A 7-year-old female spayed Shetland Sheepdog was referred to the Cornell Emergency Service in Janum-y 2014 for further workup of intermittent lethm-gy and abnormal behavior of one week duration. The referring veterinarian's physical exam and diagnostic tests were unremm-kable despite progressively worsening behavior at home; the patient was therefore referred to Cornell for further diagnostics. On presentation, the patient had episodes of mm-ked tachycm-dia and bradycm-dia: 260 and 30 beats per minute, respectively. Blood was submitted for a complete blood count, chemistry panel and DIC panel, and urine was collected for urinalysis. The patient's electrocm-diogram (ECG) documented periods of sinus arrest. She was subsequently transferred to the Cardiology Service with a presumptive diagnosis of sinus node dysfunction, and underwent emergency pacemaker implantation that evening. The pacemaker resolved the bradycm-dia, however recove1y was complicated by concomitant cm-diac thrombi and neurologic disease. This case study will focus on the biology and incidence of sinus node dysfunction, as well as treatment via pacemaker implantation.