Second degree AV block Type I identifies an AV nodal site of block with sinus impulses entering AV nodal tissue relatively and progressively earlier in the relative refractory period, with slower and slower conduction and, eventually complete block. The characteristic features are progressive lengthening of the PR interval followed by a non-conducted sinus P wave, and then repetition of the cycle (so-called grouped beating). Note in the strip, the repetitive groups of two narrow QRS complexes with an intervening pause. The QRS is usually narrow, but may be wide, due to unrelated ventricular conduction abnormalities. Type I block is usually transient and reversible. The following animation illustrates what we have just described.