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    QTi
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    QTi

    QT interval: heart-rate correction, normal and prolonged QTc, causes of long and short QT and torsade risk.

    The QT interval is measured from the start of the QRS complex to the end of the T wave. It covers the whole electrical cycle of the ventricles: depolarisation and repolarisation. The QT shortens as heart rate rises, so it is corrected for heart rate (QTc). An abnormally long or short QT increases the risk of dangerous ventricular arrhythmias, especially torsade de pointes, and of sudden cardiac death.

    ECGRepolarisationArrhythmia risk
    Prolonged QTc≥ 450 ms men, ≥ 460 ms women
    Short QTc≤ 390 ms
    High torsade riskQTc > 500 ms
    LQTS diagnosis (ESC 2022)QTc ≥ 480 ms on repeated ECGs
    One ECG beat with the QT interval highlighted from the start of the QRS complex to the end of the T wave J point P PR seg. QRS ST seg. T PR interval ST interval QT interval
    Figure 1. The QT interval (highlighted) includes the QRS complex, the ST segment and the T wave.

    Correction for heart rate

    Bazett: QTc = QT / √RR     Fridericia: QTc = QT / ∛RR    (RR in seconds)

    Bazett's formula is the most widely used but overcorrects at fast heart rates and undercorrects at slow ones. Fridericia's and Framingham's formulas are more accurate for monitoring QT during drug treatment.

    Heart-rate corrected QT interval scale: up to 390 ms short, normal up to about 450 to 460 ms, prolonged up to 500 ms, above 500 ms high risk of torsade de pointes Corrected QT interval, QTc (ms) Short ≤ 390 390 Normal men below 450, women below 460 450/460 Prolonged up to 500 500 High risk above 500
    Figure 2. QTc zones in adults. The upper limit of normal is 450 ms for men and 460 ms for women (AHA/ACCF/HRS 2009).

    Causes of a long QT

    • Medicines: antiarrhythmics (sotalol, amiodarone, quinidine), macrolide and fluoroquinolone antibiotics, antipsychotics, some antidepressants, methadone, ondansetron, antifungals
    • Electrolytes: low potassium, magnesium or calcium
    • Congenital long QT syndrome (ion channel gene variants)
    • Bradycardia, myocardial ischaemia, heart failure, hypothyroidism, hypothermia, intracranial bleeding, starvation

    A short QT can be caused by hypercalcaemia, digoxin, hyperkalaemia or acidosis, and rarely by congenital short QT syndrome.

    Normal QT interval compared with a prolonged QT interval with a delayed T wave Normal QT QT 380 ms at 60/min QT 380 ms Long QT QT 540 ms at 60/min QT 540 ms
    Figure 3. Left: normal QT. Right: prolonged QT, in which the T wave ends much later.
    Seek emergency care

    for fainting, seizures or palpitations in a person with a long QT, especially after starting a new medicine, with vomiting or diarrhoea (potassium loss) or during exercise or emotional stress.

    References

    1. Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250. doi:10.1161/CIRCULATIONAHA.108.191096
    2. Zeppenfeld K, Tfelt-Hansen J, de Riva M, et al. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. Eur Heart J. 2022;43(40):3997–4126. doi:10.1093/eurheartj/ehac262
    3. Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047–1060. doi:10.1161/CIRCULATIONAHA.109.192704
    4. Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc. 2016;5(6):e003264. doi:10.1161/JAHA.116.003264

    This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

    Published on 7 July 2019