Atrial fibrillation is a very common arrhythmia, affecting hundreds of millions of people worldwide. The diagnosis of atrial fibrillation is of great importance, as the timely initiation of oral anticoagulation can in many cases significantly reduce the risk of embolism (which is associated with this arrhythmia).
The risk of cardiac embolism is particularly high in the presence of paroxysmal atrial fibrillation, in which there is frequent alternation between sinus rhythm and atrial fibrillation.
Such a sequence is recorded here in a 3-lead ECG.

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?

This ECG is from a 65-year-old woman who had previously had no cardiac abnormalities but has now suffered several syncopes within 2 weeks. During the last syncope she suffered a fracture of the left femur.

Here we see a 30 s long strip of a 1-lead ECG rhythm strip, recorded with a modern Smartwatch (Apple watch). The ECG has been graphically processed for better visualization. The paper speed is 25 mm/s as usual. The lead shown corresponds to ECG lead I of the limb leads.

Especially in the social media, one sees again and again similar EKGS like the one shown here with the question: What type of AV block is present here? 2nd degree AVB block type I (Wenckebach) or type II (Mobitz)? It is neither one nor the other!
Here, a PAC can be seen under the blue arrow, which is not conduced because the AV node is still refractory (the refractory period of the AV node depends on the preceding heart rate). Therefore, it is not an AV block, but a physiologically non conducted PAC.

“Jerry W. Jones, MD FACEP FAAEM is pleased to announce the move from LinkedIn to his ECG education website: https://medicusofhouston.com/ . Join him there for his popular twice weekly posts on ECG topics for beginners through advanced and announcements regarding his Masterclasses and book releases.” I want to thank all of you for your interest in my teaching and in electrocardiography. I hope to see you in one of my Masterclasses soon! And I hope to hear from you in my new location! Jerry W. Jones, MD FACEP FAAEM