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8.8 About QT/QTc Interval Monitoring
The QT interval is defined as the time between the beginning of the Q-wave and the end of
the T-wave. It measures the total duration of the depolarization (QRS duration) and
repolarization (ST-T) phases of the ventricular action potential. QT interval monitoring can
assist in the detection of prolonged QT interval syndrome.
The QT interval has an inverse relationship to heart rate. Faster heart rates shorten the QT
interval and slower heart rates prolong the QT interval. Therefore, several formulas can be
used to correct the QT interval for heart rate. The heart rate corrected QT interval is
abbreviated as QTc.
QT/QTc interval monitoring is intended for adult, pediatric, and neonatal patients.
8.8.1 QT/QTc Monitoring Limitations
Some conditions may make it difficult to achieve reliable QT/QTc monitoring, for example:
R-wave amplitudes are too low
Too many ventricular beats
RR intervals are not stable
A high heart rate causes the P-wave to encroach on the end of the previous T-wave
The T-wave is very flat or T-wave are not well defined
The end of the T-wave is difficult to define because of the presence of U-waves
QTc measurements are not stable
In the presence of noise, asystole, ventricular fibrillation, and ECG lead off
For these cases you should select a lead with good T-wave amplitude and no visible flutter
activity, and without a predominant U-wave or P-wave.
Some conditions such as left or right bundle branch block or hypertrophy can lead to a
widened QRS complex. If a long QTc is observed you should verify it to ensure that it is not
caused by QRS widening.
Because normal beats followed by ventricular beats are not included in the analysis, no QT
measurement will be generated in the presence of a bigeminy rhythm.
If the heart rate is extremely high (over 150bpm for adults and over 180bpm for pediatrics
and neonates), QT will not be measured. When the heart rate changes, it can take several
minutes for the QT interval to stabilize. For reliable QTc calculation it is important to avoid
the region where the heart rate is changing.
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