Sometimes you see an ECG strip, look at it and then put it away again with the thought: I don't understand this. But if you take your time and analyze the ECG systematically, you will usually come to a good result. This is an example of such an ECG (certainly not the most difficult). The explanation is shown in the 2nd picture.

The ECG shows an example of a patient with bradycardia/tachycardia syndrome (also called sick sinus syndrome). Initially, a sinus rhythm with a heart rate of approx. 70 bpm is seen. This is followed by sinus arrest of just under 3000 ms, followed by a junctional escape beat (no preceding P wave, QRS remains narrow). After a further pause of just under 2000 ms, 2 sinus node beats follow, which merge into a sinus tachycardia or atrial tachycardia (heart rate approx. 120 bpm here).

The first 3 beats are sinus node beats, all have the same morphology of the P wave. This is followed by a PAC, which is conducted via the fast pathway in the AV node. The next PAC is conducted via the slow pathway, then the AV nodal reentry tachycardia (slow/fast) starts. The retrograde P waves are visible at the end of the QRS complex in the lower lead K3.

Why is the pause after the PVCs relatively long? We see a sinus arrhythmia in the rhythm strip. The ventricular extrasystoles penetrate the AV node retrogradely and reset the sinus rhythm, which then restarts. VA conduction can be recognized by the inverted P wave following the QRS complex of the ventricular extrasystole (PVC).

In order to differentiate supraventricular extrasystoles/tachycardias with aberrant conduction from ventricular extrasystoles/tachycardias, the search for P waves is often very important. Here are some examples of how the P waves can be positioned around the ventricular extrasystoles.

This is the ECG of a 50-year-old man with a congenital heart defect (we do not have exact details).
To interpret an ECG with several different abnormalities, you have to proceed systematically. You can see my comments in the second picture. Perhaps Dawn would like to add something?

Here you can see a long rhythm strip from a Holter ECG, written at 25 mm/s. On the left, a sinus bradyarrhythmia can be seen first, followed by an atrial tachycardia. After a few beats this changes back into a sinus bradyarrhythmia. Then follows a short VT over 3 beats, after 1 sinus node beat then a ventricular couplet. Sinus bradyarrhythmia again at the end.

For a better overview, the leads aVL and V2-V4 are not shown in this ECG. The basic rhythm is atrial fibrillation (no P waves or flutter waves visible, but fibrillation waves). When the conduction rate drops, the QRS complexes are narrow. Faster conduction results in wide QRS complexes with LBBB morphology. This is an example of phase 3 (acceleration dependant) LBBB.