QT prolongation -

Dubai Cardiology Clinic

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:

• Antipsychotics (haloperidol, sertindole), methadone for opiate addiction
• Antiarrhythmic class 3 (amiodarone and sotalol)
• Second generation antihistamines (astimizole…) ,
• Antidepressant SSRI (fluoxetine, paroxetine, sertraline) when it is taken with thiazide diuretics,
• Antibiotics: macrolide( azithromycin, clarithromycin, spiramycin), Fluoroquinolone( ciprofloxacin, levofloxacin, ofloxacin),
• Excess alcohol drinking ( high blood alcohol level).

3 – Pathological factors

• Hypothyroidism,
• Hyperkalemia,
• Hypocalcemia,
• Rheumatoid arthritis with high CRP which prolong QT and increase mortality,
• Type I diabetes about 50% of them have prolong QT interval, so in 10 tears the mortality rate 29% in diabetic with prolong QT where as it 19% with normal QT,
• Type II diabetes when QTD is more than 80 ms the cardiovascular risk increase,

 QTD (QT dispersion) = maximum QT minus minimum QT interval.

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