How do you locate ventricular tachycardia (VT) based on electrocardiogram (ECG)?

Locating Ventricular Tachycardia Based on ECG

Ventricular tachycardia (VT) can be diagnosed on ECG by identifying a wide QRS complex (>120 ms), AV dissociation, fusion beats, and specific morphological features in leads V1-V6 and aVR. 1

Key Diagnostic Criteria for VT

QRS Characteristics

• QRS duration typically >120 ms in adults, though in infants VT may present with QRS <90 ms but different from sinus QRS morphology 1
• QRS morphology different from the patient's normal QRS during sinus rhythm 1
• QRS complexes in precordial leads that are all positive or all negative (concordance) strongly suggest VT 1
• R-S interval (onset of R wave to nadir of S wave) >100 ms in any precordial lead implies VT 1

AV Relationship

• Presence of AV dissociation (ventricular rate faster than atrial rate) is pathognomonic of VT 1
• Fusion beats (combination of supraventricular and ventricular complexes) strongly indicate VT 1
• Independent P waves continuing unrelated to the QRS complexes during tachycardia 1

Lead-Specific Findings (Vereckei Algorithm for aVR)

• Presence of initial R wave in lead aVR suggests VT 1
• Initial R or Q wave >40 ms in aVR suggests VT 1
• Presence of a notch on the descending limb at the onset of a predominantly negative QRS in aVR suggests VT 1

Additional Morphological Features

• R-wave peak time ≥50 ms in lead II suggests VT 1
• QR complexes (indicating myocardial scar) are present in approximately 40% of patients with post-MI VT 1
• Similar morphology between premature ventricular beats during sinus rhythm and the tachyarrhythmia strongly suggests VT 1

Differentiating VT from SVT with Wide QRS

When distinguishing VT from supraventricular tachycardia (SVT) with aberrancy:

• SVT with aberrancy is rare in infants beyond the first 10-20 beats, so persistent aberration suggests VT 1
• QRS complexes in tachycardia that are identical to sinus rhythm suggest SVT rather than VT 1
• In infants with a QRS complex different from sinus, the diagnosis is likely VT in the majority of cases 1

Clinical Context Considerations

• Patients with prior myocardial infarction who develop wide QRS tachycardia are more likely to have VT 1
• VT rates in infants may range from 200-500 beats/min, with slight variation in R-R intervals 1
• Left bundle branch block pattern VTs are more predictive of specific endocardial sites of origin (73% accuracy) compared to right bundle branch block patterns (31%) 2
• VT associated with inferior wall infarction shows stronger correlation with specific endocardial sites (74%) compared to anterior wall infarction (37%) 2

Common Pitfalls to Avoid

• Relying solely on QRS width criteria can be misleading when patients have pre-existing bundle branch blocks or are taking certain antiarrhythmic medications 1
• Failure to obtain a 12-lead ECG may lead to misdiagnosis, as ventricular tachycardia can sometimes masquerade as SVT when only a single lead is examined 3
• Electrolyte abnormalities or metabolic disorders can cause wide QRS complexes that mimic VT 1
• When the diagnosis is unclear, it is safer to treat as VT rather than SVT 1

Advanced Diagnostic Tools

• The Mayo Clinic VT calculator (MC-VTcalc) can help estimate VT probability using computerized measurements (QRS duration, QRS axis, and T-wave axis) from standard 12-lead ECG recordings 4
• Echocardiography should be performed to determine ventricular function and potential structural abnormalities 1
• 24-hour Holter monitoring may be worthwhile in complex ventricular arrhythmias 1

Remember that accurate diagnosis of VT is crucial for appropriate management and referral to specialist services for ongoing care 5.