![]() The two main classes of antibiotics that prolong the QTc are macrolides and quinolones. If the QT exceeds half the R-R interval, we would recommend using a correctional formula that is less dependent on heart rate (e.g. Thus, it may be appropriate to initially screen the QT interval for five beats with the half R-R rule, due to its ease of use and high sensitivity. 11 Therefore, if the QT is longer than half the R-R interval, further examination is necessary, as the QTc may still be normal. This rule has been shown to have 100% sensitivity when the heart rate is 60 beats per minute or above, but it is poorly specific. 16 A “half R-R interval” method is sometimes used where it is stated that if the QT (uncorrected) is less than half of the R-R interval, then the QTc will not be prolonged. A technique of averaging the QT for five consecutive beats in the ECG has been proposed, although this would be time-consuming for the clinician. This difficulty is due to the constant flux in heart rate. ![]() There remains no consensus strategy on the measurement of the QT interval in atrial fibrillation. Useful as a “screening” test, especially in AF but not as a true diagnostic test.If the QT is greater than half the RR interval, it may still be normal.If QT is less than half the RR interval, it is normal Well described in toxicology and overdose.Needs to be physically or digitally available at point of care.Designed for use with abnormal heart rates.In any patient, especially if heart rate is 70 bpm.Least affected by abnormal heart rate 56.Useful in bradycardic patients (HR 70 bpm.Tendency to overcorrect at high heart rates (i.e.More accurate than Bazett formula at abnormal heart rates.Best used when HR is between 50 and 70 bpm 55.Tendency to over-diagnose long QT as it overcorrects at high heart rates and undercorrects at low heart rates.Thus, risk stratification of patients with a prolonged QTc is a common challenge in hospital practice.Įchinocandins (anidulafungin, caspofungin, micafungin) Long QT syndrome may also be acquired in the setting of cardiomyopathy, myocardial ischaemia, severe intracranial injury, electrolyte abnormalities and several medications including, but not limited to, various antimicrobials, antipsychotics, antidepressants, antiarrhythmics and antiemetics (Table (Table1). Beta blocker therapy is the mainstay of treatment, and the use of implantable cardioverter/defibrillators is necessary in selected patients assessed to be at high risk of TDP. The management of CLQTS includes avoidance of classic arrhythmia triggers, particularly swimming or water sports. CLQTS includes sixteen identified genetic abnormalities that give rise to abnormal myocardial repolarisation. The QTc is prolonged in patients with congenital long QT syndrome (CLQTS). ![]() The corrected QT interval is known as QTc, and it is prolongation of the QTc, rather than the QT, that determines the risk of TDP. Thus, there is a requirement to “correct” the QT interval, particularly in hospital settings where the resting heart rates of patients may not be normal. During tachycardia, the QT interval decreases, and in bradycardia, it lengthens. 10Īdditionally, the normal limit for the QT interval varies with heart rate. The tangent technique for determining the end of the QT interval. We present an approach to the ECG in this context, and a step-by-step guide to manually measuring and correcting the QT interval, and an approach to management in common hospital-based clinical scenarios. A wealth of correctional formulae have been derived, but none has proven superior. Therefore, the QT interval often requires “correction” for the current heart rate, in order to correctly stratify the risk of torsades de pointes. ![]() This is a difficult task, as the normal QT interval is typically measured relative to the heart rate. Automated ECG printouts cannot be relied upon to diagnose QT interval prolongation thus, the onus is on the clinician to identify it. Detection of a prolonged QT interval is essential as this can be a reversible problem, particularly in the context of the use of a variety of commonly prescribed medications in the hospital setting. Abnormal prolongation of the QT interval can lead to torsades de pointes, a form of potentially life-threatening polymorphic ventricular tachycardia (VT). This interval represents the time taken for the ventricles to completely repolarise after activation. One of the more challenging aspects of ECG interpretation is measurement and interpretation of the QT interval. ![]()
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