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REBEL Cast REBEL Core Cast 145.0: Understanding QTc Prolongation: Causes, Risks, and Management
Nov 17, 2025
Dr. Sanjay Mohan, an in-house toxicologist and emergency physician, delves into the critical topic of QTc prolongation and its implications. He explains the importance of recognizing prolonged QT intervals, detailing risks associated with torsades de pointes. Listeners learn about common medications that contribute to QTc issues and the necessity of monitoring electrolytes. Sanjay shares effective management strategies for torsades, highlighting emergency protocols and preventive measures to ensure patient safety.
14:47
QTc Reflects Repolarization And Risk
- The QT interval reflects ventricular depolarization through repolarization and must be HR-corrected as QTc.
- A QTc >500 ms markedly increases risk of ventricular dysrhythmia and torsades de pointes.
Use QTc And Manually Recheck When It Matters
- Use a QT correction (QTc) to compare QT across heart rates and be aware machines use different formulas.
- Manually calculate QTc when the value will change management or is borderline near 500 ms.
ECG Machines Can Mislead QTc
- Machine QTc readings vary by formula, paper speed, and sensitivity so they can be inaccurate.
- Use clinical context to decide when to trust the computer versus manually measuring the QTc.
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Intro
00:00 • 20sec
What the QT Interval Represents
00:20 • 1min
Why Use the Corrected QT (QTc)
01:25 • 1min
QTc Thresholds and Clinical Concern
02:34 • 2min
When to Manually Calculate QTc
04:05 • 1min
Why QTc Prolongation Matters (Torsades Risk)
05:06 • 39sec
Common Causes of QTc Prolongation
05:44 • 46sec
High-Risk QT-Prolonging Medications
06:30 • 41sec
When to Get an ECG Before Ordering Drugs
07:11 • 2min
Managing Torsades in the ED
09:36 • 1min
Post-Conversion Steps and Prevention
10:50 • 33sec
Approach to Asymptomatic Prolonged QTc
11:23 • 2min
Key Takeaways on QTc Management
13:49 • 46sec
Outro
14:36 • 10sec
🧭 REBEL Rundown
📌 Key Points
- 🫀 Prolonged QTc raises risk of torsades de pointes
- ⏱️ Correct for heart rate: QTc > 440 ms (men) or > 460 ms (women); > 500 ms = high TdP risk.
- 💊 Common culprits: Methadone, ondansetron, macrolides, fluoroquinolones, antipsychotics.
- 🧪 Prevention: Check & replete K, Mg, Ca and avoid QT-prolonging meds when possible.
- 🚑 If TdP develops: Defibrillate + IV magnesium and stop offending agents.
📝 Introduction
The QT interval is a vital part of ECG interpretation, reflecting the heart’s electrical recovery after each beat. When prolonged, it can set the stage for torsades de pointes. Understanding how to measure and correct the QT interval, identify high-risk medications, and act quickly when TdP occurs is essential for every clinician. This guide walks you through the physiology, interpretation, common causes, and emergency management of QTc prolongation to keep your patients safe.
🤔 Definition and Physiology
- QT evaluation is a fundamental component of EKG analysis. The QT interval reflects the time from ventricular depolarization and contraction through ventricular repolarization and relaxation.
- Clinically, QT prolongation increases the risk of torsades de pointes (TdP) – a form of polymorphic ventricular tachycardia (a non-perfusing rhythm) that is classically described as a pattern of “twisting points” or alternating amplitudes. This occurs when a premature ventricular contraction leads to an R on T phenomenon during the repolarization period.
- The differential for QT prolongation is long and varied: congenital long QT, electrolyte disturbances (hypoK, hypoMg, hypoCa), hypothermia, myocardial ischemia, and increased intracranial pressure. Moreover, a whole host of xenobiotics can prolong the QT interval: methadone, anti-microbials, anti-emetics, anti-psychotics, and anti-dysrhythmics.
🧮 ECG Interpretation
- The QT interval must be interpreted in conjunction with the patient’s heart rate. The QT interval with shorten in the context tachycardia and length in the context of bradycardia. In other words, tachycardia is protective when evaluating the patient with prolonged QT.
- With that in mind, many EKG machines will calculate a corrected QT interval or QTc. The QTc is a standardized way to account for variations in heart rate so clinicians are able to compared QT intervals at different heart rates over time and thus calculate risk.
- Generally, a QTc is considered prolonged if greater than 440ms in males or 460ms in females. Once the QTc > 500msec, the risk of TdP increases 2-3 fold.1
- A variety of different correction formulas exist: Bazett, Fridericia, Hodges, Framingham, Rautaharju.
- Manually, the QT interval should be measured from the beginning of the QRS complex to the end of the T wave – and thus should be measured in leads where all portions can be visualized, most frequently lead II or V5/V6. Ideally, the QT interval should be average over 3 or more beats.2 To determine the end of the T wave, a tangent line should be drawn through the maximum slope of the T wave – the point at which this line crosses the isoelectric line is the end of the T wave.3
💊 Commonly Used QTc Prolonging Medications
- Methadone: particularly concerning because not only does it inherently prolong QT but also induces a bradycardia
- Antiemetics: Ondansetron
- Macrolides: azithromycin, erythromycin, clarithromycin
- Fluroquinolones: ciprofloxacin, levofloxacin
- Antipsychotics: Haloperidol, Olanzapine
️ Management
- Prevention is key!
- Assess electrolytes (Mg, Ca, K) and replete as needed
- Telemetry Monitoring
- If patient happens to fall into TdP, initiate ACLS with immediate defibrillation and magnesium.
- Withdrawal of offending agents.
📚 References
- Drew BJ, Ackerman MJ, Funk M, Gibler WB, Kligfield P, Menon V, Philippides GJ, Roden DM, Zareba W. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010 Mar;121(8):1047-1060.
- Postema PG and Wilde AAM. The measurement of the QT interval. Curr Cardiol Rev. 2014 Aug;10(3): 287-294.
- https://litfl.com/qt-interval-ecg-library/
Post Peer Reviewed By: Marco Propersi, DO (Twitter/X: @Marco_propersi), and Mark Ramzy, DO (X: @MRamzyDO)
👤 Associate Editor
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