BACKGROUND: Both hypoglycemia and severe hyperglycemia constitute known risk factors for cardiac repolarization changes potentially leading to malignant arrhythmias. Patients with loss of function mutations in KCNQ1 are characterized by long QT syndrome (LQTS) and may be at increased risk for glucose-induced repolarization disturbances.

OBJECTIVE: The purpose of this study was to test the hypothesis that KCNQ1 LQTS patients are at particular risk for cardiac repolarization changes during the relative hyperglycemia that occurs after an oral glucose load.

METHODS: Fourteen KCNQ1 LQTS patients and 28 control participants matched for gender, body mass index, and age underwent a 3-hour oral 75-g glucose tolerance test with ECGs obtained at 7 time points. Fridericia corrected QT interval (QTcF), Bazett corrected QT interval (QTcB), and the Morphology Combination Score (MCS) were calculated.

RESULTS: QTc and MCS increased in both groups. MCS remained elevated until 150 minutes after glucose ingestion, and the maximal change from baseline was larger among KCNQ1 LQTS patients compared with control subjects (0.28 ± 0.27 vs 0.15 ± 0.13; P <.05).

CONCLUSION: Relative hyperglycemia induced by ingestion of 75-g glucose caused cardiac repolarization disturbances that were more severe in KCNQ1 LQTS patients compared with control subjects.