pole of that lead. B, A negative complex is seen if the depolarization wave spreads toward the negative pole (away from the positive pole) of the lead. C, A biphasic (partly positive, partly negative) complex is seen if the mean direction of the wave is at right angles (perpendicular) to the lead. These three basic laws apply to both the P wave (atrial depolarization) and the QRS complex (ventricular depolarization).
Figure 4-2 With normal sinus rhythm the atrial depolarization wave (arrow) spreads from the right atrium downward toward the atrioventricular (AV) junction and left leg.
Figure 4-3 With sinus rhythm the normal P wave is negative (downward) in lead aVR and positive (upward) in lead II. Recall that with normal atrial depolarization the arrow points down toward the patient’s left (see Fig. 4-2), away from the positive pole of lead aVR and toward the positive pole of lead II.
In summary, when the mean depolarization wave spreads toward the positive pole of any lead, it produces a positive (upward) deflection. When it spreads toward the negative pole (away from the positive pole) of any lead, it produces a negative (downward) deflection. When it spreads at right angles to any lead axis, it produces a biphasic deflection.
Mention of repolarization—the return of stimulated muscle to the resting state—has deliberately been omitted. The subject is touched on later in this chapter in the discussion of the normal T wave.
Keeping the three ECG laws in mind, all you need to know is the general direction in which depolarization spreads through the heart at any time. Using this information, you can predict what the P waves and the QRS complexes look like in any lead.
Normal Sinus P Wave
The P wave, which represents atrial depolarization, is the first waveform seen in any cycle. Atrial depolarization is initiated by spontaneous depolarization of pacemaker cells in the sinus node in the right atrium (see Fig. 1-1). The atrial depolarization path therefore spreads from right to left and downward toward the atrioventricular (AV) junction. The spread of atrial depolarization can be represented by an arrow (vector) that points downward and to the patient’s left (see Fig. 4-2).
Figure 3-7C, which shows the spatial relationship of the six frontal plane (extremity) leads, is redrawn in Figure 4-3. Notice that the positive pole of lead aVR points upward in the direction of the right shoulder. The normal path of atrial depolarization spreads downward toward the left leg (away from the positive pole of lead aVR). Therefore, with normal sinus rhythm lead aVR always shows a negative P wave. Conversely, lead II is oriented with its positive pole pointing downward in the direction of the left leg (see Fig. 4-3). Therefore, the normal atrial depolarization path is directed toward the positive pole of that lead. When sinus rhythm is present, lead II always records a positive (upward) P wave.
In summary, when sinus rhythm is present, the P waves are always negative in lead aVR and positive in lead II. In addition, the P waves will be similar, if not identical, and the P wave rate should be appropriate to the clinical context.
Four important notes about sinus rhythm:
1. Students and clinicians, when asked to define the criteria for sinus rhythm, typically mention the requirement for a P wave before each QRS complex and a QRS after every P, along with a regular rate and rhythm. However, these criteria are not necessary or sufficient. The term sinus rhythm answers the question of what pacemaker is controlling the atria. You can see sinus rhythm with any degree of heart block, including complete heart block, and even with ventricular asystole (no QRS complexes during cardiac arrest!).
2. As described later, you can also have a P wave before each QRS and not have sinus rhythm, but an ectopic atrial mechanism.
3. If you state that the rhythm is “normal sinus” and do not mention any AV node conduction abnormalities, listeners will assume that each P wave is followed by a QRS and vice versa. The more technical and physiologically pure way of stating this finding would be to say, “Sinus rhythm with 1:1 AV conduction.” Clinically, this statement is almost never used but if you try it out on a cardiology attending, she will be astounded by your erudition.
4. Sinus rhythm does not have to be strictly regular. If you feel your own pulse, during slower breathing you will note increases in heart rate with inspiration and decreases with expiration. These phasic changes are called respiratory sinus arrhythmia and are a normal variant, especially pronounced in young, healthy people with high vagal tone.
Using the same principles of analysis, can you predict what the P wave looks like in leads II and aVR when the heart is being paced not by the sinus node but by the AV junction (AV junctional rhythm)? When the AV junction (or an ectopic pacemaker in the lower part of either atrium) is pacing the heart, atrial depolarization must spread up the atria in a retrograde direction, which is just the opposite of what happens with normal sinus rhythm. Therefore, an arrow representing the spread of atrial depolarization with AV junctional rhythm points upward and to the right (Fig. 4-4), just the reverse of what happens with normal sinus rhythm. The spread of atrial depolarization upward and to the right results in a positive P wave in lead aVR, because the stimulus is spreading toward the positive pole of that lead (Fig. 4-5). Conversely, lead II shows a negative P wave.
Figure 4-4 When the atrioventricular (AV) junction (or an ectopic pacemaker in the low atrial area) acts as the cardiac pacemaker (junctional rhythm), the atria are depolarized in a retrograde (backward) fashion. In this situation, an arrow representing atrial depolarization points upward toward the right atrium. The opposite of the pattern is seen with sinus rhythm.
Figure 4-5 With atrioventricular (AV) junctional rhythm (or low atrial ectopic rhythm), the P waves are upward (positive) in lead aVR and downward (negative) in lead II.