[Diagnosis of narrow QRS complex tachycardias]

Tachycardias with normal QRS complexes (less than 0.12 seconds) may have multiple origins which may be classified in 4 groups: sinusal, atrioventricular nodal, atrioventricular junctional and finally, some ventricular tachycardias arising from near the conduction pathways. The electrocardiographic diagnosis requires analysis of the QRS complexes to detect an eventual irregularity which would suggest atrial fibrillation. When the tachycardia is regular, analysis of the P waves is fundamental (position in the ventricular cycle and morphology). The relationship of the P waves and the QRS complexes enables identification of those forms independent of the AV node and some rare types of tachycardia. Finally, the response to vagal stimulation is essential whether the tachycardia stops, slows down or persists unchanged.