A premature atrial contraction occurs when a focus in the atrium (not the sinoatrial node) generates an action potential before the next scheduled SA node action potential.
There are four main characteristics of PACs:
Premature, occurring earlier than expected if measured against previous P-P intervals.
- Ectopic, originating outside of the SA node, and thus, the P wave morphology would be different from the normal sinus P wave morphology.
- Narrow complexes; because they come from the atrium, they will eventually travel through the AV node and use the normal conduction system to spread to the ventricles. Unlike premature ventricular contractions, or PVCs, which are wide-complexed because they do not use the normal ventricular conduction system. Less commonly, PACs can conduct aberrantly in a right or left bundle pattern, which can make it challenging to differentiate them from PVCs.
- Compensatory pause following the contraction; the extra atrial action potential causes the SA node to become refractory to generating its next scheduled beat, and thus it must “skip a beat” and will resume exactly 2 P-P intervals after the last normal sinus beat.
When every other QRS complex is a premature atrial contraction, then the rhythm is referred to as “atrial bigeminy” as seen in this strip above.
If a PAC occurs when the AV node has not yet recovered from the refractory period, it will fail to conduct to the ventricles; this means there will not be a QRS complex following, or the ectopic PR interval will be prolonged. The ECG will show a premature, ectopic P wave and then no QRS complex afterward. When this occurs in a pattern of bigeminy, the interpreter may be fooled into thinking that sinus bradycardia is present, because every other beat is a nonconducted PAC, and the QRS rate is quite slow.
Ashman’s Phenomenon, also known as the Ashman Beat, occurs when a PAC or supraventricular beat occurs before the right bundle branch has recovered from its refractory period. This results in the premature beat displaying a right bundle branch block pattern and can be confused with a premature ventricular contraction.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.