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4.11

Novacode 4: marked QT prolongation (QTc ≥ 500 ms) — risk of torsade de pointes, drug and electrolyte causes, congenital long QT.

Code 4.11 (Novacode NC-4) — marked prolongation of ventricular repolarisation (long QT). The QT interval, from the start of the QRS to the end of the T wave, shows how long the ventricles take to activate and then recover electrically. When it is corrected for heart rate (QTc) and is 500 ms or longer, prolongation is considered marked. At this level the risk of a dangerous ventricular arrhythmia, torsade de pointes, rises substantially.

Novacode 4QT intervalTorsade de pointes

Upper normal QTc< 450 ms men, < 460 ms women

Marked prolongationQTc ≥ 500 ms

Also high riskQTc rise > 60 ms from baseline

Main dangertorsade de pointes

How it is measured

The QT interval shortens as the heart rate increases, so it is corrected to a standard rate of 60 beats/min. Bazett's formula (QTc = QT/√RR) is the most widely used. It overestimates QTc at fast heart rates, so the Fridericia (QT/∛RR) or Framingham formulas are often preferred. Novacode classifies repolarisation with its own rate-adjusted QT index; this article uses the AHA/ACCF/HRS QTc thresholds, which are commonly applied in practice. For details of the formulas see the article on QTc.

Figure 1. At the same heart rate (60 beats/min): normal QT of about 400 ms (left) and markedly prolonged QT of about 540 ms with a broad, late T wave (right).

Causes

Group	Examples
Medicines	Some antiarrhythmics (sotalol, dofetilide, amiodarone), macrolide and fluoroquinolone antibiotics, antipsychotics, some antidepressants, methadone, ondansetron, antifungals
Electrolyte disorders	Low potassium, low magnesium, low calcium
Heart conditions	Bradycardia and AV block, myocardial ischaemia, cardiomyopathy, heart failure
Other acquired causes	Hypothyroidism, hypothermia, anorexia, stroke or intracranial haemorrhage, liver or kidney failure (reduced drug clearance)
Congenital long QT syndrome	Inherited ion-channel disease, about 1 in 2000 people; often a family history of fainting or sudden death

Clinical significance

Torsade de pointes is a polymorphic ventricular tachycardia that can cause palpitations, dizziness, fainting, seizures or sudden cardiac arrest. Risk increases with QTc ≥ 500 ms, female sex, older age, low potassium or magnesium, slow heart rate, heart disease and use of more than one QT-prolonging drug. Treatment depends on the cause: stopping or replacing the drug, correcting electrolytes, and in congenital forms, beta-blockers and sometimes an implantable defibrillator.

Seek urgent care

if a QTc of 500 ms or more is found together with fainting, seizures, palpitations or dizziness. Do not stop prescribed medicines on your own, but contact your doctor promptly so that the medication list and blood electrolytes can be checked.

References

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
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
Roden DM. Drug-induced prolongation of the QT interval. N Engl J Med. 2004;350(10):1013–1022. doi:10.1056/NEJMra032426
Schwartz PJ, Stramba-Badiale M, Crotti L, et al. Prevalence of the congenital long-QT syndrome. Circulation. 2009;120(18):1761–1767. doi:10.1161/CIRCULATIONAHA.109.863209

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 14 May 2019

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