Corrected QT interval

Corrected QT Interval (QTc) is a measure used in electrocardiography to assess the time the heart's electrical system takes to recharge between beats. This measurement is corrected for heart rate, providing a standardized value that can be compared across different heart rates. The QT interval itself represents the time from the start of the Q wave to the end of the T wave in the heart's electrical cycle, corresponding to the period from the start of ventricular depolarization to the end of ventricular repolarization.

Calculation

Several formulas exist for correcting the QT interval for heart rate, with the most commonly used being the Bazett's formula, which divides the QT interval by the square root of the half of the RR interval (in seconds). Other formulas include the Fridericia, Framingham, and Hodges formulas, each with its own method of correction for heart rate.

Clinical Significance

The QTc is crucial in diagnosing and managing various cardiac conditions, including Long QT Syndrome (LQTS), a disorder that predisposes individuals to sudden cardiac arrest due to arrhythmias. A prolonged QTc can indicate an increased risk of these arrhythmias, while a shortened QTc can be associated with certain electrolyte imbalances or medications.

Normal Values

The normal range for the QTc varies slightly between males and females. Generally, a QTc of 350 to 440 milliseconds is considered normal for males, while for females, the normal range is 350 to 460 milliseconds. Values above these ranges are considered prolonged and may warrant further investigation.

Risk Factors for Abnormal QTc

Several factors can influence the QTc interval, including:

• Electrolyte imbalances, such as low blood potassium or magnesium
• Medications that affect heart rhythm, including certain antidepressants, antipsychotics, and antibiotics
• Genetic conditions, such as congenital Long QT Syndrome
• Myocardial infarction or other heart diseases

Treatment and Management

Management of an abnormal QTc involves identifying and addressing the underlying cause. This may include changing medications that prolong the QTc, correcting electrolyte imbalances, or implementing lifestyle changes. In cases of congenital LQTS, treatment may involve beta-blockers or an implantable cardioverter-defibrillator (ICD) to prevent sudden cardiac death.

See Also

• Electrocardiography
• Long QT Syndrome
• Cardiac Arrhythmia