What is the differential diagnosis for asymptomatic tachycardia?

Differential Diagnosis for Asymptomatic Tachycardia

Asymptomatic tachycardia requires systematic evaluation to distinguish physiologic from pathologic causes, with the ECG pattern (narrow versus wide QRS, regular versus irregular) serving as the primary diagnostic framework.

Initial ECG Classification

The differential diagnosis begins with determining whether the QRS complex is narrow (<120 ms) or wide (>120 ms), and whether the rhythm is regular or irregular 1.

Narrow Complex Tachycardias (QRS <120 ms)

Regular narrow complex tachycardias:

• Sinus tachycardia - Most common cause, with heart rate typically 100-180 bpm and normal P-wave morphology 1

  • Physiologic sinus tachycardia (exercise, anxiety, fever, hypovolemia, anemia, hyperthyroidism, medications)
  • Inappropriate sinus tachycardia - persistent resting heart rate >100 bpm without identifiable cause, predominantly affects young females (90%), mean age 38 years 1

• Atrioventricular nodal reentrant tachycardia (AVNRT) - characterized by absent or barely visible P waves, or pseudo r' waves in V1 and pseudo S waves in inferior leads 2, 1

• Atrioventricular reentrant tachycardia (AVRT) - involves an accessory pathway, may show pre-excitation pattern on baseline ECG if manifest pathway 1

• Atrial tachycardia (AT) - P waves occur in second half of tachycardia cycle, often obscured by preceding T wave, rate typically 100-250 bpm 1

  • Focal AT prevalence is 0.34% in asymptomatic patients 1
  • May be associated with digitalis toxicity, especially with AV block 1

Irregular narrow complex tachycardias:

• Atrial fibrillation - irregularly irregular rhythm without discrete P waves 1
• Multifocal atrial tachycardia (MAT) - at least three distinct P-wave morphologies 1
• Atrial flutter with variable AV conduction - sawtooth flutter waves with varying ventricular response 1

Wide Complex Tachycardias (QRS >120 ms)

Wide complex tachycardia must be assumed to be ventricular tachycardia until proven otherwise 1, 3.

• Ventricular tachycardia (VT) - AV dissociation or fusion complexes are diagnostic 1

  • Can be asymptomatic, particularly in children with structurally normal hearts 4

• Supraventricular tachycardia with aberrant conduction - QRS morphology identical to baseline bundle branch block pattern suggests SVT 1

• Pre-excited tachycardia - antidromic AVRT or atrial fibrillation with conduction over accessory pathway 1

Critical Diagnostic Considerations

Asymptomatic pre-excitation (Wolff-Parkinson-White pattern):

• Delta waves on resting ECG without documented arrhythmia symptoms 1
• Prevalence 0.15-0.25% in general population
• Risk stratification requires electrophysiology study only if high-risk occupation (pilots, drivers) or if symptoms develop 1

Postural Orthostatic Tachycardia Syndrome (POTS):

• Must be excluded before diagnosing inappropriate sinus tachycardia 1
• Excessive heart rate increase with postural change

Tachycardia-induced cardiomyopathy risk:

• Incessant forms of focal AT can lead to cardiomyopathy 1
• Risk appears low for most asymptomatic tachycardias 1

Diagnostic Approach for Asymptomatic Patients

When tachycardia is documented on resting ECG:

• Obtain 12-lead ECG during tachycardia to assess QRS width, regularity, P-wave morphology, and relationship to QRS 1, 2
• Compare to baseline ECG in sinus rhythm 1

When tachycardia is not present during evaluation:

• 24-hour Holter monitoring if episodes occur several times per week 2
• Event recorder or external loop recorder if episodes occur less than weekly but more than monthly 1
• Implantable loop recorder for infrequent episodes in high-risk patients 1

Exclude secondary causes systematically:

• Hyperthyroidism, pheochromocytoma, anemia, dehydration 1
• Medications (sympathomimetics, anticholinergics, bronchodilators)
• Caffeine, alcohol, illicit drugs
• Pulmonary embolism, infection, pain

Key Pitfall

Asymptomatic arrhythmias documented on monitoring are not accurate surrogates for symptomatic episodes - the rhythm causing symptoms may differ from incidentally detected arrhythmias 1, 5. Among patients with documented paroxysmal SVT, 6.5% had atrial fibrillation at next symptomatic episode, and 11.8% of those with documented AF had regular tachycardia at next episode 5.