A 9-year-old 21.7-kg (47.7-lb) spayed female Rottweiler was evaluated because of progressive weakness, lethargy, and diarrhea. Signs of weakness became noticeable approximately 4 weeks prior to evaluation. The dog's weakness progressively worsened, and the diarrhea had been unsuccessfully treated several times during that period. Several days prior to evaluation, the dog developed ascites. The dog was not receiving any medications other than a flea, tick, and heartworm preventative. On physical examination, bradycardia was detected and prompted echocardiographic and ECG examinations to be performed.
An initial lead II ECG tracing revealed an underlying sinus rhythm that was conducted with