ECG criteria
- Measure QT from the onset of QRS to the end of the T wave in several leads, preferably II and V5/V6; choose the longest reliably measurable QT with a clear T-wave end. In the presence of a wide QRS (bundle branch block, intraventricular conduction delay, cardiac pacing), the QT includes additional depolarization delay and may appear falsely prolonged — in these cases, interpret QTc with caution, preferably evaluating JT/JTc or using a specialized correction method for wide QRS
- Exclude the U wave from the measurement: if T and U are clearly separated, measure to the end of T; if T and U are fused and the boundary cannot be determined, use another lead without superposition or note measurement uncertainty
- Correct QT for heart rate: Bazett's formula (QTc = QT / √RR) overestimates QTc at tachycardia and underestimates at bradycardia; for drug monitoring and heart rates other than 60 bpm, Fridericia's formula is preferred
- Compare QTc with gender-specific reference values: in men, the upper limit of normal is about 450 ms; in women, about 460 ms (sources vary within 5–10 ms); values around 470/480 ms correspond to approximately the 99th percentile and a stricter definition of prolongation
- Assess dynamics: QTc ≥500 ms or an increase in QTc of approximately ≥60 ms from baseline is an indication for urgent re-evaluation of the drug, drug interactions, and electrolytes; the decision to discontinue or replace the drug is made taking into account the absolute QTc, therapy indications, and overall arrhythmia risk, rather than a single increase criterion
- Check T-wave morphology: a flattened, notched, or inverted T wave is an indirect sign of repolarization abnormality
- Look for TdP predictors: bradycardia, pauses, ventricular premature complexes, pause-dependent QT prolongation after a long pause (short-long-short sequence preceding an ectopic complex)
- Exclude secondary causes of QT prolongation: hypokalemia, hypomagnesemia, hypocalcemia, hypothyroidism, ischemia
What it looks like on ECG
The main manifestation is prolongation of the QT interval (distance from the onset of QRS to the end of the T wave) relative to normal for the given heart rate. The T wave is often altered: flattened, wide, notched (with an additional terminal hump — particularly characteristic of certain class III antiarrhythmics) or inverted. In the setting of bradycardia or after an extended pause (e.g., following a premature complex), the QT lengthens even further — so-called pause-dependence, typical of many acquired, including drug-induced, forms of QT prolongation. This supports the drug-induced mechanism but does not exclude an inherited channelopathy: for example, the LQT3 variant often manifests at rest and during bradycardia, and some congenital forms may remain clinically silent until drug exposure. Differentiation requires a history (including family history), ECGs after drug discontinuation, and, if necessary, arrhythmological and genetic evaluation.
In severe intoxication with marked QT prolongation, torsades de pointes is typically triggered by a ventricular premature complex following a short-long-short sequence (short cycle — long pause — short cycle); sometimes a premature complex falls on the peak of the preceding T ("R on T"), but this phenomenon is not mandatory and is not a diagnostic criterion for risk. TdP is a polymorphic VT with characteristic "spindle-shaped" oscillation of QRS amplitude around the isoelectric line.
| Parameter | Standard normal (upper limit) | Approx. 99th percentile / strict criterion |
|---|---|---|
| QTc, male | <450 ms | ≥470 ms |
| QTc, female | <460 ms | ≥480 ms |
The limits in the table are approximate and depend on the source and correction method. A separate threshold for high TdP risk is defined as QTc ≥500 ms or an increase of ≥60 ms from baseline — these figures, along with the clinical context, determine the level of concern rather than the mere fact of exceeding the upper limit of normal. QT should be measured in multiple leads and the longest reliably determinable value should be taken, avoiding leads with poorly distinguishable T-wave ends (often III and aVR).
Mechanism
Most QT-prolonging drugs block the rapid component of the delayed rectifier potassium current (IKr), which is encoded by the hERG gene and mediates phase 3 repolarization of the cardiomyocyte. Blockade of this current slows potassium efflux from the cell, prolonging the action potential and consequently the QT interval on the ECG. A prolonged action potential promotes early afterdepolarizations; combined with increased repolarization dispersion and triggered activity, this creates the conditions for initiating torsades de pointes. Bradycardia and long pauses enhance pause-dependent repolarization prolongation and early afterdepolarizations, making the risk particularly high immediately following pauses. The risk increases sharply with a combination of factors: an intrinsically reduced repolarization reserve (subclinical channel defect, hypokalemia), bradycardia/pauses, and an additional pharmacological impact.
Causes
- Class IA antiarrhythmics: quinidine, procainamide, disopyramide
- Class III antiarrhythmics: amiodarone, sotalol, dofetilide, ibutilide — amiodarone usually prolongs QTc but causes TdP relatively infrequently; among this group, dofetilide, ibutilide, and sotalol carry the highest risk of TdP, especially in renal impairment and electrolyte disturbances
- Antipsychotics: haloperidol (especially IV), thioridazine, quetiapine, ziprasidone
- Antidepressants: tricyclics, citalopram/escitalopram in high doses
- Macrolides: erythromycin, clarithromycin
- Fluoroquinolones: moxifloxacin, levofloxacin
- Azole antifungals: fluconazole, itraconazole — prolong QT both directly and by inhibiting the metabolism of other QT-prolonging drugs
- Antimalarials: chloroquine, hydroxychloroquine
- Antiemetics: ondansetron, domperidone
- Methadone in high doses
- Combination of multiple QT-prolonging drugs or their interaction with metabolism inhibitors (CYP3A4, etc.)
Differential diagnosis
- Congenital long QT syndrome — QT prolongation without drug intake; classically associated with adrenergic triggers (exercise, stress), but certain variants (e.g., LQT3) manifest at rest and during bradycardia; may present with a family history of syncope or sudden death and a genetic basis (Romano–Ward syndrome, Jervell and Lange-Nielsen syndrome); manifestation is also possible in adulthood, and the phenotype may remain hidden until drug provocation
- Secondary QT prolongation due to electrolyte disturbances — hypokalemia, hypomagnesemia, and hypocalcemia present a similar picture regardless of medications, often acting synergistically with drugs
- Bradycardia-dependent QT prolongation in AV block or sick sinus syndrome — with pronounced bradycardia and AV block, the measured QT is prolonged due to the long cycle, and standard correction formulas are inaccurate; multiple consecutive complexes and dynamics after pauses must be evaluated, while the contribution of drugs and electrolytes should be assessed separately
- U wave mistaken for part of T — falsely lengthens the measured QT; inspect the lead where the U wave does not superimpose on the T wave
- Brugada syndrome and early repolarization — other channelopathies with different ECG morphology; QT is typically not prolonged
Clinical significance and management
Drug-induced QT prolongation is dangerous due to the potential development of torsades de pointes, which can degenerate into ventricular fibrillation and lead to sudden cardiac death. The physician should:
- When prescribing QT-prolonging drugs — monitor the ECG and serum potassium, magnesium, and calcium levels at baseline and dynamically
- In case of QTc ≥500 ms or an increase of ≥60 ms, urgently re-evaluate the need for the causative drug, drug interactions, and electrolytes; decisions on discontinuation or dose reduction should be made taking into account the absolute QTc, therapy indications, and clinical arrhythmia risk
- Identify and correct aggravating factors: hypokalemia, hypomagnesemia, bradycardia, and drug interactions via CYP3A4
- In developed torsades de pointes (TdP): discontinue the causative drug; in case of instability or pulselessness, perform immediate unsynchronized defibrillation; administer magnesium sulfate intravenously regardless of baseline magnesium levels; correct hypokalemia (typically to high-normal values); eliminate hypomagnesemia and hypocalcemia if present (calcium is administered for confirmed hypocalcemia, not routinely to everyone); eliminate bradycardia and pauses, use temporary cardiac pacing for recurrent pause-dependent TdP; isoprenaline can only be considered for acquired pause-dependent TdP in the absence of contraindications, but not for congenital long QT syndrome or myocardial ischemia
- Consider individual risk: female sex, older age, underlying structural heart disease, renal/hepatic impairment, and subclinical channel defects increase sensitivity to QT-prolonging drugs
- If QTc does not normalize after drug discontinuation, congenital long QT syndrome should be ruled out rather than assuming the drug-induced cause is proven
Common interpretation errors
- Measuring QT in a single lead with an indistinct T-wave end instead of evaluating multiple leads and choosing the highest reliable value
- Including the U wave in the QT measurement, which falsely lengthens the interval, especially when T and U waves are fused
- Using Bazett's formula in marked tachycardia or bradycardia without considering its limitations — Fridericia's formula is more accurate for drug monitoring and extreme heart rates
- Interpreting any QTc above the approximate upper limit of normal as 'dangerous' without clinical context — clinical concern is primarily raised by QTc ≥500 ms or a significant increase
- Calculating QTc using standard formulas in the presence of a wide QRS (bundle branch block, pacing) without adjustment for depolarization delay, which can falsely overestimate prolongation
- Ignoring concomitant electrolyte abnormalities, which sharply increase the risk of TdP at the same QTc level
1. What QTc threshold in women is most commonly used as the upper limit of normal?
Approximately 460 ms is frequently used as the upper limit of normal in women; about 480 ms may correspond to the 99th percentile/stricter definition of prolongation, and 500 ms to the high-risk threshold for TdP.
2. What is the underlying mechanism of most cases of drug-induced QT prolongation?
Most QT-prolonging drugs block the IKr potassium current encoded by the hERG gene, which delays ventricular repolarization.
3. Which rhythm sequence typically precedes torsades de pointes in drug-induced QT prolongation?
Drug-induced TdP is typically preceded by a short–long–short sequence: after an extended pause, a ventricular premature beat triggers polymorphic VT.
Frequently asked questions
What QTc is considered dangerous when taking medications?
Greatest clinical concern is raised by QTc ≥500 ms or a QTc increase of approximately ≥60 ms from baseline, evaluated alongside the clinical context. Approximate thresholds of 450/460 ms (men/women) indicate prolongation or the upper limit of normal, but are not an automatic indication to stop the drug.
How to distinguish drug-induced QT prolongation from congenital syndrome?
Pause-dependence and the onset of prolongation after starting therapy support a drug-induced mechanism, and QTc normalization after drug withdrawal favors an acquired form, but does not completely prove it. Failure to normalize, a family history of syncope, or sudden death require ruling out congenital long QT syndrome, which can manifest not only in childhood but also in adulthood.
Why is the QT interval measured in different leads and the highest value taken?
The end of the T wave appears differently across leads, and in some, it merges indistinctly with the isoelectric line. To avoid missing true prolongation, measurements should be taken in multiple leads, preferably II and V5–V6, and the highest reliably determined value should be used.
Is the combination of multiple QT-prolonging drugs dangerous?
Yes, the QT-prolonging effect and the risk of torsades de pointes from drug combinations are additive and can be potentiated by drug interactions affecting drug metabolism (e.g., CYP3A4 inhibitors).
Which QT correction formula should be used in tachycardia or bradycardia?
Bazett's formula overestimates QTc during tachycardia and underestimates it during bradycardia; therefore, at heart rates other than 60 bpm, and especially for drug monitoring, Fridericia's formula is preferred.
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Related terms
More in "Pharmacology and Toxicology"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice, 6th ed.
- Hampton J. The ECG Made Easy
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
- Orlov V.N. Guide to Electrocardiography
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.