A 35-year-old woman has had palpitations, shortness of breath, poor appetite, and edema for 5 years, with worsening symptoms after an upper respiratory infection 2 weeks ago. Examination: BP 120/70 mmHg, cardiomegaly, diastolic rumbling murmur at the apex, heart rate 120/min, irregular rhythm with varying heart sound intensity, jugular vein distension, bilateral basal crackles, liver 3 cm below the costal margin with tenderness (+), no splenomegaly, lower extremity edema (+), pulse rate 80/min. The irregular rhythm in this patient is: