ECG Monitoring
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8.8 QT/QTc Monitoring
QT interval is the time from the start of QRS complex to the end of the T wave, and it represents the total time
taken for ventricular action potential depolarization (QRS interval) and repolarization (ST-T). QT monitoring is
helpful to identify long QT syndrome (LQTS).
QT interval has a negative correlation to heart rate. The QT interval is shorter when the heart rate is faster, and vice
versa. Therefore, several formulas are engaged in the algorithm to correct QT interval based on the heart rate. The
QT interval corrected on the basis of heart rate is called QTc.
QT/QTc monitoring is intended for adult, pediatric and neonate patients.
8.8.1 Restrictions on QT/QTc Measurement
The following situations may affect the accuracy of QT measurements:
Low amplitude of R wave.
Ventricular tachycardia.
Unstable RR interval.
Occupation of the end of the previous T wave by P wave due to high heart rate.
Flat T wave or unclear boundary of T wave.
Difficulty in definition of the end of T wave due to the existence of U wave.
Unstable QTc measurement.
Noise, asystole, ventricular fibrillation or ECG lead off.
In the above cases you need to choose a lead with good T wave amplitude, no visible wobble, and no dominant U
wave or P wave.
In some cases, such as left / right bundle branch block and cardiomegaly, the QRS complex may be broadened. If a
long QTc is observed, check and make sure it is not caused by QRS broadening.
QT measurement cannot be performed when there is bigeminy, because normal heart beat accompanying with
ventricular beat is excluded in the analysis.
QT measurement cannot be performed when the heart rate is extremely high (over 150bpm in adults, over 180bpm
in neonate and pediatric patients). When the heart beat changes, the QT interval becomes stable after several
minutes. Thus, in order to obtain reliable QTc calculation results, it is vital to avoid heart rate changes.
8.8.2 Enable QT/QTc Monitoring
QT/QTc monitoring is set to [OFF] by default, so you should first enable this function.
Enable QT/QTc monitoring in the following steps:
1) Select the ECG parameter area or waveform area to enter [ECG] menu.
2) Select [QT] tab → [Setup] tab.
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