Ventricular tachycardia.

The major differential diagnosis for VT is supraventricular tachycardia with a wide QRS complex due to aberrant conduction. Although VT may be slightly irregular, gross irregularities, such as those typically seen in atrial fibrillation, are uncommon during VT; such marked irregularities suggest atrial fibrillation. The QRS configuration is helpful in differentiating supraventricular tachycardia from VT. Supraventricular tachycardia with aberration generally produces QRS complexes that are less than 0.14 second in duration, whereas wider complexes are common in VT. The presence of an antiarrhythmic drug may, however, render this criterion unhelpful.