How is monomorphic ventricular tachycardia (VT) differentiated from supraventricular tachycardia (VT) on ECG?

Updated: Dec 05, 2017
  • Author: Steven J Compton, MD, FACC, FACP, FHRS; Chief Editor: Jeffrey N Rottman, MD  more...
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Answer

Polymorphic VT is easily diagnosed after exclusion of lead motion artifact. Monomorphic VT can be more difficult to sort out. The ECG will demonstrate a wide-complex tachycardia, representing either VT or supraventricular tachycardia (SVT) with aberrant conduction. If the patient is unstable, or if differentiation between VT and SVT is uncertain, treat the rhythm as VT; the majority of patients with wide-complex regular tachycardias will have VT. If the patient is stable, the ECG can be examined for clues to the mechanism underlying the arrhythmia.

Atrioventricular dissociation

AV dissociation (see the images below), is apparent in approximately half of VT episodes; when present, it is a hallmark of VT. [49]  AV dissociation occurs because the sinus node is depolarizing the atria at a rate that is slower than the pathologic, faster ventricular rate. At times, P waves can be seen in between or embedded in the QRS complexes, but the P waves and QRS complexes have their own independent rates.

The electrocardiogram shows a form of idiopathic v The electrocardiogram shows a form of idiopathic ventricular tachycardia (VT) seen in the absence of structural heart disease. This rhythm arises from the left ventricular septum and often responds to verapamil. Upon superficial examination, it appears to be supraventricular tachycardia with bifascicular conduction block. Closer examination of lead V1 shows narrowing of fourth QRS complex, consistent with fusion between the wide QRS complex and the conducted atrial beat, confirming atrioventricular dissociation and a VT mechanism.
This tracing depicts atrioventricular dissociation This tracing depicts atrioventricular dissociation.

Retrograde conduction can also exist from the ventricles to the atria via the AV node. This is not AV dissociation and reveals itself on ECG as a 1:1 correlation between the wide QRS complex and an inverted P wave, which follows the QRS complex.

Fusion and capture beats

Fusion beats and capture beats can occur in the presence of VT, depending on the refractory period of the AV node and on the timing of ventricular and atrial depolarizations, respectively (see the image below). If present, they help distinguish VT from SVT with aberrant conduction.

Fusion beats, capture beats, and atrioventricular Fusion beats, capture beats, and atrioventricular dissociation can be seen on this electrocardiogram.

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