About Qt/Qtc Interval Monitoring - Philips IntelliVue X2 Instructions For Use Manual

About QT/QTc Interval Monitoring

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 there are several formulas used to correct the QT
interval for heart rate. The heart rate corrected QT interval is abbreviated as QTc. The monitor uses as
a default the Bazett correction formula and the alternative Fridericia formula can be selected in
Configuration Mode.
For QT interval monitoring to be effective, basic or enhanced arrhythmia monitoring should be turned
on.
QT Measurement Algorithm
The QT values are updated every five minutes except in the initial phase (first five minutes) where they
are updated once per minute. Normal or atrial paced beats and beats with a similar morphology are
averaged to form a representative waveform for further processing. Normal beats followed by a
premature QRS will be excluded from the measurements to prevent the premature beat from obscuring
the end of the T-wave. If the algorithm cannot form a representative waveform, for example because
the morphology of the beats is too varied, a CANNOT ANALYZE QT INOP will be generated after 10
minutes. This is also the case if normal beats have been falsely labelled so that the algorithm does not
have enough valid beats to make QT measurements. No QT value is calculated if the QT-HR is
>150 bpm (Adult) or >180 bpm (Pedi/Neo).
Because of the different algorithm approaches, a QT/QTc measurement from a diagnostic 12-lead
program may differ from the realtime measurement on the monitor.
Where Can I Find More Information?
See the Application Note on QT/QTc Interval Monitoring and the QT Interval Monitoring Quick
Guide supplied on your documentation DVD for detailed information on the QT algorithm and
performance.
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