User's Manual

5 ECG, Arrhythmia, ST and QT Monitoring
150
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.
Indications For Use Of QT Interval Monitoring
Of special concern for QT monitoring is the administration of QT prolonging drugs to patients
identified with risk factors for Torsade de Pointe. Females, older patients and patients with
bradycardia, impaired left ventricular function (ischemia, left ventricular hypertrophy), hypokalemia
and hypomagnesemia are in this increased risk category.
Limitations For Use Of QT Interval Monitoring
Some conditions may make it difficult to achieve reliable QT monitoring, for example:
• the T-wave is very flat
• T-waves are not well defined due to atrial flutter or atrial fibrillation
• the end of the T-wave is difficult to define because of the presence of U-waves
• a high heart rate causes the P-wave to encroach on the end of the previous T-wave
• noise or high QRS morphology variation
For these cases you should select a lead with a 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 150 bpm for adults and over 180 bpm 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 a region where the heart rate is changing.
WARNING
QT/QTc measurements should always be verified by a qualified clinician.
Selecting The QT Leads
For QT Monitoring you can select one of the following three modes:
• All Leads mode - all available leads (I, II, III, V, MCL, V1 - V6) are used to produce a global QT
measurement. For EASI lead placement, directly acquired AI, AS and ES leads are used.
• Primary-Lead mode - the primary lead will be used for QT measurement. If the original primary
lead becomes unavailable or is changed, QT measurement will continue with the new primary lead.