- Start of Q-wave to end of the T-wave (time of ventricular depolarization + repolarization).
- Life threatening risk of prolonged QTc >500ms = Torsades de pointes (TdP).
- Prolonged QT/QTc interval may be a clue to electrolyte disturbances (hypocalcemia or hypokalemia), drug effects (quinidine, procainamide, amiodarone, or sotalol), or myocardial ischemia (usually with prominent T wave inversions).
- Shortened QT intervals are seen with hypercalcemia and digitalis effect.
- Each 10-millisecond increase in QTc contributes approx a 5% to 7% additional increase in risk for TdP.
- QTc of 540 milliseconds has a 63% to 97% higher risk of developing TdP than a patient with QTc of 440 milliseconds.
How do you measure it:
- Find a lead with the tallest T wave and count the little boxes from the start of the QRS complex to the point where the T wave comes back down to the isoelectric line.
- Multiply the number of little boxes by 0.04 seconds.
- Example if you counted 8 boxes then QT interval is 8 x 0.04 = 0.32 seconds (320 milliseconds).
- QT interval should be less than half the preceding R-R interval (Works for regular rates between 65-90).
Methods of calculating the QTc:
- Bazett formula, QTc = QT / √RR.
- Fridericia formula (QTc = QT / RR1/3)
- Hodges [QTc = QT + 0.00175 x (HR - 60)]
- Framingham linear regression analysis {QTc = QT + 0.154 x (1 - RR)}
- Karjalainen et al. [QT nomogram]
- Rautaharju formula, QTc = QT x (120 + HR) / 180
