What is the meaning of QTc (corrected QT interval) calculation?

From the Guidelines

QTc calculation is a critical process in assessing the risk of dangerous arrhythmias like Torsades de Pointes, and the most recent and highest quality study recommends using Bazett's formula, where QTc = QT/√(RR interval), to correct the QT interval for heart rate. The QT interval represents the time from the beginning of ventricular depolarization to the end of ventricular repolarization, and it naturally varies with heart rate, making correction necessary for meaningful comparison between measurements 1.

Importance of QTc Calculation

The QTc calculation is essential in clinical practice, particularly when prescribing medications that can prolong the QTc interval, such as certain antipsychotics, antibiotics, and antiarrhythmics. A prolonged QTc interval, generally >450ms in men and >460ms in women, indicates an increased risk of Torsades de Pointes, a potentially life-threatening arrhythmia 1.

Measurement and Correction

The QT interval should be measured from the beginning of the QRS complex to the end of the T wave, and the most common formula used for correction is Bazett's formula, although other formulas like Fridericia's and Framingham's may be more accurate at extreme heart rates 1.

Clinical Implications

Healthcare providers should routinely calculate QTc when prescribing medications that can prolong the QTc interval or when evaluating patients with syncope or palpitations. It is also crucial to monitor patients with a history of QT prolongation, relevant cardiac disease, or those treated with QT-prolonging drugs, and to consider treatment discontinuation or alternative regimens if the QTc is >500 ms or if QTc prolongation is >60 ms 1.

Key Points

• A 12-lead ECG should be recorded, and the QT interval, corrected for heart rate, should be obtained in all patients at baseline.
• Patients with a history of QT prolongation or relevant cardiac disease should be monitored by repeated 12-lead ECG.
• Consider treatment discontinuation or alternative regimens if the QTc is >500 ms or if QTc prolongation is >60 ms.
• Conditions known to provoke Torsades de Pointes, especially hypokalemia and extreme bradycardia, should be avoided in patients with drug-induced QT prolongation 1.

From the FDA Drug Label

The QT interval is used to determine patient eligibility for Sotalol AF treatment and for monitoring safety during treatment The baseline QT interval must be ≤450 msec in order for a patient to be started on Sotalol AF therapy. During initiation and titration, the QT interval should be monitored 2 to 4 hours after each dose. If the QT interval prolongs to 500 msec or greater, the dose must be reduced or the drug discontinued.

The QTc calculation is used to determine the patient's eligibility for Sotalol AF treatment and to monitor safety during treatment. The QTc interval is calculated using the patient's QT interval and heart rate. The baseline QT interval must be ≤450 msec for a patient to be started on Sotalol AF therapy. The QT interval is monitored 2 to 4 hours after each dose, and if it prolongs to 500 msec or greater, the dose must be reduced or the drug discontinued.

• The QTc calculation is crucial in determining the risk of Torsade de Pointes and other serious ventricular arrhythmias.
• The QTc interval is used to monitor the safety of Sotalol AF treatment.
• The calculation of the QTc interval is not explicitly described in the provided text, but it is implied that it is used to assess the patient's eligibility for treatment and to monitor safety during treatment 2, 2, 2.

From the Research

QTc Calculation Meaning

• The QTc interval is a measure of the efficiency of repolarization of the left ventricle, corrected for heart rate 3.
• It represents the time from the onset of ventricular depolarisation to completion of repolarisation on the electrocardiogram 4.
• A prolonged QTc interval indicates an increased risk of developing malignant ventricular tachyarrhythmias, particularly torsade de pointes 5, 4, 6, 7.
• The QTc interval is calculated using various algorithms, including Bazett's algorithm, which can produce an artificially prolonged QTc 3.
• The normal QTc interval values are:
  • 470-500 ms for males 5
  • 480-500 ms for females 5
• A QTc interval of ≥500 ms is considered excessive prolongation and can result in a potentially fatal ventricular arrhythmia 3.
• The QTc interval is influenced by heart rate, autonomic factors, electrolyte levels, gender, and age 4.

Clinical Implications

• Close monitoring of the QTc interval is necessary to prevent torsade de pointes, particularly in patients taking medications that can prolong the QT interval 5, 6.
• Patients at risk for QT interval prolongation should be educated to seek medical attention if they experience symptoms such as palpitations, lightheadedness, dizziness, or syncope 5.
• Measurement and documentation of the QTc interval is an important component of cardiac monitoring in hospital settings 6.
• Specific clinical interventions, such as dose reduction or discontinuation of the offending drug and correction of electrolytes, should be implemented to prevent the occurrence of torsade de pointes in patients with prolonged QTc interval 5, 6.