This 3-lead ECG comes from an 80-year-old gentleman who went for a cardiological examination after a syncope.
The ECG shows a sinus rhythm with wide QRS complexes, the QRS morphology in lead I suggests an LBBB, heart rate here just under 60 bpm. After 3 sinus node beats there is a pause of just under 2 seconds.
There are no P waves during this pause. Then sinus rhythm again for 6 beats, followed by a pause of just under 3 seconds, then sinus rhythm again.
How can the pauses be explained?
As there are no P waves, an AV block is ruled out, and atrial flutter or atrial fibrillation can also be ruled out with a clear conscience.
That leaves sinus arrest and SA block. Since the pause length is exactly 2 or 3 times the normal PP/RR interval and the PP intervals are constant, this is a 2nd degree SA Block type Mobitz 2