Antipsychotics and QT Prolongation: Risk & Prescribing Guide

First-generation (typical) antipsychotics like chlorpromazine, haloperidol, and droperidol primarily treat positive schizophrenia symptoms by blocking dopamine and commonly cause extrapyramidal side effects (dystonia, Parkinson's symptoms, tardive dyskinesia, akathisia), neuroleptic malignant syndrome, agranulocytosis, and anticholinergic effects; second-generation (atypical) antipsychotics like risperidone, olanzapine, and clozapine treat both positive and negative symptoms by acting as serotonin-dopamine antagonists, but carry risks of metabolic side effects including diabetes, weight gain, and increased cholesterol, along with agranulocytosis risk requiring regular blood monitoring.

First-generation (typical) antipsychotics include chlorpromazine, levomepromazine, and haloperidol. Second-generation (atypical) antipsychotics include clozapine, risperidone, olanzapine, and quetiapine. The key distinction lies in their receptor selectivity: typical antipsychotics primarily block dopamine D2 receptors, while atypical antipsychotics have higher affinity for serotonin 5-HT2 receptors compared to dopamine D2 receptors.

Antipsychotic drugs are classified into typical (first-generation) and atypical (second-generation) categories; typical antipsychotics strongly block D2 dopamine receptors in the nigrostriatal pathway, causing extrapyramidal symptoms (dystonia, akathisia, parkinsonism, tardive dyskinesia) and hyperprolactinemia, while atypical antipsychotics have a weaker D2 receptor blockade but stronger 5-HT2A receptor antagonism, resulting in fewer extrapyramidal symptoms but more metabolic side effects like weight gain, dyslipidemia, and hyperglycemia; both types are used for schizophrenia and other psychotic disorders, with typical agents being more effective for positive symptoms and atypical agents addressing both positive and negative symptoms.

Antipsychotic drugs are classified into first-generation (typical) and second-generation (atypical) agents, with typical drugs primarily blocking dopamine D2 receptors and causing extrapyramidal symptoms (EPS) through nigrostriatal pathway disruption, while atypical drugs also block serotonin 5-HT2a receptors, reducing EPS but potentially causing metabolic side effects; EPS manifestations include acute dystonia (hours), akathisia (days), parkinsonism (days), and tardive dyskinesia (months-years), with neuroleptic malignant syndrome being a life-threatening emergency requiring immediate discontinuation and supportive care.

Antipsychotic drugs treat psychotic disorders like schizophrenia by modulating dopamine function through four major brain pathways (mesolimbic, mesocortical, nigrostriatal, and tuberoinfundibular), with first-generation typical antipsychotics primarily blocking D2 receptors non-selectively and causing extrapyramidal side effects, while second-generation atypical antipsychotics additionally block serotonin 2A receptors, resulting in fewer motor side effects but potentially causing metabolic complications.

The hERG (human ether-a-go-go related gene) ion channel is a cardiac potassium channel important for the repolarization of the cardiac membrane by conducting potassium ions out of cardiomyocytes. It has been important for cardiac safety evaluation because it promiscuously binds a wide variety of structurally different compounds, and its inhibition causes delay of ventricular repolarization, resulting in QT interval prolongation. Prolonged QT interval can lead to potentially fatal arrhythmias such as torsade de pointe (TdP).

The hERG potassium channel (KCNH2), also known as HERG, is a voltage-gated potassium channel that mediates the rapid delayed rectifier potassium current (IKr) essential for cardiac repolarization. Its unique biophysical properties—slow activation, fast inactivation, and slow deactivation—make it critical for shaping the final phase of the cardiac action potential. When hERG function is impaired by mutations or drug blockade, repolarization is delayed, causing QT interval prolongation and increasing the risk of dangerous arrhythmias like torsades de pointes. Due to its druggability and susceptibility to blockade by diverse compounds, hERG testing has become mandatory in drug development to prevent cardiotoxicity.

HERG (human ether-a-go-go-related gene) encodes the KV11.1 potassium ion channel protein, which mediates ventricular repolarization through the IKr current in the cardiac action potential. Inhibition of HERG channels can result in long QT syndrome, characterized by torsades de pointes (TdP), which can degenerate into ventricular fibrillation. The cardiac electrical activity sequence includes atrial depolarization (P wave), AV node delay, ventricular depolarization (QRS complex), atrial repolarization, and ventricular repolarization (T wave). HERG assays use HEK293 cells transfected to express HERG channels, measured via patch-clamp electrophysiology. Two approaches exist: manual patch clamp (single-cell recordings in Faraday cages) and automated patch clamp (multi-well plate systems). Both methods measure millivolt changes across dose-response curves, with terfenadine commonly used as a positive control.

The HER channel (human ether-a-go-go-related gene) encodes a potassium voltage-dependent channel responsible for the IKr repolarizing current in the heart. This channel plays a crucial role in cardiac physiology, particularly in the QT interval—the time between the Q and T waves on an electrocardiogram—which represents the duration of ventricular depolarization and repolarization.

The HERG potassium channel plays a critical role in cardiac repolarization. During the ventricular action potential, after the rapid depolarization phase, the slow repolarization phase follows. The HERG potassium current represents an outward current that helps return the heart muscle to its resting state. When this channel is mutated, the outward HERG current decreases, causing prolongation of the ventricular action potential and resulting in a prolonged QT interval on the surface ECG.

This comprehensive section covers the complete clinical picture of torsades de pointes. It explains that prolonged QTc approaching 500ms creates conditions for this dangerous polymorphic VT. Etiology includes electrolyte disturbances (hypokalemia, hypomagnesemia), medications (antiarrhythmics, antipsychotics), and genetic long QT syndromes affecting L-type calcium channels. The critical management principle is immediate IV magnesium sulfate administration as the primary treatment. The section emphasizes recognizing the distinctive twisting morphology on EKG and understanding that torsades de pointes represents a medical emergency requiring prompt intervention.

Torsades de Pointes (TdP) is a life-threatening polymorphic ventricular tachycardia characterized by twisting QRS complexes around the isoelectric line, initiated by premature ventricular contractions following long pauses. Key etiologies include medications (antiarrhythmics, antipsychotics, macrolides), electrolyte disturbances (hypocalcemia, hypomagnesemia, hypokalemia), and congenital Long QT syndrome. Critical distinction: TdP requires IV magnesium sulfate, while other polymorphic VTs are treated with antiarrhythmics (contraindicated in TdP). Emergency management includes defibrillation if unconscious, IV magnesium 2g, potassium correction, benzodiazepines, and isoproterenol infusion to increase heart rate.

Torsades de Pointes (TdP) is a polymorphic ventricular tachycardia characterized by twisting QRS waves on ECG, most commonly caused by QT prolongation from medications or electrolyte abnormalities. The condition is dangerous because prolonged QT extends the relative refractory period, creating a vulnerable window where premature ventricular contractions can trigger life-threatening arrhythmias. Treatment includes intravenous magnesium sulfate as first-line therapy, correction of electrolyte abnormalities (particularly potassium), temporary overdrive pacing to shorten QT interval, electrical cardioversion for prolonged episodes, and addressing the underlying cause by discontinuing offending medications.

Torsades de pointes is a polymorphic ventricular tachycardia dependent solely on triggered activity. It occurs when hypokalemia/hypomagnesemia prolongs the QT interval, combined with QT-prolonging medications (antiarrhythmics like amiodarone, antibiotics like macrolides, antipsychotics, SSRIs/TCAs, antiemetics like ondansetron). The prolonged QT causes early afterdepolarizations that trigger abnormal electrical activity, producing the characteristic twisting QRS pattern.