A 12-year-old female was admitted to our hospital emergency department (ED) with abdominal pain and incessant vomiting. Her physical examination and blood results were normal. Transabdominal ultrasound results were normal as well, and the ED team decided to discharge the patient from the ED with a prediagnosis of acute gastroenteritis and referred her to the pediatrics department for further management. However, a day later, the patient was again admitted to the ED with more complaints of abdominal pain, headache, and chest pain. All of the patient's complaints mentioned above had also progressed in severity. The second physical examination in the ED revealed low blood pressure, and ECG results showed signs of arrhythmia. The patient consulted the Department of Pediatric Cardiology to determine the main reason for the arrhythmia findings. The patient's general condition was lethargic, and her blood pressure was 80/37 mmHg. Her ECG findings were indicative of atrial fibrillation (). A decision was made by our multidisciplinary team, including critical care, cardiology, and emergency medicine, to transfer the patient to the pediatric intensive care unit (PICU) for a higher level of care. Initial blood results at PICU admission revealed hemoglobin (HGB) of 10 g/dL, white blood cells (WBCs) of 11 × 103/μmm3, thrombocytopenia (139 × 103/μmm3), high C-reactive protein (CRP) of 45 mg/L and a high erythrocyte sedimentation rate (ESR) of 35 mm/hr. The first analyses of cardiac enzymes revealed troponin-I 1100 IU/mL (normal range 0–40), creatine kinase 450 IU/mL (normal range 22- 198), creatine kinase (CK-MB) isoenzyme 150 IU/L (normal range 5-10), amylase 185 gr/dL (normal range 30-110), and albumin 2.8 gr/dL (normal range 3.4–5.4). Serum electrolytes were unremarkable. The patient was put under strict monitoring. Bedside imaging showed AF-induced polymorphic artery tracing. The alterations in pulse oximetry were in accordance with the polymorphic artery trace (). An echocardiographic inspection revealed severe dilation of the left side of the heart, and severe mitral insufficiency was detected, with substantial systolic dysfunction (ejection fraction (EF) 40% and shortening fraction (SF) 20%). Additionally, the patient was prescribed adrenaline, milrinone, furosemide, and carnitine for her severe heart failure. Although the patient's left atrium was enlarged, thrombosis inside the left atrium was not detected. At this point, it had been more than 72 hours since the patient's first ED admission with her initial complaints of abdominal pain and incessant vomiting. Thus, cardioversion was not performed due to the probability of microthrombus formation. Warfarin treatment was promptly initiated. The thyroid hormone profile was unremarkable. Cardiac magnetic resonance imaging (MRI) was performed to explain the etiology, which resembled acute myocarditis due to the existence of T2-weighted edema images. The patient was prescribed an IVIG infusion of 1 gr/kg IV for two days in a row. The patient's condition was discussed in a council composed of two pediatric critical care specialists and two pediatric cardiologists. The council decided that the extracorporeal membrane oxygenation unit (ECMO) should be prepared if the event that the patient developed sudden cardiac arrest or her blood pressure suddenly dropped. After the administration of the second dose of IVIG in the PICU, the patient's blood pressure was mildly elevated (90/60/70 mmHg), and a small amelioration of ejection fraction 44% and a shortening fraction of 22% were noted on echocardiography. On the 3rd day in the PICU, there were no signs of atrial fibrillation improvement; however, multifocal ventricular ectopic beats were detected due to the aberrant cardiac conduction of atrial fibrillation (). There was no significant improvement or any notable development in the clinical situation of the patient throughout her stay between the 3rd day and 15th day in the PICU. On the 15th day in the PICU, a transesophageal echocardiography examination was performed, and no signs of thrombosis in the left atrium were observed. The patient was started on amiodarone of 200 mg/day. On the 3rd day of amiodarone treatment, cardioversion was performed at a dose of 0.5 Joules/kg in a total of 30 Joules. The rhythms became a sinus rhythm (). The multidisciplinary team continued both treatments of amiodarone and warfarin for two weeks, and the patient's general condition improved significantly during that time. On the 30th day of PICU admission, the patient's blood pressure was within normal range, and an echocardiographic inspection revealed significant improvement in an EF of 50% and a SF of 26% compared with her first day of PICU admission (). The patient was discharged from the hospital on the 35th day after her first admission to the ED. The patient continues to be attended at our pediatric cardiology clinic monthly and is still under treatment with amiodarone.