What Does QTc Mean?
QTc stands for the "corrected QT interval" - a heart rate-adjusted measurement of the time it takes for the ventricles of your heart to electrically reset after each heartbeat. 1, 2
Basic Definition
The QT interval measures ventricular repolarization time on an electrocardiogram (ECG), extending from the beginning of the QRS complex to the end of the T wave. 1, 2 Because the QT interval naturally lengthens when heart rate slows and shortens when heart rate increases, it must be "corrected" to a standardized heart rate of 60 beats per minute to allow meaningful comparison across different heart rates. 1
Why Correction Is Necessary
Without heart rate correction, the same absolute QT measurement could represent either normal or dangerous values depending on the patient's heart rate at the time of measurement. 1 For example, if a patient has an uncorrected QT interval of 0.44 seconds at a heart rate of 60 bpm and the same 0.44 seconds at 80 bpm, the QTc values would be 0.44 and 0.52 seconds respectively - the latter indicating significant prolongation and increased risk. 1
How QTc Is Calculated
The most commonly used formula in clinical practice is Bazett's formula: QTc = QT interval ÷ √(RR interval in seconds). 1, 2
Alternative formulas include:
- Fridericia's formula: QTc = QT ÷ ∛(RR interval), which performs better at higher heart rates (>80 bpm) where Bazett's tends to overcorrect. 1, 3
Despite its limitations, Bazett's formula remains the clinical standard and is what most automated ECG machines use. 1, 2
Normal Values and Clinical Significance
Normal QTc values are:
A QTc ≥500 ms in either sex significantly increases the risk of torsades de pointes, a potentially fatal ventricular arrhythmia. 1, 3 The vast majority of drug-induced torsades de pointes cases occur in patients with QTc ≥500 ms. 1
Clinical Applications
QTc monitoring is essential when:
- Initiating medications known to prolong the QT interval (antiarrhythmics, certain antibiotics like macrolides, antipsychotics, methadone). 1
- Evaluating patients with syncope or cardiac arrest risk. 1
- Assessing electrolyte disturbances (hypokalemia, hypomagnesemia, hypocalcemia). 3
Important Caveats
The QTc has never been validated as a predictor for torsades de pointes - it is an indirect risk marker, not a definitive predictor. 1 Notable exceptions exist, such as amiodarone, which markedly prolongs QTc but carries relatively low proarrhythmic risk. 1
Automated ECG measurements cannot be relied upon alone - visual confirmation of the computer-measured QT interval is essential before trusting the calculated QTc value. 1, 2
Measurement Technique
- Measure from the beginning of the QRS complex to the end of the T wave. 1, 2
- The end of the T wave is identified by drawing a tangent to the steepest downslope and marking where it intersects the baseline. 1, 2
- Do not include discrete U waves that occur after the T wave returns to baseline. 1, 2
- Use the same ECG lead for serial measurements in the same patient, typically lead II or V3/V5. 1, 2