This EKG shows a sinus rhythm with ventricular bigeminy and retrograde conduction leading to retrograde depolarization of the sinus node, resulting in a longer pause (sinus node reset).
Then, a polymorphic ventricular tachycardia occurs over 7 beats. The QT interval of the sinus beats does not appear prolonged, thus ruling out Torsades de Pointes tachycardia. The most likely cause of this type of polymorphic ventricular tachycardia during a stress EKG is cardiac ischemia/coronary artery disease.

Here we see the EKG of a 63-year-old man with CAD without relevant coronary stenosis. He complains of slightly reduced performance, but no other symptoms. The ECG shows the following changes:

Here we see the EKG of a 15-year-old girl. The rhythm is irregular, with the heart rate fluctuating between approximately 60 and 90 beats per minute. All P waves are identical, and the PR interval is always constant. Therefore, a sinus arrhythmia is present. In this case, it is a respiratory sinus arrhythmia, which is commonly found in younger individuals. The heart rate increases reflexively during inspiration and decreases during exspiration. This physiological sinus arrhythmia is usually no longer present in older individuals.

Why does this EKG indicate a sick sinus node? First, we observe a sinus rhythm with a rate just below 60 bpm. Then, there is a pause of approximately 3000 ms, followed not by a sinus beat, but by a junctional escape beat (retrograde/inverted P-wave immediately after the QRS complex). This ECG was recorded at the general practitioner's office, and it can be assumed that no vagal stimulus contributed to the arrhythmia.