Wide-complex tachycardias. The importance of identifying the mechanism.

Initial assessment of widecomplex tachycardias (WCTs) should begin with patient history and physical examination. If there is a history of remote myocardial infarction, WCT should be considered as ventricular tachycardia until proved otherwise. In most cases, WCT is subsequently shown by electrophysiologic evaluation to be ventricular tachycardia. The presenting symptoms and degree of hemodynamic compromise should not be used to distinguish ventricular tachycardia from supraventricular tachycardia. A 12-lead electrocardiogram (ECG) should be carefully reviewed by looking for signs of ventricular tachycardia (atrioventricular dissociation, captured and fusion beats, certain QRS shapes and concordance). If the surface ECG is inconclusive, changing the position of V1 or use of a transesophageal lead may allow assessment of atrial activity. When more QRS than P waves are documented, the diagnosis is ventricular tachycardia. Cardioversion-defibrillation is required in patients with WCT who are in unstable condition. Atrioventricular node blockers are the agents of choice for arrhythmias that are atrioventricular node-dependent, but they may cause hemodynamic collapse if administered to patients with ventricular tachycardia or with atrial fibrillation and preexcitation. Lidocaine hydrochloride (Xylocaine HCl IV), preferred for ventricular tachycardia, has been reported to cause ventricular fibrillation in patients with atrial fibrillation and preexcitation. When there is doubt about the diagnosis, elective electrical cardioversion may be considered. Medical therapy should consist of intravenous magnesium sulfate and procainamide hydrochloride (Pronestyl). A beta blocker may also be considered if more aggressive blockade of the atrioventricular node becomes necessary.