What is a QTc (QT interval corrected for heart rate)?

QTc: The Heart Rate Corrected QT Interval

The QTc (QT interval corrected for heart rate) is the measurement of the QT interval on an electrocardiogram that has been mathematically adjusted to account for variations in heart rate, as the QT interval naturally lengthens with slower heart rates and shortens with faster heart rates. 1

Definition and Measurement

• The QT interval is measured from the beginning of the QRS complex (earliest indication of ventricular depolarization) to the end of the T wave (latest indication of ventricular repolarization) 1, 2
• The end of the T wave can be difficult to determine and is often identified by drawing a tangent to the steepest downslope of the T wave and defining the intersection of this line with the baseline 1
• If the T wave is notched, the end of the T wave should be considered the end of the entire complex 1
• Discrete U waves that arise after the T wave has returned to baseline should not be included in the QT interval measurement 1
• The QT measurement should ideally be made in leads II, V5, and V6 with the longest value being used 1

Heart Rate Correction

• The most widely used formula for QT correction is Bazett's formula: QTc = QT interval divided by the square root of the RR interval (in seconds) 1
• Despite its limitations, Bazett's formula remains the standard for clinical use 1
• Bazett's formula tends to overcorrect at fast heart rates and undercorrect at slow heart rates 1
• More recent linear regression functions for QT adjustment are preferable to Bazett's formula but are less commonly used in clinical practice 1
• Rate correction of the QT interval should not be attempted when RR interval variability is large (as in atrial fibrillation) or when identification of the end of the T wave is unreliable 1

Normal Values and Clinical Significance

• In adults, a normal QTc is ≤0.46 second (460 ms) in women and ≤0.45 second (450 ms) in men 1
• A QTc of ≥0.50 second (500 ms) in either sex correlates with a higher risk for torsades de pointes, a potentially fatal ventricular arrhythmia 1, 3
• In neonates, the mean QTc on the 4th day of life is 400±20 ms with an upper normal limit of 440 ms 1
• A shortened QTc interval (<300 ms) may also be associated with malignant arrhythmias 1
• Gender differences in QTc are significant in young and middle-aged adults (women have longer QTc) but become minimal after age 40 1

Clinical Applications

• QTc monitoring is important when administering QT-prolonging medications, particularly Class III antiarrhythmic drugs 1, 4
• For patients with indications for QT monitoring, the QTc should be documented at baseline and then at least every 8-12 hours 1
• If QTc prolongation occurs during administration of a QT-prolonging drug, more frequent measurement may be needed 1
• In patients who develop QTc >500 ms, the causative drug should generally be discontinued and QTc monitoring continued until the QTc is documented to be decreasing 1, 3

Common Pitfalls in QTc Measurement

• Failing to use the same lead for serial QT measurements in the same patient (QT length varies across the 12 leads) 1
• Including U waves in the QT interval measurement when they are discrete (occurring after the T wave returns to baseline) 1
• Not accounting for QRS prolongation when measuring QT interval (in case of bundle branch block, either subtract the increased QRS length from the QT interval or measure the JT interval instead) 1
• Relying solely on automated measurements without visual validation (essential to visually validate QT-interval prolongation reported by a computer algorithm) 1
• Using Bazett's formula in situations where heart rate is particularly slow or fast, as the correction may be inaccurate 1