Abstract Body (Do not enter title and authors here): Background: Long QT syndrome (LQTS) is an arrhythmogenic channelopathy that increases the risk of fatal arrhythmias. Limited data exist regarding the arrhythmogenic risk associated with hormonal changes during pregnancy and the postpartum period in women with LQTS.

Case Description: A 25-year-old G3P3 actively breastfeeding female with reported seizures in prior postpartum periods (Figure 1) suffered ventricular fibrillation arrest while at rest nearly two months into her postpartum period. She was successfully defibrillated with return of spontaneous circulation. During her first night in the ICU, she became bradycardic with QT lengthening to 700ms, leading to polymorphic ventricular tachycardia requiring cardioversion. By electrocardiogram, she was diagnosed with suspected LQTS type 3 (LQTS3, Figure 2). Nadolol was initiated, and an ICD was placed for secondary prevention. Breast feeding was discussed as potentially arrhythmogenic and therefore discouraged. Over the following year, she had no further seizure-like activity or ICD shocks. Genetic testing is currently pending.

Discussion: LQTS is a hereditary arrhythmogenic channelopathy most often caused by genetic mutations in cardiac potassium or sodium ion channels, resulting in delayed or prolonged cardiac repolarization, thereby increasing the risk of fatal arrhythmias. LQTS3 is caused by a gain of function mutation in the SCN5A gene, which encodes the cardiac sodium channel. LQTS3 is often triggered by rest or sleep and may also be provoked by breastfeeding. Several factors, such as hormonal changes (Table 1), are hypothesized to increase the risk of malignant arrhythmias in the postpartum period. It is known that the high levels of estrogen and progesterone during pregnancy may augment adrenergic responses. In particular, estrogen contributes to the increased heart rate (HR) seen in pregnancy. Other than in LQTS type 1, elevated HRs in those with LQTS generally attenuate the QT-prolonging effects of bradycardia. Therefore, the increase in postpartum arrhythmias in LQTS3 may be related to the overall reduction in HR during this period. Furthermore, previous studies have demonstrated QT prolongation and increased action potential duration following administration of both oxytocin and prolactin in LQTS type 2 transgenic rabbits. Interestingly, regional differences in ventricular repolarization were also noted, indicating a potential electrophysiologic substrate for development of Torsades de Pointes.