Sino-atrial exit block, type I or wenckebach

This example illustrates 2nd degree sino-atrial exit block. In type 1 S-A block the conduction time between sinus firing and atrial capture progressively prolong, but this cannot be seen on the ECG tracing; type I exit block is inferred if the P-P intervals gradually shorten before the pause and if the P-P interval of the pause is less than the two preceding P-P intervals. In type II S-A block the P-P interval of the pause is twice the basic P-P interval.