Abstract
Pacemaker (PM) syndrome is an uncommon complication after PM or defibrillator implant in patients with long-standing persistent atrial fibrillation. We present a case where an unexpected and unrecognized improvement in a comorbid condition paradoxically led to worsened symptoms, ie, acute-onset persistent dyspnea, in a patient with a single-chamber implantable cardiac defibrillator. A careful review of clinical data led to diagnosis and successful treatment.