What is the significance of a blocked premature atrial contraction (PAC) on an electrocardiogram (ECG) in a newborn?

Blocked Premature Atrial Contraction on Newborn ECG

A blocked premature atrial contraction (PAC) on a newborn ECG is a premature P wave that fails to conduct to the ventricles, appearing as an early P wave (often hidden in the preceding T wave) without a following QRS complex—this is typically benign but requires careful distinction from sinus bradycardia. 1

ECG Characteristics

Blocked PACs have specific electrocardiographic features that distinguish them from other rhythm disturbances:

• The premature P wave has a different morphology and mean vector compared to normal sinus P waves 1
• The premature P wave is often hidden within or distorts the T wave of the preceding beat, requiring careful examination of T wave morphology 1, 2
• No QRS complex follows the premature P wave because it arrives during the refractory period of the AV node 1
• When blocked PACs occur in a bigeminal pattern (every other beat), they can simulate sinus bradycardia—this is a critical diagnostic pitfall 1, 2

Clinical Significance

The distinction between blocked atrial bigeminy and true sinus bradycardia has important clinical implications:

• Blocked atrial bigeminy is most often benign and does not require treatment 1, 2
• True sinus bradycardia may accompany systemic illness and requires different evaluation and management 1, 2
• Relatively long periods of blocked atrial bigeminy may falsely appear as severe bradycardia if the hidden P waves are not identified 1

Risk Stratification

While generally benign, certain patterns of blocked PACs warrant closer monitoring:

• Blocked PACs are a risk factor for development of supraventricular tachycardia, with an odds ratio of 30.3 in fetal studies 3
• PACs in bigeminy pattern carry increased risk (odds ratio 21.8) for tachyarrhythmia 3
• The presence of signs of cardiac failure alongside blocked PACs significantly increases risk (odds ratio 14.2) 3

Recommended Workup

The European Heart Journal provides clear guidance for evaluation of newborns with blocked PACs:

• Obtain a 12-lead ECG and carefully examine all T waves for hidden premature P waves to confirm the diagnosis 1, 4, 2
• Perform follow-up ECG at 1 month for patients with frequent premature atrial beats 1, 4, 2
• Consider 24-hour Holter monitoring to quantify PAC burden and identify concerning patterns 4
• Echocardiography is not routinely required for isolated blocked PACs in structurally normal hearts, but should be considered if other risk factors are present 4

Management Approach

Newborns with frequent PACs and structurally normal hearts require observation only, without pharmacologic intervention, as they are benign and typically resolve spontaneously. 4

• No antiarrhythmic therapy is indicated for isolated blocked PACs 4
• Weekly fetal heart-rate monitoring is advised when risk factors for tachyarrhythmia are identified until resolution of the PACs 3
• If tachyarrhythmia or cardiac failure develops, immediate referral for advanced evaluation is indicated 3

Critical Diagnostic Pitfalls

Avoid these common errors when interpreting newborn ECGs with suspected blocked PACs:

• Do not mistake blocked atrial bigeminy for sinus bradycardia—always examine T waves systematically for hidden P waves, as these conditions have completely different clinical implications 1, 4, 2
• Do not confuse premature atrial beats with aberrant conduction (wide QRS following premature P wave) with premature ventricular beats 2
• Do not assume all bradycardia in newborns is pathologic sinus node dysfunction without first excluding blocked atrial bigeminy 1, 2