The QT interval is a measurement that represents the total time from ventricular depolarization to complete repolarization. This process begins at the start of the Q wave and extends to the end of the T wave. The QT interval is affected by many factors. Heart rate, autonomic nervous tone, sympathomimetics, electrolytes, especially calcium and potassium, some drugs, age, sex of the patient and conditions such as sleep apnea and diabetes affect this duration. The QT interval measurement can help to recognize patients who are at risk of life-threatening arrhythmias before they happen. Because we have the need to create a number that is normal, a mathematical formula, known as Bazett's formula, was devised to correct for the heart rate of the patient. The "c" in QTc stands for corrected. If you plug in a few variables into a QTc formula, you will get a number result. You then compare this number with the two upper limits of normal values (male 0.44, female 0.45). This method allows the computer to quickly calculate the QTc for each cardiac cycle on the Holter recording.
QT or QTc intervals that are long indicate abnormal effects on the myocardium. QT prolongation is usually divided into two categories, congenital and acquired. Congenital QT prolongation is an inherited disorder that places individuals at increased risk of sudden death, usually as children or young adults. Acquired QT prolongation may be due to ischemia, electrolyte imbalance especially hypokalemia or hypomagnesemia, CVA, medications like Quinidine, Amiodarone, Sotolol and high doses of tricyclic antidepressants and Propulsid. Most acquired forms result from the side effects of medications. Extremely prolonged QT/QTc measurements can increase the patient's chances of sudden cardiac death (SCD). The arrhythmia that is associated with prolonged QT interval is Torsade de Pointes.
Torsade de Pointes |
