Clinical Characteristics and Outcomes of Malignant Neonatal Arrhythmias: A Tertiary NICU Experience

Background: Non-benign neonatal arrhythmias (NAs) can constitute a significant clinical burden in neonatal intensive care units (NICUs); however, the impact of prenatal diagnosis and postnatal persistence patterns on clinical outcomes has not been fully elucidated.Objective: To evaluate the clinical burden, acute intensive care requirement, and post-discharge antiarrhythmic treatment need in non-benign NAs according to prenatal arrhythmia history and postnatal persistence pattern.Methods: This retrospective observational study included newborns diagnosed with non-benign arrhythmia during the fetal and/or neonatal period at a tertiary NICU between July 2020 and July 2025. Patients were categorized into three groups: fetal persisting (arrhythmia detected in the fetal period and persisted postnatally), fetal resolved (fetal arrhythmia resolved after birth), and neonatal only (arrhythmia emerged exclusively during the neonatal period). Clinical outcomes, treatment requirements, and adenosine response were compared between groups.Results: Of 9,210 NICU admissions, 35 cases (0.38%) with non-benign NA were identified. Supraventricular tachycardia was the most common arrhythmia type (66.7%) Groups with active postnatal arrhythmia (fetal persisting and neonatal only) demonstrated longer hospitalization, higher comorbidity rates, and greater post-discharge antiarrhythmic treatment requirement (90.9% and 92.3%, respectively) compared to the fetal resolved group (9.1%). Potassium levels were significantly higher in adenosine-resistant cases (median 5.4 vs. 4.3 mEq/L, P = 0.044).Conclusions: In this cohort, active postnatal arrhythmia, whether persistent from the fetal period or emerging only after birth, was associated with greater clinical burden and post-discharge antiarrhythmic treatment requirement than fetal arrhythmia that resolved postnatally. The observed association between higher potassium levels and adenosine resistance should be interpreted cautiously as an exploratory finding.