QT calculates the time from the beginning of ventricular contraction to the end of relaxation(ventricular repolarization), we translate it on ECG from the beginning of Q wave to the end of T wave, it reflect some electrical action of the heart, QT prolongation may cause serious arrythmia and sudden cardiac death.
There is no clear-cut evidence about QTc prolongation by it self to increase the mortality from cardiovascular disease, and some study still going to see if it increases the mortality rate.
How we calculate corrected QT?
Sagie’s formula is the simplest method to get right reading:
QTc= 1000(QT/1000 + 0.154(1 – RR)
QTc and QT in milliseconds and RR in seconds. Most new Electrocardiogram machines calculate QTc automatically.
Normal and abnormal QTc:
Normal QTc value equal or less 420 ms and some reference till 440 ms.
Borderline QTc In male is 430-450 ms
In females 450-470 ms.
Abnormal QTc In male above 450 ms
In females above 470 ms.
For the heart rate of 60 bpm QTc= QT but QT estimation decline 20 ms for every 10-bpm elevation, so for heart rate of 70 bpm if QTc=420 ms then QT=400 ms and so on.
What is the cause of QT Prolongation?
There three main topics:
1 - Congenital
Causes or genetic (long QT syndrome) as Jervell and Lange-Nielson which is autosomal recessive syndrome with Sensorineural deafness and prolong QT.
2 – Drug adverse reaction
Many medicines may cause QT prolongation such as:
3 – Pathological factors
QTD (QT dispersion) = maximum QT minus minimum QT interval.
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