Can changes in QT interval be used to predict the onset of hypoglycemia in type 1 diabetes

Episodes of hypoglycaemia in adults and children with Type I diabetes induce abnormalities in cardiac repolarization, including lengthening of the QT interval and QT dispersion. We have used a computerised ECG system to measure overnight changes in the heart rate corrected QT interval (QTc) in diabetic patients. The aim of the study was to determine whether QT prolongation is associated with nocturnal hypoglycaemia (blood glucose <3.5 mmol/l) and to investigate whether such changes might be used as the basis for a hypoglycaemia alarm. We used a Hewlett Packard 200LX pocket PC attached to a single channel high gain amplifier with a serial data interface. Recordings were made for 30 seconds at 15-minute intervals throughout the night and the data downloaded to a PC for analysis. Patients were asked to measure their blood glucose at 23:00, 03:00 and 07:00 hours. We have studied 20 non-diabetic subjects (8F/12M). with a mean age 32 years and 14 patients with diabetes (7M/7F), with a mean age 49 years. In nondiabetic subjects, the QTc remained stable throughout the night with a mean change in QTc of 2.8 (SD 22.1) ms. In the diabetic patients, the mean change in QTc during 13 normoglycaemic nights was 6.9 (SD 10.0)ms and during hypoglycaemic nights 6.5 (SD 8.9)ms. The QTc cumulative mean change and SD was calculated throughout the night. Episodes where there were two consecutive increases in QTc above mean +2SD within an hour were considered significant. Using these criteria, there was I significant event during the normoglycaemic nights but only 4 episodes were correctly identified out of 6 hypoglycaemic nights. Automated measurements of nocturnal QT interval can be used to monitor changes in cardiac repolarization associated with hypoglycaemia but more sophisticated analysis of T wave morphology or other factors will be required if such a system is to be used to reliably detect the onset of hypoglycaemia.