QT specification and determination in mice

The duration of QRS-T complexes reflects the duration of ventricular depolarization and repolarization. Due to specific electrophysiological properties at cardiomyocytes level with predominant I_TO current, the murine action potential lacks plateau phase. Repolarization in mice starts when another cardiac territory is not yet activated^24,25,26. Combined with the short RR interval (≈100 ms), the ST segment and the T-waves are difficult to identify from ECG trace, leaving some authors to suggest that T wave in mice does not exist^24,35,36,37. Nevertheless the same concepts than in human ECG are actually used for the interpretation of theses rodent ECGs. Thus, several groups have tempted to empirically define the best estimation methods to define QT duration (as well reviewed in³⁸).

To date, the most used method to define the QT duration in mice is to consider the QT interval between the first deviation from an isoelectric PR interval until the return of the ventricular repolarisation to the isoelectric TP baseline, acquired by telemetry in vigil mice. This method included in the measure the low-amplitude portion of the T-wave and allows a complete ventricular repolarization of ventricles, as confirmed by a simultaneous record of monophasic ventricular action potential with ECG trace in mice³⁹. By this method the QT interval found in the literature ranges from 40 to 60 ms in unsedated mice with an RR interval comprised between 85 to 115 ms.