While the diagnosis of 1st and fixed 2nd degree SA block cannot be established from the body surface ECG, it is possible to make the diagnosis of type I and type II 2nd degree SA block from the body surface ECG.
This ECG is from an 82 year old patient. Note that the PP intervals before and after the pause are quite regular, that the PR intervals are normal (0.14 seconds) and constant, and that there are no P waves during the pause. Note also that the interval between beats 5 and 6 (1.72 seconds) is exactly twice the interval between beats 4 and 5 and between beats 6 and 7 (0.86 seconds). These features indicate that the pause is due to a blocked sinus impulse without prior prolongation of the SA interval, i.e. that it is due to type II 2nd degree SA block. This is depicted in the ladder diagram.