Drug-induced prolongation of the QT interval arises from disruptions to the coordinated ionic currents that repolarize ventricular myocytes. Daniel M. Roden Vanderbilt University Medical Center explains that the prototypical direct mechanism is blockade of the hERG-encoded rapid delayed rectifier potassium current known as IKr. When IKr is inhibited, ventricular action potentials lengthen, increasing the electrocardiographic QT interval and creating a substrate for abnormal impulse formation.
Ionic and cellular mechanisms
At the cellular level, prolonged repolarization increases the likelihood of early afterdepolarizations, abnormal depolarizing events that can trigger arrhythmias. Daniel M. Roden Vanderbilt University Medical Center and Shaul Viskin Tel Aviv University describe how reduced outward potassium flow from hERG blockade is the most common pathway, but other ionic changes also matter. Enhancement of the late sodium current or increased L-type calcium current can similarly extend the action potential. These ionic imbalances alter the dispersion of repolarization across the ventricular wall, promoting reentrant circuits and the polymorphic ventricular tachycardia known as torsades de pointes.
Indirect and contextual contributors
Beyond direct ion channel effects, many drugs prolong QT through pharmacokinetic interactions that raise plasma concentrations of QT-prolonging agents. Inhibition of cytochrome P450 enzymes by a second drug can transform a safe dose into a hazardous one. Genetic predisposition from variants in genes such as KCNH2 KCNQ1 and SCN5A lowers the repolarization reserve, so otherwise tolerated medications produce large QT changes. Clinical conditions and exposures including hypokalemia hypomagnesemia bradycardia and structural heart disease further reduce repolarization reserve. Shaul Viskin Tel Aviv University emphasizes that women are at higher risk for drug-induced QT prolongation, a sex difference that has implications for prescribing and monitoring.
Consequences extend from transient syncope to sudden cardiac death, so clinical practice prioritizes risk recognition and mitigation. Electrocardiographic surveillance dose adjustment avoidance of interacting drugs and correction of electrolytes are key strategies. In many regions limited access to ECG monitoring and high rates of polypharmacy raise practical challenges, and cultural practices that include herbal remedies can contribute to unrecognized interactions. Understanding the multifactorial mechanisms—molecular channel blockade, altered drug metabolism, genetic susceptibility, and clinical context—guides safer use of medications that affect the cardiac repolarization process.