AV blocks are among the ECGs that are particularly difficult to diagnose and where the most mistakes are made.
Here is the ECG of a 75-year-old lady who has not been feeling well for the last few weeks and is complaining of shortness of breath on exertion.
The ECG is irregular with alternating smaller and larger RR intervals.
QRS 2, 4 and 6 each show the same PR interval; it can be assumed that these are sinus node beats that are conducted.
QRS 1, 3, 5, 7 are junctional escape beats without reference to the P waves.
This is therefore an SR with 3:1 AVB and junctional escape beats.