Extraction of ECGs from Holter recordings with subsequent QT interval measurement requires a robust computerized approach for identifying and flaging the steady-state repolarization periods. Rapid and slow QT interval changes follow heart rate increases or decreases; it may take several minutes for the repolarization process to reach a steady-state (see Figures 1 and 2).

Figure 1. Example of the heart rate change (RR) followed by QT interval adaptation. The QT graph marked in red is the period of unstable QT.

Figure 2. Two QT measurements performed at the same preceding RR: the measurement on the left (at 2:46:27) was performed during the unstable QT-RR period resulting in an incorrect QT value.

The extraction of ECGs from Holters and measurement of the QT interval in COMPAS: Thorough QT Plus tools relie on a filtering technique whereby the algorithm is identifying and flagging periods in Holter recordings when the repolarization is unstable. Unstable repolarization periods are identified as unsuitable for ECG extraction and, if necessary, manual adjustment of QT measurement.

In addition to heart rate stability, COMPAS provides a variety of metrics regarding the quality and reliability of every beat and its associated QT measurement. These metrics are important for identifying the periods of ECG recording most suitable for subsequent measurement of the QT interval because they help reduce the variability of QT measurement.

In a study comparing COMPAS automated technique with manual reading of the ECG data by a leading core laboratory, COMPAS based technique has been shown to reduce the variability of conventional measurement of QTc by up to 20%. (see Figure 3). This reduction in variability provides the ability to reduce the number of subjects required for a well-powered TQT study.

Figure 3. iCardiac’s advanced signal processing techniques reduce the standard deviation of QTc measurement to ~7msec. This reduction in variability significantly curtails the sample size required to power a TQT study in compliance with E14 guidelines.

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