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Monitoring ECG, Arrhythmia, ST and QT QT/QTc Interval Monitoring

TM80 Telemetry Monitor Operator’s Manual 7 - 33

7.9 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 ventricular depolarization (QRS duration)

and repolarization (ST-T). QT interval monitoring can assist in the detection of long QT

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 and pediatric patients.

When the TM80 is connected to the CMS, you can set the QTc formula and enter the QT

view at the CMS. For more information, refer to "QT Monitoring" on page 13 - 4.

7.9.1 QT/QTc Monitoring Limitations

Some conditions may make it difficult to achieve reliable QT/QTc monitoring, for

example:

■ R-wave amplitudes are too low

■ The presence of frequent ventricular ectopic beats

■ Unstable RR intervals

■ P-waves tending to encroach on the end of the previous T-wave at high heart rates

■ The T-wave is very flat or T-wave are not well defined

■ The end of the T-wave is difficult to delineate because of the presence of U-waves

■ QTc measurements are not stable

■ In the presence of noise, asystole, ventricular fibrillation, atrial 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 measurements when the heart rate is changing.

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