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.

In this 12-lead ECG there is a bradycardic rhythm, which is regular, heart rate about 45 bpm. The QRS complexes are narrow.
What is this rhythm called?
There are no P waves present, and no flutter or fibrillation waves can be seen.
There are 2 possibilities:
1. There is a sinus arrest with a junctional escape rhythm. The junctional rhythm either cannot conduct retrograde to the atria (therefore no inverted retrograde P wave can be seen) or the retrograde P wave is hidden in the QRS complex (atria and ventricles are excited simultaneously).

What can we learn from this 3-channel ECG?
A sinus rhythm can be seen in the left half of the image, which merges into a ventricular tachycardia on the right. The QRS complexes of the VT change their shape and polarity, this is called polymorphic.
The question is: can this be called TdP (Torsade de Pointes) tachycardia or not? The clear answer is: no! A polymorphic VT may only be called TdP if there is a prolonged QT/QTc duration in the native rhythm. This is obviously not the case here. Therefore, the term polymorphic VT is retained here

This ECG was sent to me by a friend, I don't know if he did it himself. The question was whether a heart attack can be recognized here. The patient is a 55-year-old man who has typical angina pectoris lasting more than 1 hour.
What can you answer?

This ECG (3 rhythm strips) initially shows a sinus rhythm with 1st degree AVB grade I and wide QRS complexes (presumably RBBB pattern). A PAC (P-wave premature, different form than in sinus rhythm) appears approximately in the middle of the ECG, this is not conducted . Due to the strong prematurity, this is not surprising. But why is the 2nd PAC also blocked? The answer can be found in the second picture.

Sometimes in a simple 1-lead ECG strip, various interesting ECG phenomena can be recognized, such as here. One could briefly look over the ECG, make the diagnosis of ventricular bigemininy and then be satisfied.
On closer inspection, however, 3 questions arise:
1. Is the P wave behind the PVC a sinus node P or an atrial extrasystole?
2. + 3. after one comes to the conclusion that it is a sinus node P: why is it not conducted and why is the PP distance smaller when there is a QRS complex in between than when there is not.

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.