Premature atrial contraction (PAC): Identifying ECG features

Rhythm:  Underlying rhythm usually regular; irregular with PACs

Rate:  That of underlying rhythm

P waves:  P wave associated with PAC is premature and

Abnormal in size, shape, and direction (commonly appears small, upright, and pointed; may be inverted); abnormal P wave commonly found hidden in preceding T wave, distorting the T-wave contour PR interval: Usually normal; not measurable if hidden in

T wave

QRS complex: Premature; normal duration (0.10 second or less) ectopic site in the atrium, which interrupts the regularity of the basic rhythm (usually a sinus rhythm). The premature beat occurs in addition to the basic underlying rhythm. PACs may originate from a single ectopic pacemaker site or from multiple sites in the atria. The early beat is characterized by a premature, abnormal P wave and a premature QRS complex that’s identical or similar to the QRS complex of the normally conducted beats, and is followed by a pause.

P-wave morphology differs from sinus beats and varies depending on the origin of the impulse in the atria. If the ectopic focus is in the vicinity of the SA node, the P wave may closely resemble the sinus P wave (Figure 7-5). Its sole distinguishing feature may be its prematurity. As a rule,

Figure 7-7. Normal sinus rhythm with premature atrial contraction (PAC).

Rhythm:  Basic rhythm regular; irregular with PAC

Rate:  Basic rhythm rate 84 beats/minute

P waves:  Sinus P waves with basic rhythm; premature, abnormal P wave with PAC (The P wave of the PAC is hidden in the preceding

T wave, distorting the T-wave contour. [T wave is taller and more pointed.])

PR interval:  0.12 to 0.14 second (basic rhythm); not measurable with PAC

QRS complex: 0.06 to 0.08 second (basic rhythm); 0.06 second (PAC).

Figure 7-8. Normal sinus rhythm with one premature atrial contraction (PAC) with aberrant ventricular conduction.

Rhythm:  Basic rhythm regular; irregular with PAC

Rate:  Basic rhythm rate 68 beats/minute

P waves:  Sinus in basic rhythm; premature, abnormal P wave with PAC

PR interval:  0.16 to 0.18 second (basic rhythm); 0.24 second (PAC)

QRS complex: 0.08 second (basic rhythm); 0.12 second (PAC).

However, the P wave is different from the sinus P waves. In lead II (a positive lead), it’s generally upright and pointed (Figure 7-9), or it may be inverted (Figure 7-6) if the pacemaker site is near the AV junction. If the premature beat occurs very early, the abnormal P wave can be found hidden in the preceding T wave, causing a distortion of the T-wave contour (Figure 7-7).

The PR intervals of the PACs are usually normal, similar to those of the underlying rhythm. Occasionally the PR interval may be prolonged if the PAC is very early and finds the AV junction still partially refractory and unable to conduct at a normal rate. The PR interval will be unmeasurable if the abnormal P wave is obscured in the preceding T wave.

The QRS of the PAC usually resembles that of the underlying rhythm because the impulse is conducted normally through the bundle branches into the ventricles. The ventricles depolarize simultaneously, resulting in a normal duration QRS complex. If the PAC occurs very early, it is possible the bundle branches may not be repolarized sufficiently to conduct the premature electrical impulse normally. If the bundle branches are not sufficiently repolarized, the electrical impulse is conducted down one bundle branch (usually the left because it repolarizes quicker)

Figure 7-9.



P waves:

PR interval: QRS complex: Comment:

Normal sinus rhythm with premature atrial contraction (PAC).

Basic rhythm regular; irregular with PAC Basic rhythm rate 60 beats/minute

Sinus P waves with basic rhythm; premature, abnormal P wave with PAC

0.12 to 0.16 second (basic rhythm); 0.16 second (PAC)

0.08 second (basic rhythm and PAC)

To determine the type of pause after premature beats, measure from the QRS complex before the premature beat to the QRS complex after the premature beat. If the measurement equals two R-R intervals, the pause is compensatory. If the measurement equals less than two R-R intervals, the pause is noncompensatory. ST-segment depression is present.

Figure 7-10. Bigeminal premature atrial contractions.

Figure 7-11. Quadrigeminal premature atrial contractions.

Figure 7-12. Paired premature atrial contractions.

And not conducted down the other. The left ventricle is depolarized first, followed by depolarization of the right ventricle (sequential depolarization). Sequential ventricular depolarization is slower, resulting in a wide QRS complex of 0.12 second or greater. A PAC associated with a wide QRS complex is called a PAC with aberrancy, indicating that conduction through the ventricles is abnormal (aberrant). Figure 7-8 shows a PAC with aberrant ventricular conduction (the QRS is wide) and a long PR interval, indicating conduction through the AV node was also delayed. Aberrantly conducted PACs must be differentiated from a premature ventricular contraction (PVC), especially if the abnormal P wave associated with the PAC is obscured in the preceding T wave. PVCs are discussed in Chapter 9.

The pause associated with the PAC is usually a noncompensatory pause (the measurement from the R wave before the premature beat to the R wave after the premature beat is less than two R-R intervals of the underlying regular rhythm) (Figure 7-9). This pause is called an incomplete pause because it doesn’t equal two R-R intervals. Less commonly, the PAC may occur with a compensatory pause (a pause that is equal to two R-R intervals), but this is usually seen with the PVC. The compensatory pause is called a complete pause because it equals two R-R intervals. To differentiate between a complete pause and an incomplete pause, the underlying rhythm must be regular. Rarely, the PAC may occur with a pause that is longer than compensatory.

PACs may appear as a single beat (Figure 7-9), every other beat (bigeminal PACs, Figure 7-10), every third beat, (trigeminal PACs), every fourth beat (quadrigeminal PACs, Figure 7-11), in pairs (also called couplets, Figure 7-12), or in runs of three or more. Frequent PACs may initiate more serious atrial arrhythmias, such as paroxysmal atrial tachycardia (PAT), atrial flutter, or atrial fibrillation. Three or more beats of PACs in a row at a rate of 140 to 250 beats/ minute constitute a run of PAT.

Premature atrial beats are common. They can occur in individuals with a normal heart or in those with heart disease. PACs may be seen with emotional stress (due to an increase in sympathetic tone), or ingestion of certain substances such as alcohol, caffeine, or tobacco. Other causes include hypoxia, electrolyte imbalances, myocardial ischemia or injury, atrial enlargement, congestive heart failure, and the administration of certain drugs, such as epinephrine or nonepinephrine, that increase sympathetic tone. PACs may also occur without apparent cause.

Infrequent PACs require no treatment. Frequent PACs are treated by correcting the underlying cause: reducing stress; reducing or eliminating the consumption of alcohol, caffeine, or tobacco; administering oxygen; correcting electrolyte imbalances; treating congestive heart failure, or discontinuing certain drugs. If needed, frequent PACs may be treated with beta blockers, calcium channel blockers, or antianxiety medications. Runs of PACs may require ami-odarone to prevent more serious atrial arrhythmias from developing.

Occasionally, an ectopic atrial beat will occur late instead of early. This beat is called an atrial escape beat (Figure 7-13). Atrial escape beats usually occur during a pause in the underlying rhythm when the sinus node fails to initiate an impulse (sinus arrest) or when conduction of the sinus impulse is blocked for any reason (sinus exit block, nonconducted PAC, or Mobitz I second-degree AV block). The pause in the rhythm allows an ectopic pacemaker site in the atria to assume control of the heartbeat. The morphologic characteristics of the late beat will be the same as the PAC. Escape beats act as an electrical backup to maintain the heart rate and require no treatment.

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