What are the differential diagnoses for variable tachycardia?

Differential Diagnoses for Variable Tachycardia

Variable tachycardia most commonly indicates either multifocal atrial tachycardia (MAT) or atrial fibrillation with variable conduction, though the specific differential depends critically on whether the rhythm is regular versus irregular and whether the QRS is narrow versus wide.

Algorithmic Approach to Variable Tachycardia

Step 1: Assess QRS Width and Regularity

The differential diagnosis fundamentally divides based on two ECG features 1, 2:

For Irregular Tachycardias (Variable R-R Intervals):

• Narrow QRS (<120 ms):

  • Multifocal atrial tachycardia (MAT): Characterized by ≥3 distinct P-wave morphologies at different rates with irregular rhythm 2
  • Atrial fibrillation: Irregular atrial activity without distinct P waves and irregular R-R intervals 2
  • Atrial flutter with variable AV conduction: Variable block producing irregular ventricular response
  • Focal atrial tachycardia with variable conduction: May show irregularity especially at onset ("warm-up") and termination ("warm-down") 2

• Wide QRS (>120 ms):

  • Atrial fibrillation with bundle branch block (pre-existing or functional) 3
  • Atrial fibrillation with conduction via accessory pathways (pre-excited AF in WPW syndrome) 3
  • Polymorphic ventricular tachycardia: Always associated with hemodynamic collapse 3

For Regular Tachycardias with Variable Features:

If the tachycardia shows variable QRS morphology or RP intervals 4:

• Consider alternating bundle branch block patterns during SVT
• Antidromic AVRT with varying degrees of fusion
• Atrial tachycardia with rate-dependent aberrancy

Step 2: Identify Key ECG Features

Critical distinguishing characteristics 1, 2:

• P-wave morphology and count: MAT requires ≥3 distinct P-wave morphologies
• P-wave presence: Absent in AF, present but variable in MAT
• Atrial rate: Typical atrial flutter runs ~300 bpm (may be slower with antiarrhythmics or scarring) 2
• AV dissociation signs: Suggests ventricular origin if wide QRS 3

Step 3: Consider Clinical Context

Common pitfalls to avoid:

• Irregular wide QRS tachycardia in a hemodynamically stable patient is almost always AF with aberrancy or pre-excitation, not polymorphic VT 3
• Pre-excited AF (WPW) can be life-threatening and requires different management than typical AF
• MAT is strongly associated with underlying pulmonary disease (COPD, hypoxia) or metabolic derangements 2

Specific Differential Categories

Primary Supraventricular Causes:

• Inappropriate sinus tachycardia (variable rate >100 bpm at rest, mean 24-hour >90 bpm) 2
• Sinus node reentrant tachycardia (abrupt onset/termination with sinus-appearing P waves) 2
• Multifocal atrial tachycardia 2
• Atrial fibrillation 2
• Atrial flutter with variable block 1, 2

Accessory Pathway-Mediated:

• Pre-excited atrial fibrillation (irregular, wide QRS, very rapid rates) 3
• Antidromic AVRT with variable morphology 1

Ventricular Causes (if wide QRS):

• Polymorphic VT (hemodynamically unstable) 3
• VT with varying exit sites 5

The presence of structural heart disease, particularly prior myocardial infarction, strongly favors ventricular origin for wide complex irregular tachycardias 3.