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. The ECG is digitized, so the grid printed will allow for accurate measurement of rate (notice that the "squares" are now rectangles).
For more about sick sinus syndrome, see https://www.aafp.org/pubs/afp/issues/2003/0415/p1725.html
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