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When confronted with a tachycardia having a broad QRS complex, it is important to be able to differentiate between a supraventricular and a ventricular tachycardia. Medication given for the treatment of a supraventricular tachycardia (SVT) may be harmful to a patient with a ventricular tachycardia (VT).1 ,2 A reasonable haemodynamic condition during a tachycardia may erroneously lead to the wrong diagnosis of SVT.3 Familiarity with the ECG signs allowing the diagnosis of a VT is therefore essential. But as will be discussed here, the ECG should not only tell you how to distinguish VT from other tachycardias with a broad QRS complex, but also to suspect its aetiology and its site of origin in the ventricle. Both aspects are important in decision making about the prognostic significance of VT and correct treatment.
Classification of tachycardias with a broad QRS complex
As shown in fig 1, broad QRS tachycardia can be divided in three groups.
SVT with bundle branch block—Bundle branch block (BBB) may be pre-existing or can occur when the refractory period of one of the bundle branches is reached because of the heart rate of the SVT (so called tachycardia related or phase 3 block). BBB can also occur because of retrograde invasion in one of the bundle branches.4 These causes of BBB can be found in patients with atrial tachycardia, atrial flutter, atrial fibrillation, atrioventricular (AV) nodal tachycardia, and also during orthodromic circus movement tachycardia (with AV conduction over the AV node and ventriculo-atrial (VA) conduction over an accessory AV pathway).
SVT with AV conduction over an accessory AV pathway—This may occur during atrial tachycardia, atrial flutter, atrial fibrillation, AV nodal tachycardia, and during antidromic circus movement tachycardia (with AV conduction over an accessory AV pathway and VA conduction over the AV node or a second accessory AV pathway). It is …