QT interval prolongation has been reported to occur during induced hypoglycaemia.  Such QT changes may be associated with ventricular arrhythmias.  The effect of “spontaneous” hypoglycaemia on QT interval during free-living conditions is less well established.  To explore this phenomenon, simultaneous Holter and Continuous Glucose Monitoring (CGM) were performed.

Fourteen patients with diabetes (3 with T1DM; 11 insulin treated T2DM) were studied.  Their mean age was 62 ± 7.2 years, with duration of diabetes 18.4 ± 10.0 years and HbA1c 8.6 ± 1.5%.  Holter and CGM were performed simultaneously for a mean of 38 hours (range 21 to 46).  Patients were asked to continue their regular daily activities and diet.  Episodes of hypoglycaemia, euglycaemia and hyperglycaemia were defined as glucose readings ≤ 4mmol/L, 4.1-14.9mmol/L and ≥15mmol/L, respectively.  For each patient, periods of hypoglycaemia, euglycaemia and hyperglycaemia were pooled into respective categories and each analysed as a composite period.  Heart rate and QTc at five minute intervals were determined for each period.  Results are presented as mean ± SD or range and analysed by paired t test.

There was no significant difference in QTc between periods of euglycaemia and hyperglycaemia.  However, five patients with a total of eight episodes of hypoglycaemia with a mean duration of 100 minutes (range 20 to 165) showed an increase in QTc during hypoglycaemia compared to euglycaemia.  The magnitude of change in QTc during hypoglycaemia was inversely proportional to the baseline QTc (r= -0.81, p<0.05).

This study showed that prolongation of QT interval could occur during periods of spontaneous hypoglycaemia in an ambulatory setting.  Although the magnitude of the QT change is small, it may be associated with life threatening arrhythmia in high risk individuals.  A larger study is required to confirm this phenomenon and determine if there is any group of patients particularly at risk.