Irritability in the conduction system that results in an atrial ectopic beat is known as a premature atrial contraction (PAC)  (see Figure 4.2). The PAC occurs earlier than the next anticipated normal beat and originates within the atria. 

When reading a 6-second ECG strip for PACs, it is important to remember that they occur as a single beat and not as an entire rhythm.

Figure 4.2. Premature Atrial Contraction (PAC)

Premature Atrial Contraction

Rhythms with PACs are irregular because the underlying rhythm is disrupted by the ectopic beat. When evaluating a patient with PACs, it is important to measure the R-R intervals on the ECG strip when no PACs are visible to be certain whether the rhythm is regular or irregular. 

Because the ectopic beat originates in the atria, the P waves may be flattened, notched, peaked, or biphasic. Conduction from the AV nodes to the ventricles is normal; therefore, the PR interval is normal (120–200 milliseconds). 

The PAC appears earlier than the next expected P wave in the ECG strip and is often seen soon after the preceding QRS complex (see Figure 4.3). The P wave may also manifest near the T wave, or it can be lost entirely in the T wave.

Related Video – ECG Rhythm Review – Rhythm Premature Atrial Complex (PAC)

Figure 4.3. Blocked Premature Atrial Contraction

Blocked contraction is hidden at preceding T Wave, not followed by QRS complex, and followed by compensatory pause.

Characteristics of an ECG with Premature Atrial Contractions

1. Regularity depends on the underlying rhythm and is interrupted by the PAC.
2. Rate depends on the underlying rhythm.
3. The P wave is flattened, notched, peaked, or biphasic. 
4. The P wave may be buried within the preceding T wave and not be distinguishable.
5. The PR interval may be normal (120–200 milliseconds) or prolonged (> 200 milliseconds).
6. The QRS complex is normal (< 120 milliseconds).