The Q-T interval represents the time of ventricular activity including both depolarization and repolarization. It is measured from the beginning of the QRS complex to the end of the T wave. Normally, the QT interval is 0.36 to 0.44 seconds (9-11 boxes). The QT interval will vary with patient gender, age, and heart rate. Another guideline is that normal QT Intervals is usually less than half of the R-R Interval for heart rates below 100 bpm.
Long QT syndrome (LQTS) describes a group of disorders characterized by QT interval prolongation secondary to delayed cardiac repolarization that may either be genetic (otherwise known as ‘congenital’) or acquired. The substrate of abnormal cardiac repolarization predisposes to the development of torsades de pointes (TdP), the characteristic form of polymorphic ventricular tachycardia, and cardiac arrest.
This post will focus on the common causes of acquired Long QT syndrome. Acquired LQTS has traditionally been associated with electrolyte abnormalities and exposure to QT-prolonging drugs, with antiarrhythmic medications being the most common precipitant.
- Drugs: amiodarone, TCA’s, many antibiotics, fluconazole, erythromycin, metoclopramide, quinidine, haloperidol, droperidol, methadone, ondansetron, SSRI’s
- Electrolytes: low Ca2+, low K+, low Mg2+
The concern about a prolonged QT-syndrome is the precipitation of Torsades de Pointes (TdP). TdP by definition is: (1) a polymorphic ventricular tachycardia that occurs specifically under conditions of QT prolongation; and (2) it is almost always initiated by R-on-T ectopic beats. Clinical manifestations of TdP include syncope (fainting), seizure (epilepsy), or sudden cardiac death.
First of all, all LQTS patients should avoid any of the QT prolonging drugs or those such as catecholaminergic drugs that can facilitate the development of TdP. There are hundreds of cardiac and noncardiac drugs that can aggravate QT prolongation or trigger TdP. These drugs include (1) antiarrhythmic drugs such as dofetilide and sotalol; (2) antibiotics such as erythromycin and levofloxacin; (3) psychotropics like haloperidol; and (4) narcotics such as methadone.
Here’s a helpful mnemonic to help remember the common classes of medications that may prolong the Q-T interval:
T – Thiazide diuretics
O – Ondansetron (antiemetics), Opioids
R – Risperidone (antipsychotics)
S – Sotalol (antiarrhythmic)
A – Antihistamines, Antibiotics (Quinolones)
D – anti Depressants (TCAs)
E – Erythromycin (Macrolides)
S – SSRIs
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