QT Interval Prolongation On ECG
•Normal QTc interval is less than 440 ms.
• QTc interval greater than 440 ms is considered prolonged.
• QTc interval greater than 550 ms is markedly prolonged.
1. The QT interval is measured form the beginning of the QRS complex to the termination of the T wave (or U wave if present) and should be measured in the EKG lead with the longest-appearing QT interval that has a distinct T wave with a clear termination point.
2. The QT interval will increase with bradycardia and decrease with tachycardia, thus it is important to use the corrected QT interval (QTc = QT/μR-R interval) for heart rates other than 60 (in which the QTc = QT).
3. Prolonged QT intervals may be congenital. The vast majority are acquired, usually due to medications or electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia, and hyperphosphatemia).
4. Numerous medications may prolong the QT interval in therapeutic or toxic doses.
5. Prolongation of the QT interval predisposes to torsades de pointes.
6. Look carefully for prolonged QT intervals in patients who present with syncope.