![]() These medications bind to the human ether-related gene (hERG) channels and reduce electrical conduction through the potassium ion channels. However, the degree of QT prolongation is not severe enough to warrant caution in healthy patients. ![]() Many commonly used medications, such as diphenhydramine and azithromycin, exhibit QT-prolonging effects. Caution is advised when combining QT-prolonging medications or when using these medications in those with electrolyte abnormalities. This is specifically medication dependent. They are preferably classified based on the degree of QT prolongation they induce. Others: Methadone, sumatriptan, ondansetron, cisaprideĪ vast number of medications prolong the QT interval. Pharmacological agents are the most common cause of QT prolongation given the broad range of medications that may induce it. As well, torsade, which is drug-induced, is reversible by the discontinuation of the offending drug.Īntipsychotics: Haloperidol, ziprasidone, quetiapine, thioridazine, olanzapine, risperidoneĪntiarrhythmics: Amiodarone, sotalol, dofetilide, procainamide, quinidine, flecainideĪntibiotics: Macrolides, fluoroquinolonesĪntidepressants: Amitriptyline, imipramine, citalopram, amitriptyline However, subsets of cardiac patients may have an increased risk of mortality if they suffer from QT prolongation. No major evidence in the general population indicates changes in mortality are associated with QT prolongation. This is also true in those with heart disease. Literature has shown that people who have diabetes and those suffering from certain inflammatory diseases may suffer from mildly prolonged QT. Isoproterenol can also be used to increase the heart rate, thereby decreasing the absolute QT interval. It is generally accepted that QT prolongation past 500 ms carries an increased risk of torsades de pointes however, if the QT prolongation is severely prolonged, ventricular fibrillation is a certainty. The most common symptom is syncope. Patients with torsade are administered a 2-4g bolus of magnesium sulfate and must undergo cardioversion if hemodynamically unstable. This rhythm may cease on its own and go into sinus rhythm, or it may degenerate into ventricular fibrillation. It is the most common form of polymorphic ventricular tachycardia, an unstable cardiac rhythm. Those with prolonged QT are at risk for one of the potentially deadly arrhythmias known as torsades de pointes. Ī normal QTc in a male is 440 ms or less, and in a female, it is 460 ms or less. Its accuracy is largely limited to heart rates of 60 to 100 beats per minute, and clinicians must factor in the heart rate when assessing the QTc. Bazett's is automatically calculated on most ECG machines, and its limitations are underestimation and overestimation of the QTc in the cases of bradycardia and tachycardia, respectively. Framingham is a more complex formula, but the literature has shown it may be the most superior formula. Fridericia is a similar formula, except it uses the cube root of the R-R interval. ![]() īazett is the most commonly used formula and is done by dividing the QT interval by the square root of the R-R interval. There are different formulas used to obtain the QTc interval. This allows for an accurate QT interval at lower and higher heart rates. When measured on an ECG, the QT interval lengthens when the heart is beating slower and shortens when the heart is beating faster. That is why an adjusted version of the QT interval is used: QTc. It is measured from the Q wave until the T wave, and the QT interval clinically represents the repolarization of the ventricles. The focus of this article is on the QT interval. Lastly, the T wave shows the repolarization of the ventricles. The QRS complex shows the depolarization of the ventricles. The P wave represents the electrical activity of the atrium. This electrical activity is clearly outlined on an electrocardiogram (ECG) with P waves, the QRS complex, and T waves. The electrical activity is conducted through the sinoatrial (SA) node and atrioventricular (AV) node and into the ventricles. A key aspect of this electrical system is depolarization and repolarization. ![]() The heart has an electrical system that allows it to contract in a rhythm.
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