*2.6.1 Narrow QRS complex, regular rhythm with short RP interval*

Atrioventricular nodal reentrant tachycardia (AVNRT) is a regular tachycardia resulting from a reentry circuit that occurs inside the atrium and perinodal tissue. AVNRT is a paroxysmal regular tachycardia characterized by two electrical pathways (fast and slow) near or inside the AV node, **Figure 11**. Fast pathways conduct quickly and exhibit a relatively long refractory period. In contrast, the slow pathway conducts relatively slowly and has a shorter refractory period [9, 10].

Approach to ECG findings: Heart rate > 100 (total R waves on long lead × 6) → Narrow QRS complex (QRS < 120 ms) → Regular RR intervals (RR intervals are equal) → Short RP interval (RP interval < half of RR interval).

If P waves are visible, they exhibit retrograde conduction with P wave inversion in leads II, III, and aVF. They may be buried within the QRS complex, visible afterward, or appear before the QRS. There will be pseudo S wave in Lead II and pseudo R wave in V1.
