Long QT Syndrome
Clinical Snapshot
Long QT syndrome (LQTS) is a disorder of the heart's electrical repolarization, seen on the surface ECG as a prolonged QT interval (corrected for heart rate as QTc, upper limits of normal 440 msec in men and 460 msec in women using Bazett's formula) and clinically important because it predisposes to torsades de pointes — a distinctive polymorphic ventricular tachycardia that twists around the isoelectric line — which can self-terminate (causing transient syncope) or degenerate into ventricular fibrillation and sudden cardiac death.[1] A documented prevalence of 1 in 2,000 live births likely understates the true figure, since the original prospective newborn-ECG study did not capture genotype-positive, phenotype-negative individuals whose resting QTc is normal.[1]
This chapter is organized around the distinction that drives everything else: congenital versus acquired LQTS. Congenital LQTS arises from a pathogenic variant in a cardiac ion-channel gene, most often KCNQ1 (LQT1, roughly 50% of genotyped cases), KCNH2 (LQT2, roughly 40%), or SCN5A (LQT3, roughly 10%) — and each genotype carries genuinely different triggers, a different expected response to therapy, and different counselling priorities, which is why "LQTS" is never really one condition to manage the same way twice.[1] Acquired LQTS develops in a structurally and often genetically normal-appearing heart under the combined pressure of a QT-prolonging drug, hypokalemia or hypomagnesemia, bradycardia, or heart block — and correcting the offending factor is usually sufficient to prevent recurrence, provided it is actually identified.[2][1]
Nonselective beta-blockade — nadolol or propranolol specifically, not a beta-1-selective agent — is the mainstay of therapy for congenital LQTS and has been since the mid-1970s, with efficacy confirmed independent of genotype.[1] For pharmacists specifically, this chapter gives particular weight to a role that is easy to underestimate: a very large share of acquired LQTS and drug-induced torsades de pointes is genuinely preventable at the point of dispensing, by recognizing a QT-prolonging drug, checking it against the rest of a patient's regimen and their LQTS status, and catching an interaction or a dose that exceeds a specific, evidence-based ceiling (as Health Canada has set explicitly for citalopram and domperidone) before it reaches the patient.[3][4]
Recognize the Condition
Typical presentation
- Syncope, classically abrupt, without a clear vasovagal prodrome, and often — though not always — triggered by exertion, strong emotion, a sudden loud noise, or sleep, depending on genotype (see the genotype table below). Short-duration syncope or vertigo in a patient with known QT prolongation should be taken as a possible harbinger of torsades de pointes, not dismissed as benign lightheadedness.[1]
- Seizure-like activity from transient cerebral hypoperfusion during a self-terminating arrhythmia — a genuine and recurring source of misdiagnosis as epilepsy, particularly in children and teenagers, especially when the episode has an exertional, emotional, or startle trigger, or occurs during sleep.
- Palpitations, sometimes preceding a syncopal event.
- Cardiac arrest or sudden cardiac death as the first presentation — the reason timely diagnosis matters so much; making the diagnosis after the fact, rather than before, is described in the primary literature as "diagnostic or therapeutic errors" that current, highly effective therapies should make inexcusable.[1]
- Entirely asymptomatic, found on a routine ECG, family cascade screening after a relative's diagnosis, or incidentally during another work-up. Roughly a quarter of individuals with a confirmed pathogenic LQTS variant have a normal baseline QTc (under 440 msec) — concealed LQTS — so a single normal resting ECG does not exclude the diagnosis.[1]
Congenital vs. acquired LQTS — the first branch point
Two different problems, two different first questions
Congenital LQTS
A pathogenic variant in a cardiac ion-channel gene (or, less often, a calcium-channel or calmodulin gene) delays ventricular repolarization from birth. The QTc may be prolonged at baseline or entirely normal (concealed), and symptoms are typically genotype-specific in their triggers. The first question is: which genotype, and what is this person's specific trigger and treatment profile?
Acquired LQTS
QT prolongation develops under the pressure of a QT-prolonging drug, hypokalemia, hypomagnesemia, bradycardia, or heart block, in a heart that may or may not carry an underlying genetic predisposition. The first question is: what is the offending factor, and can it be identified and corrected? Age under 40 years, a baseline QTc over 440 msec, and an actual arrhythmic episode together suggest a latent congenital predisposition may have been unmasked, and should prompt genetic testing rather than assuming the drug alone fully explains the event.[1]
The three major congenital genotypes
The three genes account for roughly 90% of genotyped congenital LQTS, and matter clinically because their triggers, ECG appearance, and response to specific therapies genuinely differ — this table is referenced repeatedly through the rest of this chapter.[1]
| Genotype | Gene / current affected | Approx. share of cases | Classic triggers | T-wave appearance | Beta-blocker / LCSD response | Mexiletine response |
|---|---|---|---|---|---|---|
| LQT1 | KCNQ1 (IKs current) | ~50% | Adrenergic — strenuous exercise, especially swimming; strong emotion | Broad-based T waves | +++ / +++ (both highly effective) | Unknown / not established |
| LQT2 | KCNH2 (IKr current) | ~40% | Sudden loud noises (alarm clocks, ringing phones), especially at rest or on waking; low serum potassium; the postpartum period | Notched or bifid, low-voltage T-wave alternans | ++ / ++ | ++ (~70% show meaningful QTc shortening) |
| LQT3 | SCN5A (INa current, gain of function) | ~10% | Sleep or rest — QT prolongation is most pronounced at slow heart rates | Long ST segment, high-amplitude narrow T wave | ++ / +++ (LCSD particularly effective) | +++ (the original gene-specific indication) |
Rarer causes include homozygous or compound heterozygous KCNQ1 or KCNE1 variants, which cause the recessive Jervell and Lange-Nielsen syndrome — a more severe phenotype associated with congenital sensorineural deafness — and CACNA1C variants, which cause Timothy syndrome, a multisystem disorder that also includes skeletal and neurodevelopmental abnormalities. Variants in the calmodulin genes CALM1, CALM2, and CALM3 cause calmodulinopathies, a less common but often severe cause of life-threatening arrhythmia, including in infants.[1]
Risk Factors
Congenital risk factors
- A pathogenic or likely pathogenic variant in KCNQ1, KCNH2, or SCN5A — together accounting for roughly 90% of genotyped cases — or, less commonly, CACNA1C (Timothy syndrome), CALM1/2/3 (calmodulinopathy), or homozygous/compound heterozygous KCNQ1/KCNE1 (Jervell and Lange-Nielsen syndrome).[1]
- A first-degree relative with confirmed LQTS, or unexplained sudden cardiac death in an immediate family member under age 30 — both are explicit criteria in the Schwartz diagnostic score (see Assessment/Diagnosis).[1]
- Modifier genes — common genetic variants that are not disease-causing on their own but can meaningfully increase or decrease arrhythmic risk in a person who already carries a disease-causing variant — are an active area of refinement in individual risk stratification.[1]
- A genotype-negative genetic test does not exclude the diagnosis: molecular testing is negative in 10-15% of patients who otherwise meet clinical criteria for LQTS, and the available outcome data show their arrhythmic risk is similar to genotype-positive patients — they should be treated the same way, based on the clinical phenotype, not held to a lower standard of concern because a gene panel came back negative.[1]
Acquired LQTS risk factors — the pharmacist-critical differential
The probability that a given QT-prolonging drug will actually produce clinically significant QT prolongation in a specific patient depends on the intrinsic potency of that drug's IKr-blocking effect and on the individual's own "repolarization reserve" — itself modulated by genetic factors ranging from ultra-rare to common variants.[1] The following raise that risk and deserve specific attention during medication review:
- Electrolyte disturbance — hypokalemia, hypomagnesemia, and (less directly) hypocalcemia — from diuretics, GI losses (vomiting, diarrhea, laxative use), poor oral intake, or an eating disorder.
- Bradycardia or high-grade AV block, which itself prolongs the QT interval and — critically — makes QT-prolonging drugs more dangerous, since torsades de pointes in acquired LQTS is classically pause-dependent.[2]
- Female sex — women have a longer baseline QTc than men and are substantially overrepresented among cases of drug-induced torsades de pointes.
- Combining more than one QT-prolonging drug, or combining a QT-prolonging drug with a drug (often a CYP3A4 or CYP2D6 inhibitor) that raises its level or slows its clearance — a pharmacokinetic interaction is often the actual mechanism behind an otherwise unremarkable dose becoming dangerous.
- Renal or hepatic impairment reducing clearance of a renally or hepatically eliminated QT-prolonging drug.
- Older age, mainly through its association with polypharmacy, comorbid structural heart disease, and reduced renal/hepatic clearance rather than age itself.
- A latent congenital predisposition unmasked by a drug: three variables — age under 40 years, a baseline QTc over 440 msec, and an actual arrhythmic episode — together suggest genetic testing should be offered, since acquired LQTS can be the first clinical sign of an underlying, previously silent congenital channelopathy.[1]
Red Flags & Refer
Escalate or refer urgently for any of the following:
- Torsades de pointes or cardiac arrest — a medical emergency; treat per the acute algorithm below and do not wait for an outpatient work-up.
- Syncope, particularly abrupt syncope without a clear vasovagal prodrome, or syncope triggered by exertion (especially swimming), strong emotion, a sudden loud noise, or occurring during sleep — treat as a possible cardiac event until proven otherwise and refer urgently rather than reassure.[1]
- QTc greater than 500 msec, from any cause — this threshold meaningfully discriminates patients at moderate-to-high arrhythmic risk and warrants urgent specialist evaluation.[1]
- New or worsening QT prolongation after starting or increasing the dose of a QT-prolonging drug — hold the drug and reassess urgently; do not simply "watch and wait" on an outpatient basis once the QTc is significantly abnormal.
- Unexplained seizure-like activity in a child, teenager, or young adult, especially with an exertional, emotional, or startle trigger, or occurring during sleep, and especially with a family history of unexplained sudden death, unexplained drowning, or a "seizure disorder" that has never had a clear neurological cause established — consider LQTS before accepting an epilepsy diagnosis at face value.
- Family history of unexplained sudden cardiac death under age 30, particularly during exercise, sleep, or associated with drowning or a single-vehicle accident — this is itself a Schwartz score criterion, and it is a reason to evaluate the patient in front of you, not just to note it for the family.[1]
- Postpartum patient (up to roughly 9 months after delivery) with new palpitations, presyncope, or syncope — the postpartum period is a specific, genotype-linked (LQT2) high-risk window that is easy to attribute to normal postpartum fatigue or anxiety instead.[1]
- Any patient with known congenital LQTS being considered for a new QT-prolonging medication, or any patient with a QTc already in the abnormal range being considered for one — this should trigger a pharmacist/prescriber consult before dispensing, not after a problem develops (see the Clinical Decision Algorithm).
Assessment & Diagnosis
Measuring the QT interval
Measure the QT interval from the start of the Q wave to the point the T wave returns to the isoelectric baseline, using the tangent method — a line drawn along the steepest slope of the T wave's descending limb, extrapolated to baseline — which is widely used because it is reproducible, though it can underestimate the true length of ventricular repolarization; when several complexes are available, the longest QT interval is the most clinically important value for risk assessment, not an average.[1] Correct for heart rate using Bazett's formula (QTc = QT / √RR); despite its known limitations at very fast or very slow heart rates, it remains the most widely used and clinically useful correction, including in infants.[1] The upper limits of normal are 440 msec in men and 460 msec in women; a QTc over 500 msec discriminates patients at moderate-to-high arrhythmic risk.[1]
Pattern recognition beyond the raw QTc number
Certain T-wave morphologies should raise suspicion for LQTS independent of the measured QT duration itself: T-wave alternans (beat-to-beat alternation in T-wave amplitude or morphology) is a marker of major electrical instability and a genuine prefibrillatory sign; notched or bifid T waves, particularly typical of LQT2; broad-based T waves, typical of LQT1; and a long isoelectric ST segment with a high-amplitude, narrow T wave, typical of LQT3.[1]
The Schwartz diagnostic score
For clinicians without specific LQTS expertise, the Schwartz score is a validated, widely used tool for a preliminary probability assessment; it should not substitute for specialist evaluation in an ambiguous case, but it is a genuinely useful structure for organizing the history, ECG, and family history findings that matter.[1]
| Criterion | Points |
|---|---|
| ECG findings (in the absence of a QT-prolonging medication or a condition known to affect the QT interval) | |
| QTc ≥480 msec | 3 |
| QTc 460-479 msec | 2 |
| QTc 450-459 msec (male patients) | 1 |
| QTc ≥480 msec at the 4th minute of recovery from an exercise stress test | 1 |
| Torsades de pointes | 2 |
| T-wave alternans | 1 |
| Notched T wave in 3 leads | 1 |
| Low heart rate for age (resting heart rate below the 2nd percentile for age) | 0.5 |
| Clinical history | |
| Syncope with stress | 2 |
| Syncope without stress | 1 |
| Congenital deafness | 0.5 |
| Family history | |
| ≥1 family member with confirmed LQTS | 1 |
| Unexplained sudden cardiac death in an immediate family member under age 30 | 0.5 |
Interpretation: 0-1 point, low probability; 1.5-3 points, intermediate probability; ≥3.5 points, high probability (a clinical diagnosis of LQTS). Torsades de pointes and syncope are mutually exclusive criteria — do not count both for the same event. The same family member cannot be counted twice.[1]
Additional testing
- Exercise stress testing is the single most useful ancillary test: a marked, specifically 4th-minute recovery-phase QTc prolongation is highly specific for LQTS and can unmask the diagnosis in a patient with a normal or borderline resting ECG.[1]
- Ambulatory (24-hour Holter) ECG monitoring can unmask T-wave alternans or notching, particularly overnight, that a single resting ECG misses.[1]
- Do not use an epinephrine (adrenaline) provocation challenge to help establish the diagnosis outside a center with specific expertise — it carries genuine arrhythmogenic potential in its own right and can profoundly and misleadingly alter repolarization in a person with an entirely normal heart, risking a false-positive result; the 2022 ESC guidelines explicitly recommend against its use for this purpose.[5][1]
- Genetic testing, ideally via a multigene LQTS panel, identifies a pathogenic variant in a majority of clinically diagnosed patients and directly enables efficient cascade testing of relatives; predictive testing is recommended for all at-risk family members from birth onward, and is especially prioritized under age 18, since the risk of a cardiac event is greatest in childhood.[6]
- Do not let a negative genetic test override a convincing clinical picture — 10-15% of clinically diagnosed patients are genotype-negative and should be risk-stratified and treated the same as genotype-positive patients.[1]
An important overdiagnosis trap: exercise-induced repolarization changes
Excessive physical training, especially in teenagers, can produce marked QT prolongation that mimics LQTS. These individuals are typically asymptomatic, genotype-negative, and have no family history of LQTS; the abnormality is reversible with 3 to 4 months of detraining. The arrhythmogenic potential of this training-induced pattern is not well characterized, but sports physicians and clinicians alike should be aware of it specifically to avoid prematurely and permanently labelling a young athlete with a genetic diagnosis they do not actually have.[1]
Acquired LQTS-specific assessment
Identify the offending drug(s), and check serum potassium, magnesium, and calcium, renal and hepatic function, and the heart rate/rhythm for bradycardia or high-grade AV block. Where age under 40 years, a baseline QTc over 440 msec, and an actual arrhythmic episode are all present, offer molecular genetic testing for the definitive LQTS-associated genes — this combination suggests a latent congenital predisposition may have been unmasked rather than fully explained by the acquired factor alone.[1]
Treatment Targets
- Prevent torsades de pointes, ventricular fibrillation, and sudden cardiac death — the overarching goal for every patient, symptomatic or not.
- In congenital LQTS: ensure essentially every patient is on an adherent, adequately dosed nonselective beta-blocker (nadolol or propranolol), including genotype-positive/phenotype-negative individuals, since current therapy is highly effective and undiagnosed or untreated LQTS is considered inexcusable given how well it responds to treatment.[1]
- Layer genotype-specific trigger avoidance onto pharmacotherapy rather than treating drug therapy as sufficient on its own — the specific triggers differ meaningfully between LQT1, LQT2, and LQT3 (see the genotype table above).[1]
- Maintain serum potassium at or above 4 mmol/L — genuinely contributes to lowering arrhythmic risk, and is specifically emphasized for LQT2.[1]
- Escalate breakthrough symptoms through mexiletine and/or left cardiac sympathetic denervation before defaulting to an ICD, and reserve ICD implantation for the specific indications in the Clinical Decision Algorithm — genuine regional overuse of ICDs in LQTS is a recognized problem, not a reason to avoid a genuinely indicated device.[1]
- In acquired LQTS: identify and remove or correct the offending drug or electrolyte disturbance promptly, and treat active torsades de pointes with IV magnesium regardless of the measured serum magnesium level.[2][1]
- Screen every new prescription, and every clinically significant OTC or herbal request, against the patient's LQTS status and current QT-active drug list before it is dispensed — the specific, practical target this chapter emphasizes for pharmacists.
- Offer first-degree relatives clinical and, where appropriate, genetic cascade screening after every new diagnosis.[6]
Choose Initial Treatment
First-line, essentially all patients with congenital LQTS a nonselective beta-blocker — nadolol 1.0-1.5 mg/kg/day or propranolol 2.0-3.5 mg/kg/day (in divided doses) — specialist-titrated to heart-rate response and symptom control. These are the only two beta-blockers with confirmed efficacy in LQTS, and their benefit is independent of genotype.[1] Long-acting nadolol is often favoured specifically because once-daily dosing supports adherence, which matters enormously here: nonadherence to beta-blocker therapy, together with exposure to QT-prolonging drugs, is responsible for most of what looks like "beta-blocker failure" in LQTS.[1]
Do not substitute a beta-1-selective agent (metoprolol, atenolol, bisoprolol) — these have not been confirmed effective in LQTS and should not be used in place of nadolol or propranolol.[1]
Beta-blockade should be prescribed even for genotype-positive, phenotype-negative individuals (a normal resting QTc with a confirmed pathogenic variant), with only narrow, specialist-directed exceptions (for example, an asymptomatic man with LQT1 who has reached age 25 without therapy). Withholding or stopping beta-blocker therapy in a patient with a confirmed pathogenic variant is a decision for the treating specialist, not a default.[1]
First step, always confirm adherence before assuming the beta-blocker has genuinely failed — nonadherence, not pharmacologic failure, explains most "breakthrough" events.[1]
Genuine breakthrough on a confirmed-adherent, adequately dosed beta-blocker escalate to what is often described as "triple therapy": add mexiletine, particularly for LQT3 (the original gene-specific indication) and for LQT2, where roughly 70% of patients show a clinically meaningful QTc response; assess candidacy with the acute oral drug test — 6-8 mg/kg of oral mexiletine, with a QTc shortening of more than 40 msec within 2 hours predicting a good long-term response — before committing to chronic therapy.[1] Add or refer for left cardiac sympathetic denervation (LCSD) in parallel or as an alternative, particularly when ICD therapy is declined or contraindicated, or when beta-blockers are not effective, not tolerated, or not accepted.[1][6]
Recommended survived cardiac arrest, regardless of whether beta-blocker therapy had been adequate — the recurrence risk after a cardiac arrest remains high even on treatment.[6][1]
Reasonable syncope despite full-dose beta-blocker therapy, when optimization with LCSD and/or mexiletine is not available; and syncope despite full-dose beta-blocker therapy and optimized LCSD.[1]
Not indicated by default asymptomatic patients who have not yet had a trial of beta-blocker therapy, or asymptomatic patients whose only risk marker is a family history of sudden death. There is large, well-documented international variation in ICD use for LQTS — some centers implant a device in roughly half their LQTS patients, while the largest specialist LQTS centers in the world implant one in only about 5% — and excessive ICD use is a recognized problem, not the safer default.[1] An ICD is preferentially placed after a genuine, informed discussion of LCSD as an alternative or complement, not automatically ahead of it.[1]
Immediate identify and stop or substitute the offending QT-prolonging drug (do not simply reduce the dose without reassessing whether it is genuinely needed); correct potassium to at least 4 mmol/L and replace magnesium even if the level is reported as "normal" when torsades de pointes is present or strongly suspected.[2][1]
Active torsades de pointes IV magnesium sulfate is first-line, given regardless of the measured serum magnesium level. If the arrhythmia is bradycardia- or pause-dependent, increasing the heart rate — with temporary (often overdrive) pacing, or isoproterenol as a bridge — shortens the QT interval and reduces the substrate for recurrence.[2][1]
Do not treat drug-induced torsades de pointes with another Class Ia or Class III antiarrhythmic (e.g., procainamide, amiodarone, sotalol) — these themselves prolong the QT interval and can make the arrhythmia worse rather than better; this is one of the more dangerous, and avoidable, errors in acute arrhythmia management.
Non-Pharmacologic Treatment
- Genotype-specific trigger avoidance — the single most actionable non-drug intervention, and genuinely different by genotype: LQT1 patients should limit strenuous exercise and avoid unsupervised swimming (supervision by an adult who can swim is specifically recommended); LQT2 patients should keep serum potassium at or above 4 mmol/L and avoid abrupt loud noises in the bedroom (alarm clocks, phones left on ring); LQT3 patients, whose risk is highest during sleep or rest, may benefit from home automated external defibrillator access and bedroom-sharing with a partner for sleep protection.[1]
- Left cardiac sympathetic denervation — now most often performed thoracoscopically, removing the lower half of the stellate ganglion (to avoid Horner syndrome) and the first four thoracic ganglia (T1-T4) — reduces sympathetic drive to the ventricle without the heart-rate reduction of a beta-blocker; in patients with recurrent ICD shocks or arrhythmic storms despite beta-blocker therapy, LCSD has shown an approximately 90% reduction in the mean annual number of ICD shocks and should be considered and discussed without hesitation once syncope recurs despite full-dose beta-blocker therapy.[1]
- ICD implantation for the specific indications above — a specialist decision made through shared decision-making, not a default response to a diagnosis of LQTS on its own.
- Genetic counselling and cascade family screening for every first-degree relative of a newly diagnosed patient is itself a core management intervention, not an optional add-on, given the disease's inherited basis.[6]
- Individualized, specialist-guided sports and activity counselling — the historically very conservative approach (blanket exclusion from competitive sport) is being progressively replaced by a more individualized conversation, particularly for patients with LQT2 or LQT3; decisions should still account for the fact that sports participation is regulated by specific law in some jurisdictions.[1]
- Avoidance of QT-prolonging drugs and situations wherever a reasonable alternative exists (see Pharmacologic Treatment Options and Drug Tables) — a lifestyle-level intervention in its own right for a patient with known LQTS.
- Newborn ECG screening in the first month of life is used in some programs specifically because up to 10% of sudden infant death syndrome cases, and some sudden deaths in utero, are now known to carry an LQTS-causing variant, and infants with a cardiac event in the first year of life are at very high risk and are poorly protected by traditional therapies — this remains a program-level, regional decision rather than a universal recommendation, but the rationale is genuine.[1]
Special Populations
Pregnancy and the postpartum period
Continue beta-blocker therapy through pregnancy in a patient with known LQTS — unsupervised discontinuation "to be safe" removes genuinely effective protection at a time when arrhythmic risk has not gone away. The postpartum period is a specific, elevated-risk window, particularly for LQT2, plausibly related to sleep disruption and the arrhythmogenic potential of rapid-eye-movement rebound sleep; a specific, low-cost recommendation for this period is for a partner to share the bedroom to provide sleep protection and be present if an event occurs.[1] Avoid new QT-prolonging drugs in pregnancy and postpartum wherever a reasonable alternative exists, and coordinate any necessary exception with the treating specialist.
Pediatrics and infants
- Dose beta-blockers by weight and re-titrate regularly as the child grows — a fixed adult dose is never appropriate.
- Infants with a cardiac event in the first year of life are at very high risk and are seldom fully protected by traditional therapies — this age group needs specialist-level management, not routine outpatient follow-up.[1]
- Predictive genetic testing is recommended for all at-risk family members from birth onward, with particular priority under age 18, because the risk of a cardiac event is greatest in childhood — do not wait until adolescence or adulthood to test a child from a known LQTS family.[6]
- Consider LQTS in a child or teenager with unexplained "seizures," particularly with an exertional, emotional, or startle trigger, or occurring during sleep — this is a genuine and recurring source of missed diagnosis.
Older adults
Age itself is not a primary LQTS risk factor, but older adults are disproportionately exposed to the drivers of acquired LQTS — polypharmacy, reduced renal and hepatic clearance of QT-prolonging drugs, comorbid structural heart disease, and a higher baseline likelihood of bradycardia or conduction disease. Medication reconciliation carries proportionally more weight in this population.
Renal and hepatic impairment
Reduce the dose of renally cleared QT-prolonging drugs (for example, sotalol) in renal impairment, since accumulation itself raises QT risk independent of any other factor. Domperidone is specifically contraindicated in moderate or severe hepatic impairment per Health Canada, in addition to its dose ceiling below.[4] Consult the Drug Database and a pharmacist for renal/hepatic dosing of mexiletine and other agents used in this chapter.
Sex differences
Women have a longer baseline QTc than men (reflected directly in the different Bazett upper-limit thresholds, 460 msec vs. 440 msec) and are substantially overrepresented among reported cases of drug-induced torsades de pointes — a genuine reason for extra vigilance when screening a new QT-prolonging drug for a female patient, independent of any known LQTS diagnosis.
Monitoring & Follow-Up
- Yearly specialist visit for risk reassessment is recommended for every congenital LQTS genotype, regardless of how well-controlled the patient currently appears — arrhythmic risk in LQTS is a genuinely moving target over time, not a one-time determination made at diagnosis.[1]
- Confirm adherence at every visit — this is not a formality here: nonadherence to beta-blocker therapy, together with exposure to QT-prolonging drugs, explains most of what looks like treatment failure in LQTS.[1]
- For patients on methadone maintenance therapy: obtain a baseline ECG before starting or soon after initiation, a follow-up ECG around 30 days or after a significant dose increase, and periodic ECGs thereafter per program protocol — well-established practice given methadone's dose-dependent QT effect.
- Obtain a follow-up ECG after starting any new QT-prolonging drug in a patient with known LQTS or a borderline baseline QTc, particularly after a dose increase.
- Monitor serum potassium and magnesium periodically in any patient on a diuretic, with GI losses, or with a known eating disorder, and correct proactively rather than waiting for a QTc to become abnormal.
- Confirm that first-degree relatives have actually completed recommended cascade screening — this is easy to lose track of over time and represents a genuine, correctable gap in care.[6]
- For patients with an ICD or after LCSD: confirm device/wound follow-up is arranged, counsel on recognizing signs of a device or lead complication, and provide driving-restriction guidance per local/provincial requirements after any syncopal or arrhythmic event.
- Reinforce genotype-specific counselling at every visit — the potassium target for LQT2, the swimming-supervision and confrontation-avoidance points for LQT1, and the home-AED/bedroom-sharing conversation for LQT3 are easy to mention once at diagnosis and then never revisit.[1]
Common Errors to Avoid
- Dispensing or prescribing a QT-prolonging drug without checking it against the patient's LQTS status and current medication list — the central, avoidable error this chapter is built around; use a maintained resource such as CredibleMeds/AZCERT's QTdrugs list rather than relying on memory for a list that changes as new evidence accumulates.[7]
- Choosing metoprolol, atenolol, or bisoprolol for LQTS "because it's a beta-blocker" — only nadolol and propranolol have confirmed efficacy in this specific condition.[1]
- Treating drug-induced torsades de pointes with another Class Ia or Class III antiarrhythmic (procainamide, amiodarone, sotalol) — these themselves prolong the QT interval and can worsen, rather than resolve, the arrhythmia. Use IV magnesium instead.
- Stopping a chronic LQTS beta-blocker abruptly, or during pregnancy "out of caution" — unsupervised discontinuation removes genuine protection, and nonadherence is already the leading driver of breakthrough events.[1]
- Relying on a single normal resting ECG to exclude LQTS — roughly a quarter of patients with a confirmed pathogenic variant have a normal baseline QTc (concealed LQTS), and exertional or postexercise recovery-phase prolongation is easy to miss without exercise testing.[1]
- Using an epinephrine provocation challenge to try to confirm the diagnosis outside a specialist center — it carries genuine arrhythmogenic risk of its own and can produce a misleading result in a person with a normal heart.[5][1]
- Dismissing exertional-, emotional-, or startle-triggered "seizures" or fainting spells in a child or teenager without considering LQTS, especially with a family history of unexplained sudden death or unexplained drowning.
- Screening only for a drug's direct QT effect and missing a clinically important interacting drug that raises its level or effect instead — for example, a strong CYP3A4 inhibitor added to a patient already taking domperidone or a macrolide.
- Assuming a genotype-negative genetic test rules out LQTS in a patient with a convincing clinical (Schwartz score) phenotype — 10-15% of clinically diagnosed patients test negative and carry a similar arrhythmic risk to genotype-positive patients.[1]
- Defaulting to ICD implantation as the "safe" first escalation rather than first optimizing beta-blocker adherence and adding mexiletine and/or left cardiac sympathetic denervation — genuine regional overuse of ICDs in LQTS is a recognized problem, and a device carries its own complication burden.[1]
- Overlooking the specific Health Canada dose ceilings for two very commonly dispensed QT-prolonging drugs — citalopram (maximum 40 mg/day under age 65, 20 mg/day at age 65 or older, or with hepatic impairment, poor CYP2C19 metabolizer status, or a concomitant CYP2C19 inhibitor) and domperidone (maximum 30 mg/day, with added caution and dose review over age 60) — particularly in a patient who already has another QT risk factor.[3][4]
Clinical Decision Algorithm
Drug Tables
How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.
Beta-Blockers for Congenital LQTS
Only nadolol and propranolol have confirmed efficacy in LQTS specifically — do not substitute a beta-1-selective agent.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Nadolol | 1.0-1.5 mg/kg/day PO, typically once daily; specialist-titrated to heart-rate response and symptoms | Bradycardia, fatigue, bronchospasm, masked hypoglycemia symptoms | Additive bradycardia with a non-dihydropyridine calcium channel blocker or digoxin | Long-acting, once-daily dosing supports adherence — nonadherence is the leading cause of apparent "beta-blocker failure" in LQTS.[1] |
| Propranolol | 2.0-3.5 mg/kg/day PO in 2-3 divided doses; specialist-titrated | Bradycardia, fatigue, bronchospasm, vivid dreams | Additive bradycardia with a non-dihydropyridine calcium channel blocker or digoxin | Equally effective to nadolol; more frequent dosing may be a real adherence disadvantage for some patients.[1] |
| Beta-1-selective beta-blockers (e.g., metoprolol, atenolol, bisoprolol) | Not appropriate as LQTS therapy | N/A | N/A | Efficacy in LQTS specifically has not been confirmed — do not substitute for nadolol or propranolol even though these are also "beta-blockers."[1] |
Sodium-Channel Blocker — Add-On for LQT3 and Refractory LQT2
Reserved for breakthrough symptoms despite an optimized nonselective beta-blocker.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Mexiletine | Assess with an acute oral drug test (6-8 mg/kg); if QTc shortens by more than 40 msec within 2 hours, start specialist-directed long-term dosing | Nausea, tremor, dizziness; hepatotoxicity with long-term use (monitor) | CYP1A2/CYP2D6-mediated interactions — review before adding a new interacting drug | Directly targets the SCN5A gain-of-function mechanism in LQT3; also shortens QTc in roughly 70% of LQT2 patients. Only patients with a positive acute drug test are started on chronic therapy.[1] |
Acute Management of Torsades de Pointes and Electrolyte Correction
IV magnesium is first-line for active torsades de pointes regardless of the measured serum magnesium level.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Magnesium sulfate | IV, per institutional torsades de pointes protocol | Flushing, hypotension with rapid administration, respiratory depression at high doses in renal impairment | Additive hypotension with other vasodilators/negative inotropes | First-line for active torsades de pointes — give even if the measured level is reported as normal.[2] |
| Potassium chloride (IV) | IV, per institutional protocol and degree of deficit | Phlebitis with peripheral administration, hyperkalemia with over-replacement | Additive hyperkalemia risk with potassium-sparing diuretics, ACE inhibitors/ARBs | Correct potassium to at least 4 mmol/L during an acute event.[2] |
| Potassium chloride (oral) | Oral maintenance dosing per institutional protocol, titrated to serum level | GI upset, hyperkalemia with over-replacement, especially in renal impairment | Additive hyperkalemia risk with potassium-sparing diuretics, ACE inhibitors/ARBs | A genuinely simple, high-value chronic maintenance measure, especially for LQT2.[1] |
| Isoproterenol | IV infusion, titrated to heart-rate response, as a bridge in bradycardia- or pause-dependent torsades de pointes | Tachyarrhythmias, ischemic chest pain, hypertension | Additive tachyarrhythmia risk with other sympathomimetics | Increasing the heart rate shortens the QT interval and reduces the arrhythmic substrate; overdrive pacing is the definitive bridge where available.[2] |
Antiarrhythmics That Themselves Prolong the QT Interval
Do not use one of these to try to treat drug-induced torsades de pointes — they can make it worse. Screen for these specifically before adding a rate- or rhythm-control agent in a patient with known or suspected LQTS.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Amiodarone | Per indication; not an LQTS-specific dose | QT prolongation, thyroid dysfunction, pulmonary/hepatic toxicity with chronic use | Numerous CYP3A4/CYP2C9-mediated interactions; raises digoxin levels | Long half-life means its QT effect can persist for weeks after the last dose.[7] |
| Sotalol | Per indication; not an LQTS-specific dose | QT prolongation, bradycardia, torsades de pointes (dose-dependent) | Renally cleared — accumulates and raises QT risk further in renal impairment | A known-risk QT-prolonging drug in its own right; requires renal dose adjustment and ECG monitoring even outside LQTS.[7] |
| Procainamide | Per indication; not an LQTS-specific dose | QT prolongation, hypotension, drug-induced lupus with chronic use | Additive QT effect with other QT-prolonging drugs | Avoid for treating drug-induced torsades de pointes — it prolongs the QT interval itself.[7] |
| Quinidine | Per indication; not an LQTS-specific dose | QT prolongation, "quinidine syncope" from torsades de pointes, GI upset | Raises digoxin levels; additive QT effect with other QT-prolonging drugs | A classic, historically significant cause of drug-induced torsades de pointes.[7] |
| Dofetilide | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes (dose- and renal-function-dependent) | Renally cleared — requires in-hospital initiation with renal-function-based dosing | Initiated only in a monitored inpatient setting specifically because of this risk.[7] |
Psychotropic Drugs Commonly Implicated in QT Prolongation
Screen carefully in any patient with known LQTS or another QT risk factor; a safer alternative agent should be genuinely considered before proceeding.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Haloperidol | Per indication; particular caution with IV administration | QT prolongation, torsades de pointes (dose-dependent, higher risk IV), extrapyramidal symptoms | Additive QT effect with other QT-prolonging drugs | IV administration and higher doses carry the greatest QT risk; continuous ECG monitoring is reasonable when used IV in an at-risk patient.[7] |
| Chlorpromazine | Per indication; not an LQTS-specific dose | QT prolongation, orthostatic hypotension, sedation | Additive QT effect with other QT-prolonging drugs | One of the older, well-documented QT-prolonging antipsychotics.[7] |
| Droperidol | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes — carries a specific regulatory boxed-warning history | Additive QT effect with other QT-prolonging drugs | Requires baseline ECG assessment per its product labelling in many jurisdictions before use.[7] |
| Pimozide | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes | Numerous CYP3A4/CYP2D6-mediated interactions raise levels and QT risk further | A known-risk QT-prolonging antipsychotic with a particularly narrow margin — avoid combining with other QT-prolonging or CYP-inhibiting drugs.[7] |
| Quetiapine | Per indication; not an LQTS-specific dose | QT prolongation (dose-dependent), sedation, orthostatic hypotension | Additive QT effect with other QT-prolonging drugs; CYP3A4 substrate | Risk rises meaningfully with overdose or rapid dose escalation.[7] |
| Ziprasidone | Per indication; not an LQTS-specific dose | QT prolongation — carries one of the more prominent QT warnings among second-generation antipsychotics | Additive QT effect with other QT-prolonging drugs | Baseline and interval ECG monitoring is reasonable in a patient with any other QT risk factor.[7] |
| Citalopram | Maximum 40 mg/day under age 65; maximum 20 mg/day at age 65 or older, with hepatic impairment, in poor CYP2C19 metabolizers, or with a concomitant CYP2C19 inhibitor | Dose-dependent QT prolongation | CYP2C19 inhibitors (e.g., omeprazole, fluconazole) raise citalopram levels and QT risk further | Health Canada issued specific, still-current dose ceilings after a dedicated thorough QT study showed a clear dose-response relationship — confirm the patient's dose and age against these limits at every dispense.[3] |
| Escitalopram | Per product monograph; lower maximum recommended in older adults, consistent with the citalopram QT signal | Dose-dependent QT prolongation, generally less pronounced than citalopram at equivalent doses | CYP2C19 inhibitors raise escitalopram levels and QT risk further | The active enantiomer of citalopram — carries a related, generally smaller QT signal; still deserves the same screening attention.[3] |
Other Commonly Encountered QT-Prolonging Drugs to Screen Before Dispensing
This is a representative, not exhaustive, list — check a maintained resource such as CredibleMeds/AZCERT for any drug not shown here before assuming it is safe.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Azithromycin | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes (uncommon but reported) | Additive QT effect with other QT-prolonging drugs | Macrolides as a class carry a QT signal; azithromycin's is generally the smallest of the three below but is not zero.[7] |
| Clarithromycin | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes | Strong CYP3A4 inhibitor — raises levels of many co-prescribed QT-prolonging drugs, compounding risk beyond its own direct effect | The CYP3A4 interaction risk is often more clinically important than clarithromycin's own direct QT effect — screen the whole regimen, not just this drug in isolation.[7] |
| Erythromycin | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes, especially with IV administration | Strong CYP3A4 inhibitor — same compounding risk as clarithromycin | IV erythromycin carries a particularly well-documented QT risk.[7] |
| Levofloxacin | Per indication; not an LQTS-specific dose | QT prolongation (dose-dependent) | Additive QT effect with other QT-prolonging drugs | A lower-risk fluoroquinolone than moxifloxacin, but not risk-free — still screen in a patient with other QT risk factors.[7] |
| Moxifloxacin | Per indication; not an LQTS-specific dose | QT prolongation — the highest-risk fluoroquinolone in common use | Additive QT effect with other QT-prolonging drugs | Consider a non-fluoroquinolone or a lower-risk fluoroquinolone alternative in a patient with known LQTS where clinically reasonable.[7] |
| Fluconazole | Per indication; risk rises with higher doses | QT prolongation (dose-dependent) | CYP3A4/CYP2C9 inhibitor — raises levels of many co-prescribed drugs, including other QT-prolonging agents | Azole antifungals as a class (itraconazole, ketoconazole, voriconazole, posaconazole) carry a similar combined direct-QT-effect-plus-CYP-inhibition risk profile.[7] |
| Ondansetron | Per indication; avoid single IV doses above the labelled maximum | Dose-dependent QT prolongation, most notable with rapid or high-dose IV administration | Additive QT effect with other QT-prolonging drugs | A single high-dose IV push is the highest-risk administration pattern — confirm the dose and infusion rate, not just that the drug was ordered.[7] |
| Domperidone | Maximum 30 mg/day; additional caution and dose review over age 60; contraindicated in moderate-severe hepatic impairment | QT prolongation, ventricular arrhythmia, sudden cardiac death (rare but the basis of the Health Canada advisory) | Contraindicated with potent CYP3A4 inhibitors and with other QT-prolonging drugs per Health Canada labelling | Confirm the daily dose and the patient's age against the specific Health Canada ceiling at every dispense — this is one of the more commonly missed QT-risk checks in routine community practice.[4] |
| Methadone | Per program protocol; QT risk is dose-dependent, most notable above approximately 100 mg/day | Dose-dependent QT prolongation, torsades de pointes | Numerous CYP3A4/CYP2D6-mediated interactions alter methadone levels; additive QT effect with other QT-prolonging drugs | Baseline ECG before or soon after initiation, a follow-up ECG around 30 days or after a significant dose increase, and periodic ECGs thereafter is well-established practice.[7] |
| Loperamide | Per labelled OTC dose; QT risk arises specifically with supratherapeutic/misuse doses | QT prolongation and torsades de pointes reported specifically with high-dose misuse ("the poor man's methadone") | Additive QT effect with other QT-prolonging drugs at high doses; interacting drugs that raise loperamide's CNS/cardiac exposure increase risk | At labelled OTC doses this is not a major QT concern — the risk is specifically with large supratherapeutic doses taken for opioid-like effect or to blunt opioid withdrawal; ask about actual quantity used, not just "loperamide, yes/no."[7] |
| Hydroxyzine | Use the lowest effective dose for the shortest duration; regulatory advisories have specifically restricted maximum daily doses because of QT risk | QT prolongation, torsades de pointes (dose-dependent), sedation | Additive QT effect with other QT-prolonging drugs | Easy to overlook as a QT risk because it is often used briefly and considered a low-stakes antihistamine — the QT signal is real and dose-related.[7] |
| Hydroxychloroquine | Per indication; not an LQTS-specific dose | QT prolongation, torsades de pointes (uncommon at standard doses, more notable with other QT drugs on board or renal impairment) | Additive QT effect with other QT-prolonging drugs, notably azithromycin when the combination is used | Chronic-use QT risk is generally modest at standard rheumatologic doses, but review the rest of the regimen before adding another QT-prolonging drug on top.[7] |
References
- Long QT Syndrome — New England Journal of Medicine, Massachusetts Medical Society (Schwartz PJ, Crotti L). N Engl J Med 2025;393(20):2023-2034
- The 2023 Canadian Cardiovascular Society Clinical Practice Update on Management of the Patient With a Prolonged QT Interval — Canadian Journal of Cardiology, Canadian Cardiovascular Society (Davies RA, Ladouceur VB, Green MS, Joza J, Juurlink DN, Krahn AD, McMurtry MS, Roberts JD, Roston TM, Sanatani S, Steinberg C, MacIntyre C). Can J Cardiol 2023;39(10):1285-1301
- Celexa (citalopram hydrobromide) - Association with Abnormal Heart Rhythms - For Health Professionals — Health Canada. Health Canada Recalls and Safety Alerts, 2012
- Domperidone Maleate - Association with Serious Abnormal Heart Rhythms and Sudden Death (Cardiac Arrest) - For Health Professionals — Health Canada. Health Canada Recalls and Safety Alerts, 2015
- 2022 ESC Guidelines for the Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death — European Heart Journal, European Society of Cardiology (Zeppenfeld K, Tfelt-Hansen J, de Riva M, et al.). Eur Heart J 2022;43(40):3997-4126
- Long QT Syndrome Overview — GeneReviews, University of Washington, Seattle (Groffen AJ, Bikker H, Christiaans I). GeneReviews, 2003 Feb 20 [Updated 2024 Mar 21]
- The CredibleMeds QTdrugs List — AZCERT, Inc. (Arizona Center for Education and Research on Therapeutics). CredibleMeds, maintained and updated on an ongoing basis
Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.