Supraventricular tachycardia commonly presents in two forms - AV reentry and AV nodal reentry. In AV reentry (below), the SVT presents as a regular tachycardia originating outside the ventricular myocardium. In this type of SVT, the AV node is used for impulse conduction to the ventricles, while an accessory pathway is used to return electrical conduction back to the atria. The heart rate is usually regular, at a rate of 170 to 250 bpm (below = 188 bpm). In this type of SVT, P waves are always present outside of the QRS complex, while their polarity depends on the atrial insertion of the accessory pathway. The QRS complex is narrow with a duration less that 0.2 seconds and an atrioventricular conduction ratio of 1:1. In 25% - 30% of patients demonstrating AV reentry, QRS alternans is present (varying amplitudes of the QRS complex in all leads except V4). AV reentry is not usually associated with structural heart disease and commonly presents as a variety of symptoms including palpitations, nervousness, anxiety, syncope or heart failure.