The Normal Electrocardiogram

Three Basic “Laws” of Electrocardiography

To answer these questions, you need to understand three basic ECG “laws” ( Fig. 5.1 ):

• 1.

  A positive (upward) deflection appears in any lead if the mean (overall) wave of depolarization spreads toward the lead’s positive pole. Thus, if the mean atrial stimulation path 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. 5.2 and 5.3 ). If the mean ventricular stimulation path is directed to the left, a positive deflection (R wave) is seen in lead I (see Fig. 5.1 A).

  
  

• 2.

  A negative (downward) deflection appears in any lead if the mean wave of depolarization spreads toward the negative pole of that lead (or away from the positive pole). Thus, if the mean atrial stimulation path spreads downward and to the left, a negative P wave is seen in lead aVR (see Figs. 5.2 and 5.3 ). If the mean ventricular stimulation path is directed entirely away from the positive pole of any lead, a negative QRS complex (QS deflection) is seen (see Fig. 5.1 B).

• 3.

  A biphasic deflection (consisting of positive and negative deflections of equal size) is usually seen if the mean depolarization path is directed at right angles (perpendicular) to any lead axis. Thus, if the mean atrial stimulation path spreads at right angles to any lead, a biphasic P wave is seen in that lead. Similarly, if the mean ventricular stimulation path spreads at right angles to any lead, the QRS complex is biphasic (see Fig. 5.1 C). A biphasic QRS complex may consist of either an RS pattern or a QR pattern.

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 deferred until 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.

Physiologic (Normal) Sinus P Wave

The P wave, which represents atrial depolarization, is normally the first waveform seen in any cycle. Atrial depolarization is initiated by spontaneous depolarization of pacemaker cells in the sinus (sinoatrial [SA]) node, located in the high 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. 5.2 ).

Fig. 4.7 C, which shows the spatial relationship of the six frontal plane (extremity) leads, is redrawn in Fig. 5.3 . 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 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. 5.3 ). Therefore the normal atrial depolarization path is directed toward the positive pole of that lead (at about +60 degrees) with respect to the frontal plane. When sinus rhythm is present, lead II always records a positive (upward) P wave. ^a

a As a more advanced question, can you think of a setting where the P wave would be positive in lead II and sinus rhythm not present? One answer is an atrial tachycardia (AT) originating near but outside the sinus node proper (see Chapter 14 ). A number of clues may help identify this mimic of sinus tachycardia: (1) AT usually starts and stops abruptly, whereas the rate in sinus tachycardia usually speeds up and slows down more gradually; (2) AT is usually initiated by a premature atrial complex (PAC); (3) AT may be associated with varying degrees of AV nodal block (e.g., 2:1 block; AV Wenckebach, etc.), which is unusual for sinus tachycardia; (4) the P wave in AT is most often subtly different from the sinus P waves. However, a previous ECG for comparison may not be available.

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.