What causes premature atrial contractions (PACs) on an electrocardiogram (ECG)?

Causes of Premature Atrial Contractions (PACs) on ECG

PACs are extremely common in the general population and occur in virtually everyone, with only 1% of individuals showing no PACs during 24-hour monitoring. 1

Primary Risk Factors and Associations

Age-Related Factors

• Age is the strongest independent predictor of PAC frequency, with PAC burden increasing progressively from a median of 0.8 PACs/hour in those aged 50-55 years to 2.6 PACs/hour in those ≥70 years (risk ratio per standard deviation: 1.80). 1
• The frequency of PACs increases significantly with each decade of life, representing a natural consequence of atrial aging and remodeling. 1

Cardiovascular Disease and Structural Factors

• Prevalent cardiovascular disease is strongly associated with increased PAC frequency (risk ratio: 2.40), making it one of the most significant modifiable risk factors. 1
• Height is independently associated with PAC frequency (risk ratio per SD: 1.52), likely reflecting larger atrial dimensions and increased atrial tissue mass. 1
• Elevated N-terminal pro B-type natriuretic peptide levels correlate with increased PAC burden (risk ratio per SD: 1.27), suggesting atrial stretch and hemodynamic stress as contributing mechanisms. 1

Metabolic and Lifestyle Factors

• Lower high-density lipoprotein (HDL) cholesterol is associated with more frequent PACs (risk ratio per SD: 0.80), suggesting a protective role of favorable lipid profiles. 1
• Physical activity ≥2 hours per day is associated with reduced PAC frequency (risk ratio: 0.69), indicating a protective effect of regular exercise. 1
• Contrary to popular belief, caffeine consumption (coffee, tea, chocolate) shows no association with PAC frequency in large population studies using 24-hour Holter monitoring. 2

Cardiac Conditions Associated with PACs

Structural Heart Disease

• Valvular heart disease, particularly mitral valve disease, creates atrial pressure and volume overload that promotes PAC formation. 3
• Cardiomyopathies of any etiology can produce atrial remodeling and increased PAC burden. 3
• Congenital heart disease, especially in patients with prior surgical repair and residual hemodynamic abnormalities, warrants monitoring for PACs. 4

Acute Cardiac Conditions

• Myocarditis and pericarditis can trigger PACs through inflammatory mechanisms affecting atrial tissue. 3
• Acute myocardial ischemia or infarction may produce PACs, though ventricular arrhythmias are more common. 3

Post-Surgical States

• Cardiac surgery is associated with high rates of atrial arrhythmias, with PACs serving as precursors to postoperative atrial fibrillation in 32% of coronary bypass patients and up to 64% in combined valve procedures. 3
• PACs typically peak on postoperative days 2-4, with 14% of episodes occurring after hospital discharge. 3

Non-Cardiac Causes

Endocrine Disorders

• Hyperthyroidism is a well-established cause of increased atrial ectopy and should be excluded in patients with new or increased PAC burden. 4
• Thyroid dysfunction assessment is recommended in the initial evaluation of symptomatic PACs. 4

Pharmacologic Triggers

• Multiple cardiovascular and non-cardiovascular drugs can induce atrial arrhythmias, including PACs as precursors to atrial fibrillation. 3
• Anticancer drugs represent an increasingly recognized cause of drug-induced atrial arrhythmias, including PACs. 3
• Psychotropic agents (phenothiazines, tricyclic antidepressants, lithium), anti-infective agents (erythromycin, pentamidine), and various antihypertensive medications can produce ECG changes and trigger PACs. 3

Electrolyte and Metabolic Disturbances

• Hypokalemia, hypomagnesemia, and hypocalcemia can increase atrial irritability and PAC frequency. 3
• Acid-base disturbances may contribute to altered atrial electrophysiology. 3

Important Clinical Caveats

Common Pitfalls in Diagnosis

• Atrial bigeminy with blocked PACs can mimic various conduction disorders, including second-degree AV block, requiring careful ECG analysis to distinguish nonconducted PACs from true AV block. 5, 3
• PACs with aberrant conduction may be confused with premature ventricular contractions, necessitating careful P-wave identification. 5

When PACs Signal Underlying Pathology

• Frequent PACs (≥2,000 per 24 hours) are associated with substantially increased risk of atrial fibrillation (HR 2.96), stroke (HR 2.54), and mortality (HR 2.14), warranting comprehensive evaluation. 6, 4
• Multiple PACs (≥2) on a single 12-lead ECG in athletes require extensive evaluation including Holter monitoring, echocardiography, and exercise testing to exclude structural heart disease. 3, 4

Benign vs. Pathologic PACs

• In healthy, asymptomatic individuals without structural heart disease or thyroid dysfunction, isolated PACs require no further evaluation or therapy. 4
• PACs causing symptoms (palpitations, fatigue, chest discomfort, dyspnea, lightheadedness, or syncope) warrant diagnostic workup regardless of frequency. 4