Rhythm strip A shows atrial fibrillation with a rapid ventricular rate that is between 120 and 160 beats per minute
Rhythm strips B1 and B2:
These show sinus rhythm with atrial ectopic beats (4th and 9th beats in B1, 5th and 6th beats in B2) and a ventricular rate of about 80 beats per minute
Rhythm Strip B3:
The first three QRS complexes are each preceded by a P wave; the first P wave has a narrow base and a pointed tip, the second P wave has a broader base but a lower amplitude while the third P wave has a broad base and a pointed tip.
The shape of these P waves differs from the most common P wave shape seen in Rhythm strips B1 and B2, so they are probably all atrial ectopic beats.
The third P wave in B3 is located at the end of the ST segment of the second beat ("P on T", by analogy with the "R on T" terminology used for ventricular ectopic beats), and is followed by the development of atrial fibrillation.
Atrial fibrillation alternating with periods of sinus rhythm in a patient with (acute) right atrial strain due to pulmonary embolism. Atrial fibrillation initiated by atrial ectopic beats