How is premature atrial contraction (PAC) diagnosed?

Diagnosis of Premature Atrial Contractions (PACs)

PACs are diagnosed by ECG documentation showing premature P waves with different morphology from sinus P waves, which may be conducted normally, with aberration, or blocked. 1, 2

ECG Confirmation Required

• A 12-lead ECG or single-lead ECG tracing of ≥30 seconds is required to establish the diagnosis of PACs. 3
• The ECG must capture the arrhythmia during its occurrence—any suspected PAC episode requires ECG documentation for definitive diagnosis. 3
• PACs appear as premature P waves with altered morphology and mean vector compared to normal sinus P waves. 2

Key Diagnostic Features on ECG

• Premature P waves that occur earlier than expected in the cardiac cycle. 1, 2
• P wave morphology differs from normal sinus P waves due to different atrial origin. 2
• PACs may conduct to ventricles in three ways: normally, with aberrant ventricular conduction, or as blocked PACs (not conducted). 2

Critical Diagnostic Pitfall to Avoid

• Blocked atrial bigeminy can mimic sinus bradycardia—carefully examine T waves for hidden premature P waves. 4, 2
• This distinction is clinically important because blocked atrial bigeminy is typically benign, while severe sinus bradycardia may indicate systemic illness. 2
• Failing to identify blocked PACs hidden in T waves can lead to misdiagnosis of the underlying rhythm. 2

Extended Monitoring for Quantification

• 24-hour Holter monitoring should be obtained to quantify PAC burden, correlate symptoms with PAC occurrence, and identify concerning patterns. 1, 4
• Holter monitoring is particularly valuable when PACs are symptomatic or when episodes are suspected but not captured on standard ECG. 3
• Event recorders or transtelephonic monitoring can be used if PAC episodes are infrequent. 3

Diagnostic Workup Based on Clinical Context

For symptomatic PACs (any frequency):

• Obtain 24-hour Holter monitoring regardless of PAC frequency on initial ECG. 1
• Any PAC causing symptoms including palpitations, dizziness, fatigue, chest discomfort, dyspnea, lightheadedness, or syncope warrants diagnostic evaluation. 1

For frequent PACs (>2,000 per 24 hours):

• Comprehensive evaluation is required due to substantially increased risk of atrial fibrillation, stroke, and adverse atrial remodeling. 1
• This threshold represents a critical cutoff where clinical significance dramatically increases. 5, 6

For athletes with ≥2 PACs on single 12-lead ECG:

• Extensive evaluation including Holter monitoring, echocardiogram, and exercise stress testing is indicated. 1
• This lower threshold in athletes reflects the need to exclude underlying structural heart disease in this population. 3

For healthy, asymptomatic individuals with isolated PACs:

• No further evaluation or therapy is required if there is no structural heart disease and normal thyroid function. 1

Additional Diagnostic Considerations

• Chest radiograph is less important than echocardiography for routine PAC evaluation but may detect pulmonary pathology. 3
• Two-dimensional echocardiography should be obtained during initial workup to assess left atrial and ventricular dimensions, wall thickness, and function. 3
• Identify potential triggers through specific inquiry: alcohol, sleep deprivation, emotional stress, caffeine, large meals, or exercise. 3
• Screen for reversible causes including hyperthyroidism, drug-induced causes (adenosine, theophylline/aminophylline, chemotherapeutic agents), and underlying structural heart disease. 3, 1