What are the common ECG (electrocardiogram) arrhythmias confused with atrial fibrillation (A.Fib) and what are the key differences between each of them compared to A.Fib?

Common ECG Arrhythmias Confused with Atrial Fibrillation

The four arrhythmias most commonly confused with atrial fibrillation are atrial flutter, multifocal atrial tachycardia, focal atrial tachycardia, and AV nodal/AV reentrant tachycardias, each distinguished by specific P wave characteristics and rhythm regularity that differ fundamentally from AF's irregularly irregular rhythm and absent P waves. 1

Core ECG Features of Atrial Fibrillation (Reference Standard)

Before distinguishing mimics, understand AF's defining characteristics:

• Absent distinct P waves replaced by rapid oscillations (fibrillatory waves) that vary in amplitude, shape, and timing 1, 2
• Irregularly irregular R-R intervals with no discernible pattern 2
• Irregular atrial activity representing chaotic, uncoordinated atrial activation 2
• Ventricular response typically rapid (when AV conduction intact) but always irregular unless concurrent AV block or pacemaker present 1

Common pitfall: In patients with pacemakers, diagnosis may require pacemaker inhibition to expose underlying fibrillatory activity 1

---

1. Atrial Flutter

Key Distinguishing Features:

• Regular "saw-tooth" pattern of flutter (ƒ) waves, most visible in leads II, III, aVF, and V1 1
• Regular atrial rate typically 240-320 beats per minute 1
• Regular or regularly irregular ventricular response (depending on AV conduction ratio: 2:1,3:1,4:1, or variable) 1
• ƒ waves typically inverted in leads II, III, aVF and upright in V1 (typical counterclockwise flutter) 1

How It Differs from AF:

• Organized vs. chaotic: Flutter shows organized, repetitive atrial activity; AF shows disorganized fibrillatory waves 1
• Regularity: Flutter usually has regular ventricular response (with fixed AV block); AF is irregularly irregular 1
• P wave morphology: Flutter has consistent saw-tooth waves; AF has variable fibrillatory waves 1

Critical caveat: Misdiagnosis occurs when fibrillatory atrial activity in AF is prominent in multiple ECG leads, mimicking flutter waves 1. Additionally, atrial flutter may degenerate into AF, and AF may convert to flutter, sometimes coexisting 1

---

2. Multifocal Atrial Tachycardia (MAT)

Key Distinguishing Features:

• At least 3 distinct P wave morphologies on a single ECG lead 1
• Irregular R-R intervals (can mimic AF's irregularity) 1
• Visible P waves separated by isoelectric baseline (unlike AF's absent P waves) 1
• Variable P-R intervals reflecting multiple atrial foci 1
• Atrial rate typically >100 bpm 1

How It Differs from AF:

• P waves present vs. absent: MAT has identifiable (though varying) P waves; AF has none 1, 2
• Isoelectric baseline: MAT shows clear baseline between P waves; AF shows continuous fibrillatory activity 1
• Organized foci: MAT arises from multiple discrete atrial sites; AF represents complete atrial disorganization 1

Clinical context: MAT commonly occurs in patients with severe pulmonary disease, making this distinction clinically important for management decisions 1

---

3. Focal Atrial Tachycardia

Key Distinguishing Features:

• Distinct P waves with consistent morphology (single focus) 1
• P waves separated by isoelectric baseline 1
• Regular or slightly irregular rhythm (much more regular than AF) 1
• P wave morphology may localize origin of arrhythmia 1
• Atrial rate typically 100-250 bpm 1

How It Differs from AF:

• Organized vs. disorganized: Focal AT shows organized atrial activity from single focus; AF shows chaotic multi-wavelet reentry 1
• P wave presence: Focal AT has clear P waves; AF does not 1, 2
• Rhythm regularity: Focal AT typically regular or minimally irregular; AF always irregularly irregular 1, 2

Important note: Focal atrial tachycardias may trigger AF episodes, so both can coexist or transition between patterns 1

---

4. AV Nodal Reentrant Tachycardia (AVNRT) and AV Reentrant Tachycardia (AVRT)

Key Distinguishing Features:

• Regular, narrow-complex tachycardia (unless aberrant conduction) 1
• P waves may be hidden in QRS (AVNRT) or visible shortly after QRS (AVRT with orthodromic conduction) 1
• Perfectly regular R-R intervals (no beat-to-beat variability) 1
• Sudden onset and termination (paroxysmal) 1
• Heart rate typically 150-250 bpm 1

How They Differ from AF:

• Regularity is key: AVNRT/AVRT are perfectly regular; AF is irregularly irregular 1, 2
• P wave relationship: When visible, P waves have fixed relationship to QRS in AVNRT/AVRT; AF has no P waves 1, 2
• Mechanism: AVNRT/AVRT involve AV node in reentry circuit; AF is purely atrial 1

Critical distinction: These tachycardias may trigger AF episodes, so careful ECG analysis before and after arrhythmia termination is essential 1

---

Special Diagnostic Challenges

AF with Regular Ventricular Response

When AF coexists with complete AV block, ventricular tachycardia, or AV junctional tachycardia, the ventricular rhythm (R-R intervals) may appear regular, potentially masking the diagnosis 1. Always examine atrial activity carefully in leads II, III, aVF, and V1 to identify fibrillatory waves 1

Wide-Complex Irregular Tachycardia

An irregular, sustained, wide-QRS-complex tachycardia suggests either:

• AF with bundle branch block (most common) 1
• AF with conduction over accessory pathway (pre-excited AF) 1
• Polymorphic ventricular tachycardia (life-threatening) 1

Extremely rapid rates (>200 bpm) suggest accessory pathway conduction or ventricular tachycardia and require immediate expert evaluation 1

Coarse vs. Fine AF

• Coarse AF (fibrillatory wave amplitude ≥0.1 mV) may be confused with atrial flutter due to prominent atrial activity 3
• Fine AF (<0.1 mV) may be mistaken for sinus rhythm with artifact if ventricular response is controlled 3
• Coarse AF is associated with higher cardioversion success but also higher stroke risk 3

---

Practical Diagnostic Algorithm

Step 1: Assess rhythm regularity

• Irregularly irregular → Consider AF, MAT, or atrial flutter with variable block 1, 2
• Regular → Consider atrial flutter, focal AT, AVNRT/AVRT 1

Step 2: Identify P waves

• Absent P waves with fibrillatory waves → AF 1, 2
• Saw-tooth pattern → Atrial flutter 1
• Multiple distinct P wave morphologies → MAT 1
• Single P wave morphology with isoelectric baseline → Focal AT 1
• P waves hidden or immediately after QRS → AVNRT/AVRT 1

Step 3: Examine leads II, III, aVF, and V1 specifically These leads best demonstrate atrial activity and distinguish flutter waves from fibrillatory waves 1

Step 4: Measure ventricular rate

• Extremely rapid (>200 bpm) with irregular wide QRS → Consider pre-excited AF (accessory pathway) requiring emergent management 1