Commentary—QT prolongation due to roxithromycin

This case and supporting discussion highlight the circumstances in which drug induced prolongation of the QT interval may occur and they serve as a reminder of the associated risk of the serious complication of polymorphic ventricular tachycardia—also known astorsade de pointes—which can lead to ventricular fibrillation and cardiac arrest.1–3 The arrhythmia is a non-sustained wide QRS complex (usually >160 ms in duration) tachycardia which tends to occur in repetitive bursts of 4–20 complexes at fast rates (generally 200–250/min) with characteristic variation in QRS amplitude and axis leading to the impression in certain leads of the electrocardiogram that the QRS complexes are twisting around the isoelectric baseline.1 3 4 In routine clinical practice, a simple correction is used for the rate dependency of the QT interval (Bazett’s formula of QTc = QT/R−R⁠, where QTc is the rate corrected QT interval in ms, QT is the measured QT interval in ms, and R-R is the R-R interval in seconds).4 The formula is simple, but imperfect in the sense that it tends to overestimate QT at fast heart rates and underestimate QT at slow heart rates.4 The upper limit of normal for QTc can be taken as 430 ms in men and 450 ms in women.4 QTc prolongation is easier to spot if computer interpreted 12 lead electrocardiograms are available, with their routine printout of QT and QTc intervals. QTc values >500 ms should prompt review of the patient’s drug therapy in the light of the catalogue of potential troublemakers presented here by Woywodt et al. Any clinical features which might indicate bursts of ventricular tachycardia (features such as dizziness, lightheadedness, syncope, or palpitation) or bursts of polymorphic ventricular tachycardia seen on continuous electrocardiographic monitoring demand immediate measurement of QT and calculation of QTc on the best electrocardiographic tracing possible. The true point of termination of the T wave can be quite difficult to identify, especially at faster heart rates,4 so in clinically critical situations it should be interpreted liberally.