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Het QT interval geeft de ventriculaire depolarisatie en repolarisatie weer. Een verlengde QT tijd kan aanleiding geven tot Torsades de Pointes, een ventriculaire tachycardie. Het QTc interval is het QT interval gecorrigeerd voor de hartfrequentie (zie kader). Men spreekt van een QT-verlenging bij een QTc boven de 470ms. In de praktijk zien we dat Torsades de pointes vaak pas bij de QTc boven de 500ms optreedt.2 Alternatief kan er gebruik worden gemaakt van het QT Nomogram om te zien of er op basis van QT-tijd en hartfrequentie sprake is van verlengde QT-tijd (zie kader).3 Met name bij een lage hartfrequentie is deze accurater (ondercorrectie bij Bazet).
Oorzaken
Zeer veel medicijnen kunnen QT verlenging veroorzaken. Bijvoorbeeld:
Het risico op Torsades de Pointes in geval van QT verlenging verschilt per middel (bijv hoog risico: amiodaron, sotalol, chloorpromazine en erytromycine).1 Door nierfunctiestoornissen of interacties kan het QT-verlengend effect versterkt worden. Bij vrouwen, hypokaliëmie, hypomagnesiëmie, bradycardie, digitalis en patiënten met een congenitaal verlengd QTc syndroom kan eerder een Torsades de Pointes optreden. Voor een compleet overzicht en risicocategorie van QT-verlenging zie https://crediblemeds.org/. Let op: Bij intoxicaties kunnen middelen die normaal maar minimaal invloed op QT hebben wel significante QT verlenging geven.
QT verlenging op zich geeft geen symptomen. Het is een pseudomarker voor de kans op het krijgen van Torsades de Pointes (polymorfe ventriculaire aritmie). Torsades de Pointes uit zich als: recidiverende episodes palpitaties, duizeligheid, tachycardie (150-250/min) en syncope. Torsade de pointes kan overgaan in ventrikelfibrilleren en leiden tot acute hartdood.4 Tevens kan er sprake zijn van een ernstige bradycardie of hartblok. Bij een bradycardie is de kans op het ontstaan van Torsades de Pointes groter.
Risicofactoren Torsades de Pointes: bradycardie, elektrolytstoornissen (diureticagebruik, verminderde nierfunctie, diarree), onderliggende hartziekte (myocardinfarct, AF), digoxine-gebruik, aanwijzingen CVA of cerebraal trauma.
Lichamelijk onderzoek
ABCDE-opvang. Speciaal aandacht voor: temperatuur (hypothermie) en polsfrequentie (bradycardie).
Laboratorium onderzoek
Elektrolyten (met name kalium, magnesium, calcium), Creatinine.
Aanvullend onderzoek
ECG: QT-tijd. Aanwijzingen Torsade de Pointes, ectopieën of coupletten. Let op bradycardie.
Ritmestrook met Torsades de Pointes: snelle polymorfe tachycardie met draaien van QRS om de basislijn [Yates, 2012].
Beleid
Aanbevolgen benadering van geïntoxiceerde patiënt met een risico op QT verlenging en Torsades de Pointes
Aanbevolen behandeling van Torsades de Pointes
Referenties
1 - Derick M.G. et al. Medication-Induced QT-Interval Prolongation and Torsades de Pointes. US Pharm. 2011;36(2):HS-2-HS-8.
2 - Drew BJ, Ackerman MJ, Funk M, Gibler WB, Kligfield P, Menon V, 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. J Am Coll Cardiol 2010: 55: 934-47.
3 - Isbister GK, Page CB. Drug induced QT prolongation: the measurement and assessment of the QT interval in the clinical practice. Br. J. Pharmacol 2013; 76: 48-57.
4 - Kan AA, de Lange DW, Donker DW, Meulenbelt J. Management of prolonged QT and torsades de pointes in the intoxicated patient. Neth J Med 2014; 72: 119-26.
5 - Kenigsberg DN, Khanal S, Kowalski M, Krishan SC. Prolongation of the QTc interval is seen uniformly during early transmural ischemia. J Am Coll Cardiol 2007; 49: 1299-305.
6 - Priori SG, Blomstrom-Lundqvist C. 2015 ESC guidelines for the management of patients with ventricular arrythmias and the prevention of sudden cardiac death. Eur heart J 2015; 36: 2757-9
7 - Yates C, Manini AF. Utility of the Electrocardiogram in Drug Overdose and Poisoning: Theoretical Considerations and Clinical Impli-cations. Curr Cardiol Rev 2012; 8: 137-51.
Diagnosis of Torsades de Pointes Ventricular Tachycardia
Normal QTc values are about 0.40 second for men and 0.41 second for women and are considered prolonged when > 0.47 second for men or > 0.48 second for women. A family history may suggest a congenital syndrome.
Torsades de Pointes is a type of polymorphic ventricular tachycardia characterized on electrocardiogram by oscillatory changes in amplitude of the QRS complexes around the isoelectric line. Torsades de Pointes is associated with QTc prolongation, which is the heart rate adjusted lengthening of the QT interval.
Torsades de Pointes can lead to a heart rate anywhere between 150 to 300 beats a minute. People who have Long QT interval (seen on the electrocardiogram), an electrical problem with their heart, tend to get Torsades de Pointes. Genetic abnormalities or sometimes certain medicines can cause Long QT interval.
Predisposing factors known to increase the likelihood of developing torsade de pointes are: electrolyte imbalance (hypokalemia, hypomagnesemia, or both) and slow heart rate induced either by sinus bradycardia or heart block. Treatment of torsade de pointes is aimed at shortening the QT interval.
The normal range of the QT/QTc interval in adults varies between 350 ms and 470 ms or greater (for men) and 480 ms or greater (for women). Both short and long QT intervals can cause a variety of life-threatening ventricular arrhythmias.
Due to the variation of QT interval with heart rate (higher heart rate has shorter QT interval, lower heart rate has longer QT interval), it is important to correct the QT interval for the heart rate. This is known as QTc. QTc is prolonged if it is greater than 440 ms in men or greater than 460 ms in women.
Torsade de pointes, often referred to as torsade, is associated with a prolonged QT interval, which may be congenital or acquired. Torsade usually terminates spontaneously but frequently recurs and may degenerate into ventricular fibrillation, and therefore can be fatal if not diagnosed and managed.
The first thing a doctor will do after diagnosing torsades de pointes is to check the person's calcium, magnesium, and potassium levels. If these levels are low, healthcare professionals may recommend supplements. Magnesium can also be an effective treatment in people who already have typical magnesium levels.
If your QT interval is longer than 450 milliseconds, you may have long QT syndrome. Your doctor will ask you if you have a: Family history of long QT syndrome. Family history of unexplained fainting, seizures or cardiac arrest.
LQTS is usually a lifelong condition. The risk of having an abnormal heart rhythm that leads to fainting or cardiac arrest may lessen as you get older, especially in men after age 40. However, the risk never completely goes away. In boys who have LQTS, the QT interval often returns to normal after puberty.
Inherited long QT syndrome (LQTS) is an arrhythmogenic disorder predisposing to sudden cardiac death (SCD) secondary to polymorphic VT; mostly torsades de pointes. The mean age at presentation is 14 years of age, whilst the median age of individuals who die of LQTS is 32 years, with men predominantly affected.
Beta blockers used to treat long QT syndrome include nadolol (Corgard) and propranolol (Inderal LA, InnoPran XL). Mexiletine. Taking this heart rhythm medicine with a beta blocker might help shorten the QT interval. It can help lower the risk of fainting, seizure or sudden cardiac death.
Common causes for torsades de pointes include drug-induced QT prolongation and less often diarrhea, low serum magnesium, and low serum potassium or congenital long QT syndrome. It can be seen in malnourished individuals and chronic alcoholics, due to a deficiency in potassium and/or magnesium.
In the case of torsades de pointes (TdP), the heart's two lower chambers, called the ventricles, beat faster than and out of sync with the upper chambers, called the atria. An abnormal heart rhythm is called an arrhythmia.
The electrocardiographic QTc is approximately normally distributed in the general population. Normal values for the QTc range from 350 to 450 ms for adult men and from 360 to 460 ms for adult women; however, 10%-20% of otherwise healthy persons may have QTc values outside this range.
In subject area: Medicine and Dentistry. 23 QTc is the QT interval corrected for heart rate (as a faster heart rate shortens the QT interval) and a rate-corrected QT interval is necessary to accurately assess the true length of the QT interval.
A prolonged QT interval is an irregular heart rhythm. It is a change in how the heart's bottom chambers send signals. In a prolonged QT interval, it takes longer than usual for the heart to recharge between beats. A prolonged QT interval can be seen on a heart test called an electrocardiogram.
The most frequently used method for calculating the QTc is the Bazett formula, QTc = QT / √RR. Bazett published the first version of this formula in 1920 using ECGs from 39 young subjects and was subsequently updated by Taran and Szilagyi in 1947 (5,6).
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