Case Presentation: An 80-year-old male with a history of atrial fibrillation post-ablation on apixaban, COPD, hypertension, and Stage IIIC malignant melanoma post-immunotherapy with nivolumab complicated by non-resolving pneumonitis presented with worsening dyspnea and tachycardia. In the ED, he was found to have atrial flutter with rapid ventricular response (RVR), elevated high-sensitivity troponin, and brain natriuretic [...]