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Ventricular tachycardia is a tachycardia from the ventricles that is usually classified based on morphology, duration, and clinical presentation. The morphology can be either monomorphic or polymorphic. The duration is either sustained or nonsustained, with the cutoff being 30 seconds. For example, nonsustained ventricular tachycardia (NSVT) is >3 beats of VT but <30 seconds. The clinical presentation is divided into clinically stable or unstable. Monomorphic VT is usually from a single focus within the ventricles and produces uniform wide QRS complexes. The rate is typically 130-220bpm; those with rates less than this are unlikely to be VT (SVT with aberrancy).
Polymorphic VT has multiple foci and, therefore, has QRS complexes with different morphologies. Torsades de pointes is the most commonly recognized polymorphic VT that occurs with a prolonged QTc and displays an axis that is 'twisting'. Torsades usually results from the 'R on T phenomenon', where a QRS complex (commonly a PVC) is initiated on top of the prior T wave. Although torsades de pointes is usually a short-lived rhythm, it can often result in hemodynamic instability.
There are numerous criteria that are used to differentiate VT from SVT with aberrancy (which can also produce a wide complex tachycardia). The Brugada algorithm is the most commonly cited, though the Vereckei criteria are also commonly used. R wave peak time in lead II is another common method to differentiate them. The sensitivities for the detection of VT for the algorithms are 89% (Brugada), 94% (Griffith), 89% (Bayesian), 87% (Vereckei), and 60% (lead II). If one is not completely sure based on the EKG, the safest treatment modality is usually electrical cardioversion, if possible. Patients that are not young (>35yo) or with CAD or cardiac risk factors are more likely to have VT vs SVT with aberrancy. Extreme axis deviation (extreme right axis deviation) is much more commonly from VT. Additionally, a very wide QRS complex (>160ms) is likely to be VT vs SVT with a bundle branch block, though QRS complexes >200ms are more likely to be from a metabolic process (ie hyperkalemia).
Fusion and capture beats are very specific findings that the wide complex tachycardia is ventricular in origin. Capture beats are when the sinus node 'captures' the conduction system, with a resultant normal sinus beat. Fusion complexes are when the sinus node attempts to capture the conduction system and occurs at the same time as a ventricular beat; these will fuse together to create a mixture of the 2.
Examples:
Source: LITFL
A PVC that occurs on a T wave (R on T) that results in torsades de pointes. Note the wide complex tachycardia with various QRS morphologies and axises.
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