Measure QT and RR intervals
Correct for Heart Rate
Correction Formulas:
- Bazzett: QTc = QT*(HR/60)1/2
- Fridericia: QTc = QT*(HR/60)1/3
Obvious Long QT: QT>1/2 RR
Check QTc versus Normal Limits
Risk Factors
- ECG from LQT2 patient: "Current evidence suggests that 5 to 10 percent of persons in whom torsade de pointes develops on exposure to QT-interval-prolonging drugs harbor mutations associated with the long-QT syndrome." (Roden, NEJM, 2004)
- Example of Hypokalemia: Imbalance in K, Ca, or Mg can lead to higher risks. In this example, lead II exhibits TP fusion while in V4-V6 these waves are clearly separate
- Brady-Arrhythmia: Atrial fibrillation with low heart rate and prolonged QT. Long, followed by short RR, leads to Torsades de Pointes.
- Poor Liver or Heart Function:
- Liver: Poor function can lead to toxic levels of prescribed drug
- Heart: Cardiac ischemia can prolong QT. Low ejection fraction can increase risk of arrhythmic death due to prolonged QT.
- Drugs:
- Anti-depressants
- Anti-psychotics
- Anti-biotics
- Anti-arrhythmics
- Etc.
Tools and strategies for measuring difficult ECGs
Artifact
- Raw Rhythm Tracing: The artifact in tracing A is too excessive. The T-wave is not even visible. Advise repeat ECG. Tracing B exhibits a T-wave but the measurement of QT could be inconsistent due to baseline sway and muscle artifact. Select the best complex for measurement
- Median Complexes: A median complex is formed by taking the median value, at each sample time, from the superimposed raw beats. The results is a cleaner, representative complex for measurement.
Flat T-wave in Limb Leads - Use All Leads
Measurements based on a single lead will result in error, especially if the lead has a flat T-wave. Check all leads. Use longest interval- GE´s Marquette 12SL program superimposes median complex from each lead. The automated QT measurement is from earliest QRS onset to latest T-offset.
Bi-phasic T-wave - Include Terminal Portion
Do not define baseline on the end of the T-wave. This can lead to the mistake of defining T offset withing T´. QRS onset should define baseline. See V2. The entire wave of repolarization includes T and T´. Use all leads to assist in defining the end of repolarization.
Calculating QTc in the Presence of Varying Heart Rate
QT is varying with heart rate. An overall QTC needs to be determined. Some recommend on average obtained from the shortest and longest precedding RR intervals. Others perform the average over 3 to 5 consecutive beats. Instead, GE´s Marquette 12SL program uses QT from the median beat and the average RR.
