A middle-aged, depressed, coughing, exercise-intolerant Doberman pinscher is presented. The dog has a rapid and irregular heart rate. What disease is at the top of the differential diagnosis list？一只中年，沉郁，咳嗽以及运动不耐受的杜宾犬前来就诊。该犬心动过速且心律不齐。你的鉴别诊断列表最顶端的诊断是什么？