Qt interval

QT Interval

The QT interval is a measure of time in the heart's electrical cycle. It represents the time taken for the ventricles to depolarize and repolarize, or in simpler terms, to contract and relax. The QT interval is measured from the start of the Q wave to the end of the T wave on an electrocardiogram (ECG).

Measurement

The QT interval is measured in milliseconds (ms) and can be affected by heart rate, age, and sex. It is usually corrected (QTc) for heart rate as the QT interval tends to shorten at faster heart rates. The most commonly used correction formula is the Bazett's formula, although it tends to overcorrect at high heart rates and undercorrect at low heart rates.

Clinical significance

The length of the QT interval is of clinical importance. A prolonged QT interval, known as Long QT syndrome (LQTS), can lead to life-threatening arrhythmias, including Torsades de Pointes. LQTS can be congenital, caused by mutations in the genes that regulate heart's electrical activity, or acquired, often due to certain medications, electrolyte imbalances or heart disease.

A shortened QT interval is less common and its clinical significance is less clear. It may be associated with Hypercalcemia or Digitalis toxicity.

See also

• Electrocardiography
• Cardiac action potential
• T wave
• R-R interval