What does an ECG show in atrial fibrillation?

ECG Findings in Atrial Fibrillation

An ECG in atrial fibrillation shows replacement of P waves with rapid, irregular fibrillatory waves that vary in amplitude, shape, and timing, accompanied by an irregularly irregular ventricular response when AV conduction is intact. 1

Key ECG Characteristics of Atrial Fibrillation

Atrial Activity

• Absence of distinct P waves - Normal P waves are completely absent
• Fibrillatory waves - Rapid oscillations of the baseline that:
  • Vary in amplitude and morphology
  • Have irregular timing
  • Are most visible in leads V1, II, III, and aVF
  • Have no isoelectric baseline between deflections

Ventricular Response

• Irregularly irregular RR intervals - The hallmark of AF with intact AV conduction
• Variable ventricular rate depending on:
  • AV node conduction properties
  • Autonomic tone (vagal and sympathetic)
  • Medication effects
  • Presence of accessory pathways

Differential Diagnostic Considerations

Atrial Flutter vs. Atrial Fibrillation

Atrial flutter can sometimes be misdiagnosed as AF, especially when:

• Atrial activity is prominent in multiple ECG leads 1
• The ECG pattern fluctuates between atrial flutter and AF 1

Key differences:

• Atrial flutter shows regular "saw-tooth" pattern (flutter waves)
• Flutter waves are typically at 240-320 beats/minute
• Flutter often has more regular ventricular response (commonly 2:1 conduction)

Other Considerations

• Regular RR intervals during AF may indicate:

  • AV block
  • Ventricular or junctional tachycardia 1
  • Pacemaker activity

• Wide QRS complexes during AF suggest:

  • Conduction over an accessory pathway
  • Underlying bundle branch block 1

• Extremely rapid rates (>200 beats/minute) suggest:

  • Accessory pathway conduction
  • Ventricular tachycardia 1

Special Situations

Pacemaker Patients

• May require temporary inhibition of the pacemaker to expose atrial fibrillatory activity for diagnosis 1

Minimal Recording Requirements

• Standard 12-lead ECG or
• Single-lead ECG tracing of ≥30 seconds showing characteristic findings 1

Hemodynamic Impact of Irregular Rhythm

The irregular sequence of RR intervals in AF has independent adverse hemodynamic consequences beyond rate alone, including:

• Decreased cardiac output
• Increased pulmonary capillary wedge pressure
• Increased right atrial pressure 2

This explains why patients with AF may experience symptoms even when the average heart rate is controlled.

Clinical Significance

Early recognition of AF on ECG is critical as it:

• Identifies patients at increased risk of stroke (5-fold higher than general population)
• Identifies patients at risk for heart failure (3-fold increased risk)
• Identifies patients with doubled risk of all-cause mortality 3

Even short episodes of irregular SVT without P waves may represent early stages of AF and predict future stroke risk 4.