ABC of clinical electrocardiography: Broad complex tachycardia---Part II

This article continues the discussion, started last week, on ventricular tachycardias and also examines how to determine whether a broad complex tachycardia is ventricular or supraventricular in origin.Fascicular ventricular tachycardia (note the right bundle branch block pattern and left axis deviation) Fascicular tachycardiaFascicular tachycardia is uncommon and not usually associated with underlying structural heart disease. It originates from the region of the posterior fascicle (or occasionally the anterior fascicle) of the left bundle branch and is partly propagated by the His-Purkinje network. It therefore produces QRS complexes of relatively short duration (0.11-0.14 s). Consequently, this arrhythmia is commonly misdiagnosed as a supraventricular tachycardia.Right ventricular outflow track tachycardiaThe QRS complexes have a right bundle branch block pattern, often with a small Q wave rather than primary R wave in lead V1 and a deep S wave in lead V6. When the tachycardia originates from the posterior fascicle the frontal plane axis of the QRS complex is deviated to the left; when it originates from the anterior fascicle, right axis deviation is seen. Right ventricular outflow tract tachycardiaThis tachycardia originates from the right ventricular outflow tract, and the impulse spreads inferiorly. The electrocardiogram typically shows right axis deviation, with a left bundle branch block pattern. The tachycardia may be brief and self terminating or sustained, and it may be provoked by catecholamine release, sudden changes in heart rate, and exercise. The tachycardia usually responds to drugs such as β blockers or calcium antagonists. Occasionally the arrhythmia stops with adenosine treatment and so may be misdiagnosed as a supraventricular tachycardia. Torsades de pointes tachycardiaTorsades de pointes (“twisting of points”) is a type of polymorphic ventricular tachycardia in which the cardiac axis rotates over a sequence of 5-20 beats, changing from one direction to another and back again. The QRS amplitude varies similarly, such that the complexes appear to …