This EKG shows the classic features of sick sinus syndrome. Initially, there is an accelerated atrial rhythm/atrial tachycardia. After a pre-automatic pause of 2609 ms, a ventricular premature beat occurs, followed by a junctional escape rhythm. Pre-automatic pause is a pause after a tachycardia and before an automatic rhythm, like sinus rhythm or, in this case, junctional escape rhythm. Therefore, there is an alternation between tachycardic phases and very bradycardic rhythms, the classic bradycardia/tachycardia syndrome. Paper speed is 12.5 mm/sec.
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.