SA block

How can you detect a second degree SA block type Mobitz 1 in a surface ECG?

Because firing of the sinoatrial node is not visible on the ECG tracing, the delay between excitation of the sinus node and the atrial myocardium cannot be seen on the surface ECG. Paradoxically, the P-P distance shortens with each excitation. This is because although the delay between sinus node and atrial myocardium increases with each sinus depolarization, the degree of increase becomes less with every beat.