P1 Patient Monitor User’s Manual
8-25
The J point position is given relative to the R-wave peak and helps locating
the ST point. Position the J point at the end of the QRS complex and the
beginning of the ST segment.
The ST point is located at a fixed distance relative to the J point, and the J
point is moved so that the ST point is located in the middle of the ST segment.
The ST point can be located at the positions of J+0, J+20, J+40, J+60, and
J+80.
8.8. QT/QTc Monitoring
QT interval is the time from the beginning of QRS complex to the end of T wave, that
is, the whole period of ventricular action potential depolarization (QRS interval) and
repolarization phase (ST-T). QT test can help you to judge long QT interval syndrome.
QT interval is negatively correlated with heart rate. As heart rate increases, the QT
interval shortens, while at lower heart rates QT interval gets longer. We can use several
formulas to correct QT interval according to heart rate. The QT interval corrected by heart
rate is called QTc.
QT/QTc monitoring is applicable for adults, pediatric and neonatal patients.
8.8.1. QT/QTc measurement limitation
The following conditions may affect the accuracy of QT measurement:
The amplitude of R wave is too low.
Excessive ventricular heartbeat.
RR interval is unstable.
High heart rate causes P wave to invade the end of the previous T wave.
T wave is too flat or t wave boundary is unclear.
The existence of U wave makes the end of T wave difficult to define.
QTc measurement is unstable.
In the presence of noise, asystole, ventricular fibrillation, and ECG lead off.
In the above situation, you need to select leads with good T wave amplitude, no
visible flutter, and no dominant U wave or P wave. In some cases, such as left and right
bundle branch block or cardiac hypertrophy, QRS complex may widen. If a long QTc is
observed, this should be confirmed to ensure that it is not caused by QRS broadening.
QT measurement cannot be performed in the presence of bigeminy rhythm because
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