What are the characteristics of ventricular tachycardia?

Characteristics of Ventricular Tachycardia

Ventricular tachycardia is defined as three or more consecutive ventricular beats at a rate exceeding 100 bpm (cycle length <600 ms) with a wide QRS complex (≥120 ms), originating from the ventricles below the atrioventricular node. 1, 2

Core Defining Features

Rate and Duration Criteria

• Rate >100 bpm (cycle length <600 ms), though clinically significant VT typically presents at rates >120 bpm 1, 2
• Nonsustained VT: ≥3 beats terminating spontaneously in <30 seconds 1
• Sustained VT: Duration >30 seconds or requiring termination due to hemodynamic compromise in <30 seconds 1

QRS Complex Characteristics

• QRS duration ≥120 ms (0.12 seconds) in all leads—this must be verified in multiple leads as the QRS may appear deceptively narrow in 1-2 leads 3
• QRS width >140 ms with RBBB pattern or >160 ms with LBBB pattern strongly favors VT over SVT with aberrancy 1
• The QRS morphology differs from the patient's normal sinus QRS 4

Critical Diagnostic ECG Features

Pathognomonic Findings

• AV dissociation with ventricular rate faster than atrial rate is diagnostic of VT (present in ~30% of cases but highly specific) 1
• Fusion complexes (merger of conducted sinus impulses with ventricular depolarization) are pathognomonic for VT 1
• Capture beats (occasional normally conducted sinus beats during tachycardia) confirm VT 5

Highly Suggestive ECG Criteria

The ACC/AHA/ESC guidelines provide specific morphologic criteria that strongly suggest VT 1:

• Absence of RS complexes in all precordial leads V1-V6 (Brugada criteria) 1
• RS interval >100 ms (onset of R wave to nadir of S wave) in any precordial lead 1
• R-wave peak time ≥50 ms in lead II 1
• QRS concordance (all positive or all negative) in precordial leads V1-V6—negative concordance is diagnostic for VT 1
• Initial R wave in aVR or initial R or Q wave >40 ms in aVR (Vereckei algorithm) 1
• Notch on descending limb of predominantly negative QRS in aVR 1
• QR complexes indicate myocardial scar and are present in ~40% of post-MI VT 1

Morphologic Subtypes

Monomorphic VT

• Single, stable QRS morphology throughout the tachycardia 1
• Most commonly associated with structural heart disease, particularly prior myocardial infarction 1

Polymorphic VT

• Continuously changing or multiform QRS morphology at cycle length 180-600 ms 1
• Torsades de pointes: specific polymorphic VT associated with prolonged QT/QTc, characterized by twisting of QRS peaks around the isoelectric line 1

Specialized Forms

• Ventricular flutter: Regular rhythm ~300 bpm (cycle length ~200 ms) with monomorphic appearance and no isoelectric interval between QRS complexes 1
• Bidirectional VT: Beat-to-beat alternans in QRS frontal plane axis, classically associated with digitalis toxicity 1
• Bundle-branch reentrant VT: Reentry involving His-Purkinje system, usually LBBB morphology, occurring with cardiomyopathy 1

Physical Examination Findings

When P waves are not visible on ECG, physical examination can reveal evidence of AV dissociation 1:

• Irregular cannon A waves in jugular venous pulse
• Variable intensity of first heart sound
• Beat-to-beat variability in systolic blood pressure

Clinical Context

Hemodynamic Presentation

• VT can present with the patient being asymptomatic to hemodynamically unstable 6
• Pulsed VT: Patient maintains cardiac output but may have symptoms of reduced output from poor ventricular filling 6
• Pulseless VT: No effective cardiac output—treated as cardiac arrest 1

Common Underlying Substrates

VT occurs most frequently in 1, 7, 2:

• Chronic coronary heart disease and prior myocardial infarction
• Cardiomyopathies (dilated, hypertrophic, arrhythmogenic right ventricular)
• Heart failure
• Electrolyte disturbances (hypokalemia, hypomagnesemia)
• Medication toxicity (antiarrhythmics, digitalis)
• Structurally normal hearts (idiopathic VT)

Critical Diagnostic Pitfalls

Common Misdiagnoses

The differential diagnosis of wide-complex tachycardia includes 1:

• SVT with pre-existing bundle branch block
• SVT with aberrant conduction due to tachycardia
• SVT with conduction over accessory pathway (pre-excitation)
• SVT with wide QRS from electrolyte/metabolic disorders
• Paced rhythm
• Artifact

Key Clinical Decision Rule

When wide-complex tachycardia cannot be definitively diagnosed despite careful ECG evaluation, treat as VT—this is the safest approach as most wide-complex tachycardias are ventricular in origin 1. A history of prior myocardial infarction with first occurrence of wide-complex tachycardia after the infarct strongly indicates VT 1.

Limitations of Morphologic Criteria

Width and morphologic criteria become less specific in patients taking 1:

• Class Ia or Ic antiarrhythmic drugs
• Those with hyperkalemia
• Those with severe heart failure

Adenosine should be used with extreme caution when diagnosis is unclear, as it may precipitate ventricular fibrillation in patients with coronary artery disease 1.