Automatic QT measurements and dynamic QT behaviour on a 2 channel 24 hour recording: importance of lead selection and QT offset determination

Lead selection could influence the accuracy of automatic QT measurements from a 24 hour ECG recording. QT/RR slopes derived from QTapex and QTend may be different. Therefore, 20 consecutive two-channel (modified bipolar V1 and V5) Holter recordings were reviewed on "intention to analyse" basis to assess the influence of lead selection and QT offset measurement. Reliable measurements over 24 hour derived from mV1 could be obtained in 7/20 (35%) recordings. mV5 could be used in 14/20 (70%) recordings (difference=35%; 95% Cl: 6 to 64%). The root mean square value (RMS) of mV1 and mV5 was reliable for analysis in 18/20 (90%) recordings (difference with mV1=55%; 95% Cl: 30 to 80%; difference with mV5/spl ap/20%; 95% Cl: -4 to 44%). There was no difference in QT measurements or in slope measurements derived from mV1, mV5 and RMS. The slopes derived from QTapex and QTend measurements were not different in mV5 and the RMS measurements over 24 hour. In conclusion, lead selection significantly influences the ability for correct automatic QT measurements. However, between the 3 selected measurements there is no difference in QT interval or slope measurements over 24 hour if leads were reliable. There exists no major difference in slopes derived from QTapex and QTend over 24 hour.