Understanding the Normal ECG

Chapter 4 Understanding the Normal ECG

Please go to expertconsult.com for supplemental chapter material.

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):

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:

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.

Jun 11, 2016 | Posted by in CARDIOLOGY | Comments Off on Understanding the Normal ECG
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