Right Atrial Abnormality

Overload of the right atrium (either dilation or actual hypertrophy) may increase the voltage of the P wave. To recognize a large P wave, you must know the dimensions of the normal P wave.

When the P wave is positive, its amplitude is measured in millimeters from the upper level of the baseline, where the P wave begins, to the peak of the wave. A negative P wave is measured from the lower level of the baseline to the lowest point of the P wave. (Measurement of the height and width of the P wave is shown in Fig. 6-1.)

Figure 6-1 The normal P wave is usually less than 2.5 mm in height and less than 0.12 sec in width.

Normally, at rest, the P wave in every lead is less than 2.5 mm (0.25 mV) in amplitude and less than 0.12 sec (three small boxes) in width.

Overload of the right atrium may produce an abnormally tall P wave (2.5 mm or more). Occasionally, right atrial abnormality (RAA) will be associated with a deep (negative) but narrow P wave in lead V₁, due to the relative inferior location of the right atrium relative to this lead. This finding may cause confusion with left atrial abnormality (LAA).

However, because pure RAA generally does not increase the total duration of atrial depolarization, the width of the P wave is normal (less than 0.12 sec). The abnormal P wave in RAA is sometimes referred to as P pulmonale because the atrial enlargement that it signifies often occurs with severe pulmonary disease (Figs. 6-2 and 6-3).

Figure 6-2 Tall narrow P waves may indicate right atrial abnormality or overload (formerly referred to as P pulmonale pattern).

Figure 6-3 Tall P waves (arrow) are seen in leads II, III, aVF, and V₁ from the ECG of a patient with chronic lung disease. This finding is sometimes called the P pulmonale pattern.

The tall, narrow P waves characteristic of RAA can usually be seen best in leads II, III, aVF, and sometimes V₁. The ECG diagnosis of P pulmonale can be made by finding a P wave exceeding 2.5 mm in any of these leads. Echocardiographic evidence, however, indicates that the finding of a tall, peaked P wave does not consistently correlate with RAA. On the other hand, patients may have actual right atrial overload and not tall P waves. In other words, tall peaked P waves are of limited sensitivity and specificity in the diagnosis of right atrial enlargement (see Chapter 23).

RAA is seen in a variety of important clinical settings. It is usually associated with right ventricular enlargement. Two of the most common clinical causes of RAA are pulmonary disease and congenital heart disease. The pulmonary disease may be either acute (bronchial asthma, pulmonary embolism) or chronic (emphysema, bronchitis). Congenital heart lesions that produce RAA include pulmonary valve stenosis, atrial septal defects, Ebstein’s anomaly (a malformation of the tricuspid valve), and tetralogy of Fallot.

Left Atrial Abnormality

Enlargement of the left atrium by dilation or actual hypertrophy also produces predictable changes in the P wave. Normally the left atrium depolarizes after the right atrium. Thus, left atrial enlargement should prolong the total duration of atrial depolarization, indicated by an abnormally wide P wave. Left atrial enlargement (LAE) characteristically produces a wide P wave with duration of 0.12 sec or more (at least three small boxes). With enlargement of the left atrium the amplitude (height) of the P wave may be either normal or increased.

Some patients, particularly those with coronary artery disease, may have broad P waves without detectable enlargement of the left atrium. The abnormal P waves probably represent an atrial conduction delay in a normal-sized chamber. Therefore, rather than left atrial enlargement, the more general term left atrial abnormality is recommended to describe these abnormally broad P waves.

Figure 6-4 illustrates the characteristic P wave changes seen in LAA. As shown, the P wave sometimes has a distinctive humped or notched appearance (Fig. 6-4A). The second hump corresponds to the delayed depolarization of the left atrium. These humped P waves are usually best seen in one or more of the extremity leads (Fig. 6-5). The older term P mitrale is sometimes still used to describe wide P waves seen with LAA because these waves were first described in patients with rheumatic mitral valve disease.

Figure 6-4 Left atrial abnormality/enlargement may produce the following: A, wide, sometimes notched P waves in one or more extremity leads (formerly referred to as P mitrale pattern), and B, wide biphasic P waves in lead V₁.

Figure 6-5 Broad, humped P waves from the ECG of a patient with left atrial enlargement (abnormality).

In patients with LAA, lead V₁ sometimes shows a distinctive biphasic P wave (see Figs. 6-4B, and 6-6). This wave has a small, initial positive deflection and a prominent, wide negative deflection. The negative component is longer than 0.04 sec in duration or 1 mm or more in depth. The prominent negative deflection corresponds to the delayed stimulation of the enlarged left atrium. Remember that anatomically the left atrium is situated posteriorly, up against the esophagus, whereas the right atrium lies anteriorly, against the sternum. The initial positive deflection of the P wave in lead V₁ therefore indicates right atrial depolarization, whereas the deep negative deflection is a result of left atrial depolarization voltages directed posteriorly (away from the positive pole of lead V₁).

In some cases of LAA, you may see both the broad, often humped P waves in leads I and II and the biphasic P wave in lead V₁. In other cases, only broad, notched P waves are seen. Sometimes a biphasic P wave in lead V₁