ECG criteria
- Bazett's QTc >450 ms (men) / >460 ms (women) represents borderline prolongation requiring attention; >500 ms indicates a high risk of proarrhythmia (both thresholds are used in different guidelines)
- QT measurement: from the onset of the QRS complex to the end of the T wave (return to the isoelectric line), preferably in lead II or V5–V6 where the end of the T wave is clearly visible; interpret QTc with caution in the presence of a wide QRS (see the «Common Pitfalls» section)
- Amiodarone: moderate QTc prolongation, often with a flattened, broad T wave; sinus bradycardia and AV block are possible
- Sotalol (class III + non-selective β-blocker): dose-dependent QTc prolongation, bradycardia, high risk of torsades de pointes when QTc ≥500 ms
- Class IC (flecainide, propafenone): widening of the QRS complex and prolongation of PQ as the primary effects; non-specific ST-T changes may occur, but these are not the main criteria
- Quinidine/procainamide (class IA): simultaneous QT and QRS prolongation, bradyarrhythmia possible; a U wave may appear but is non-specific and requires assessment of heart rate and electrolytes
- Serial evaluation: comparison of QTc and QRS width with the baseline ECG prior to therapy—the absolute value matters, but the increase from baseline is equally significant
- Fredericia's formula is preferred over Bazett's in marked tachy- or bradycardia and during drug QT monitoring; in any case, the correction formula used must be specified
What it looks like on ECG
Effects depend on the drug class and its electrophysiological mechanism.
| Class | Drugs | Primary ECG effect |
|---|---|---|
| IA | quinidine, procainamide | ↑QT, ↑QRS, U wave possible |
| IC | flecainide, propafenone | ↑QRS, ↑PQ |
| III | amiodarone, sotalol | ↑QTc, flat/broad T |
| IV | verapamil, diltiazem | bradycardia, ↑PQ (not QT) |
Digoxin stands apart — it does not belong to the Vaughan Williams classes, but is often discussed alongside antiarrhythmics due to frequent combined use in atrial fibrillation. Its typical ECG effects include PR prolongation, characteristic scooping (sagging) ST-segment depression, T-wave flattening or inversion, and frequently shortened QT.
With amiodarone, the T wave often becomes flattened and broadened. QT may prolong shortly after therapy initiation; the magnitude and dynamics depend on the dose, tissue loading, drug interactions, and individual factors, while a sustained stable effect is linked to drug accumulation in tissues. Sinus bradycardia and AV block may occur. The proarrhythmic potential of amiodarone in the setting of QT prolongation is considered lower than that of pure potassium channel blockers — a distinct characteristic of the drug.
With sotalol, QTc prolongation is more predictably dose-dependent and clinically significant: ECG monitoring is mandatory during titration because the risk of torsades de pointes is higher than with amiodarone.
For class IC drugs, QRS width and the PQ interval are paramount: slowed intraventricular conduction due to sodium channel blockade manifests as QRS widening and PQ prolongation, especially at higher heart rates. A practical threshold for proarrhythmic risk is a QRS increase of more than 25% from baseline (taking into account the specific product labeling and significant absolute widening, typically over 120 ms); such changes require dose reduction or drug discontinuation and assessment for toxicity. Severe sodium channel blocker overdose can cause marked QRS widening, monomorphic or polymorphic ventricular tachycardia, ventricular fibrillation, and a terminal sine-wave pattern.
Mechanism
Class III antiarrhythmics block potassium channels (IKr, functionally similar to the hERG channel), delaying phase 3 ventricular repolarization — this prolongs the action potential and the QT interval, creating conditions for early afterdepolarization-mediated triggered activity (torsades de pointes). Class IA agents affect both sodium and potassium channels, thereby prolonging both QRS and QT. Class IC drugs block fast sodium channels (phase 0 depolarization) and slow conduction in the His-Purkinje system and ventricular myocardium, hence QRS widening and PQ prolongation with relatively preserved repolarization. Amiodarone is distinguished by its multi-channel action (K+, Na+, Ca2+ channel blockade, antiadrenergic effect), which partly explains its relatively low proarrhythmic profile despite marked QT prolongation.
Causes
- Amiodarone therapy (effects may appear early in treatment, steady-state levels achieved after tissue saturation, often after weeks of administration)
- Sotalol therapy: dose-dependent effect; renal function assessment is mandatory prior to initiation and during titration (the drug is cleared renally), with dose adjustment for reduced clearance
- Quinidine, procainamide, disopyramide (class IA)
- Flecainide, propafenone (class IC)
- Combination with other QT-prolonging drugs (macrolides, antipsychotics, certain antiemetics)
- Electrolyte disturbances during therapy (hypokalemia and hypomagnesemia potentiate QT prolongation)
- For amiodarone, clinically significant drug interactions (CYP substrates, warfarin, digoxin) and the need to monitor liver and thyroid function are essential; for sotalol and dofetilide, renal function monitoring is required as they are primarily eliminated by the kidneys
Differential diagnosis
- Congenital long QT syndrome — administration of a QT-prolonging drug does not rule out latent congenital LQTS, which may be unmasked by the treatment; differentiated by prior baseline ECGs before therapy, clinical LQTS criteria, family history of syncope/sudden death, and QTc dynamics after drug discontinuation; if suspected, an arrhythmologist consultation and genetic testing are warranted
- Hypokalemia/hypomagnesemia — also prolong the QT interval and produce a U wave; check serum electrolytes; the effect frequently coincides with drug administration
- Hypothermia — QT prolongation plus an Osborn wave, which is absent in drug-induced effects
- Bundle branch block — widens QRS without drug causation; check morphology (typical for LBBB/RBBB) and medication history
- Tricyclic antidepressant overdose — also widens QRS and may produce an R wave in aVR, but the clinical presentation and history differ
- Digoxin effect — PR prolongation and scooping ST-segment depression with QT shortening rather than prolongation
Clinical significance and management
QTc prolongation and QRS widening during antiarrhythmic therapy warrant serial ECG monitoring and dose adjustment or drug discontinuation. The clinician should: obtain a baseline ECG prior to therapy; for flecainide and propafenone, serially evaluate QRS width relative to baseline (an increase of more than 25% is a universally accepted trigger to reconsider the dose or discontinue the drug) and follow the specific product labeling; for sotalol, dofetilide, and other QT-prolonging agents when QTc approaches or exceeds 500 ms, immediately rule out reversible causes (electrolytes, bradycardia, drug interactions) and reduce the dose or discontinue the drug according to protocol; prior to starting or resuming sotalol, ensure monitored supervision with assessment of QTc, renal function, and electrolytes, repeating the ECG after every dose adjustment; monitor serum potassium and magnesium, keeping them within the normal range; remain aware of drug interactions that increase QT prolongation.
In the event of torsades de pointes (TdP), the clinician's actions include: immediate discontinuation of the offending agent and any other QT-prolonging drugs; intravenous administration of magnesium regardless of baseline serum Mg levels; correction of hypokalemia targeting a potassium level typically around 4.5–5.0 mmol/l; elimination of pauses and bradycardia via temporary pacing or isoproterenol for acquired, bradycardia-dependent TdP; in the event of hemodynamic instability or pulselessness, immediate unsynchronized defibrillation per life-threatening arrhythmia algorithms (ACLS). For sustained monomorphic ventricular tachycardia occurring in the setting of an antiarrhythmic drug with a different mechanism (e.g., class IC agents), provide emergency treatment per standard ventricular tachyarrhythmia algorithms with mandatory discontinuation of the provocative drug.
Common interpretation errors
- Measuring QT in a lead with a low-amplitude or notched T wave where the end of the wave cannot be distinguished from the U wave
- Failing to correct QT for heart rate (using the 'raw' QT instead of QTc) and failing to specify the correction formula used
- Mistaking the U wave for a portion of the T wave and obtaining a falsely elevated QTc
- In case of a widened QRS (bundle branch block, drug-induced widening), interpret QTc without accounting for QRS width; in this scenario, assessing JT/JTc or using a QRS-corrected QT formula is preferred
- Ignoring the baseline ECG prior to therapy and missing significant parameter increases
- Failing to account for potassium and magnesium levels when interpreting QT prolongation on antiarrhythmics
1. Which ECG sign is most typical for class IC agents (flecainide, propafenone)?
Class IC blocks sodium channels and slows intraventricular and AV conduction, which manifests as QRS widening and PQ prolongation.
2. Which of the listed antiarrhythmics carries the highest dose-dependent risk of torsades de pointes?
Sotalol is a potassium channel blocker with predictable dose-dependent QTc prolongation and a higher risk of torsades de pointes compared to amiodarone.
3. In which leads is it recommended to measure the QT interval for QTc calculation?
In leads II and V5–V6, the end of the T wave is usually visible most clearly, which reduces measurement error.
Frequently asked questions
Which antiarrhythmic carries a higher risk of torsades de pointes — amiodarone or sotalol?
Sotalol is considered more proarrhythmic regarding QT prolongation: its risk of torsades de pointes is higher and more dose-dependent. Amiodarone prolongs QT significantly, but due to its multichannel action, it less frequently provokes torsades de pointes at a comparable QTc.
How is the ECG monitored during sotalol dose titration?
Before starting or resuming therapy, evaluate the baseline QTc, renal function (the drug is excreted by the kidneys), and potassium/magnesium levels. Initiation and dose titration should be performed under monitored observation with repeat ECGs after each dose change, in accordance with instructions and local protocols.
What to do if the QRS widens significantly during flecainide therapy?
An increase in QRS width by more than 25% from the baseline ECG is a universally accepted reason to review the dose or discontinue the drug, as this is a sign of pronounced sodium channel blockade and an escalating risk of proarrhythmia.
How to distinguish drug-induced QT prolongation from congenital syndrome?
Taking a QT-prolonging drug does not rule out congenital LQTS — it may simply unmask a latent predisposition for the first time. They are distinguished by previous ECGs, clinical criteria for LQTS, family history, and QTc dynamics after drug withdrawal; the final decision is made by an arrhythmologist and genetic testing.
Why might a U wave appear during antiarrhythmic therapy?
The U wave does not have a definitively established mechanism and is not a specific effect of any particular drug, including amiodarone and quinidine. It more often becomes prominent in bradycardia or hypokalemia; when it merges with the T wave, it should not be automatically included in the QT measurement.
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Related terms
More in "Pharmacology and Toxicology"
Sources
- Goldberger A.L. "Clinical Electrocardiography", current edition
- Surawicz B., Knilans T. «Chou's Electrocardiography in Clinical Practice»
- Orlov, V.N. «Electrocardiography Guide»
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the ECG, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.