QTc interval

Detailed article on the QTc interval in cardiology

QTc Interval

The QTc interval is a heart rate-corrected measure of the QT interval on an electrocardiogram (ECG). It represents the time taken for the ventricles of the heart to depolarize and repolarize, which is crucial for maintaining normal cardiac rhythm. The QTc interval is an important parameter in assessing the risk of arrhythmias, particularly Torsades de Pointes.

Physiology

The QT interval on an ECG reflects the total time for ventricular depolarization and repolarization. It begins at the start of the Q wave and ends at the end of the T wave. The duration of the QT interval can be influenced by the heart rate; therefore, the QTc interval is calculated to correct for heart rate variations.

Calculation

Several formulas exist to calculate the QTc interval, with the most common being the Bazett's formula:

\[ QTc = \frac{QT}{\sqrt{RR}} \]

where QT is the measured QT interval in seconds, and RR is the interval between two consecutive R waves (also in seconds). Other formulas include the Fridericia's formula, Framingham formula, and Hodges formula. Each has its own advantages and limitations, and the choice of formula may depend on clinical context.

Clinical Significance

A prolonged QTc interval is associated with an increased risk of ventricular arrhythmias, particularly Torsades de Pointes, which can lead to sudden cardiac death. Factors that can prolong the QTc interval include:

• Electrolyte imbalances (e.g., hypokalemia, hypomagnesemia)
• Medications (e.g., certain antiarrhythmics, antibiotics, and antipsychotics)
• Congenital long QT syndrome
• Myocardial ischemia or infarction

Conversely, a shortened QTc interval can also be clinically significant, though it is less common.

Monitoring and Management

Patients with a prolonged QTc interval should be monitored closely, especially if they are on medications known to affect the QT interval. Management strategies may include:

• Discontinuation or adjustment of QT-prolonging medications
• Correction of electrolyte imbalances
• Use of beta-blockers in congenital long QT syndrome
• Implantation of a cardioverter-defibrillator in high-risk patients

Also see

• Electrocardiogram
• Torsades de Pointes
• Long QT syndrome
• Cardiac arrhythmia
• Sudden cardiac death

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