QT measurement for TU fused ECG morphology as exhibited during hypoglycaemia

During hypoglycaemia, there is typically a change in the ECG characterised by a flattened and prolonged T wave, often accompanied by a fused U wave. In the presence of such U waves, there is no widely accepted method of measuring the QT interval. This study compares two possible methods: using the intersection of the isoelectric line and (1) the T wave, and (2) a tangent to the T wave. The methods were tested on 306 sets of high resolution ECG data recorded during hyperinsulinaemic hypoglycaemic clamp studies. A value of measurement subjectivity was also recorded for each measurement. Results show that the tangent method is more consistent and reproducible than the non-tangent method. Further, the non-tangent method was shown to be significantly more subjective than using a tangent. The tangent method is less reliant on observer experience, is more easily defined and should be considered when ECG data sets have a large number of U waves.