QT interval prolongation during spontaneous episodes of hypoglycaemia in type 1 diabetes: The impact of heart rate correction

Aims/hypothesis: Prolongation of the heart rate corrected QT interval (QTc) is seen during episodes of hypoglycaemia in type 1 diabetes. We studied the relationship between spontaneous hypoglycaemia and the QT interval and hypothesised that the choice of heart rate correction affects the observed change in QTc. Methods: Twenty-one participants with type 1 diabetes (aged 58∈±∈10 years with duration of diabetes 34∈±∈ 12 years) had continuous glucose and ECG monitoring for 72 h. QT and RR intervals were measured during hypoglycaemia (blood glucose or continuous glucose measurements ≤ 3.5 mmol/l) and compared with euglycaemia (5-12 mmol/l). QT intervals were measured using the semi-automated tangent method from signal-averaged ECG and corrected using Bazett's formula, Fridericia's formula, the nomogram method and a linear subject-specific method. Results: Hypoglycaemia was present in 14 participants. With Bazett's formula, QTc changed significantly from euglycaemia to hypoglycaemia (422∈±∈30 vs 432∈±∈33 ms; p∈=∈0.02). Heart rate, QT intervals and QTc corrected with formulas other than Bazett's were not associated with a significant change (p∈=∈0.07-0.29). During hypoglycaemia, significantly lower values of QTc compared with the subject-specific method were seen for Fridericia's formula (p∈=∈0.02) and the nomogram method (p∈=∈0.04). Conclusions/interpretation: Spontaneous hypoglycaemia was associated with a modest increase in QTc. Bazett's formula resulted in overcorrection of QTc while both Fridericia's formula and the nomogram method undercorrected the QTc compared with the subject-specific method during hypoglycaemia. The results may indicate that the use of a fixed heart rate correction formula can lead to misleading results in investigations of spontaneous hypoglycaemia.