Long QT Syndrome: What You Need to Understand

The Scene

The following scene is drawn from the composite of patients I have cared for in clinic. All identifying details are changed.

Elena is 17 years old when she collapses during competitive swim practice. She had been in the water for 40 minutes. She did not feel anything unusual before it happened; she was mid-lap when she went under. Her coach was there. They pulled her out of the water in under 30 seconds. She had no pulse. CPR was started and an AED was used. It delivered one shock. She had a pulse within 90 seconds.

At the hospital, her ECG shows a QTc interval of 490 milliseconds. Normal is below 440 ms in females. A genetic panel is sent. The result comes back four weeks later: a pathogenic variant in KCNQ1, the gene encoding the cardiac potassium channel. The diagnosis is Long QT Syndrome Type 1.

Her mother and older brother are tested. Both carry the same variant. Neither had symptoms before.

Two family members. One variant. The same gene. Three different clinical situations. This is the complexity of inherited channelopathy medicine.

Long QT syndrome is one of the most important inherited cardiac conditions a cardiologist encounters. It is silent on most days, then lethal on one. It is responsible for a substantial fraction of sudden cardiac deaths in children and young adults. It is also one of the most manageable inherited cardiac conditions if identified before the first event. The QTc interval on a routine ECG is the entry point for diagnosis, and the implications extend to every medication decision a patient or physician makes for the rest of that patient’s life.

What It Is

Long QT syndrome (LQTS) is a disorder of cardiac ion channels that prolongs the QT interval on the ECG, reflecting delayed ventricular repolarization. The QT interval represents the time from the beginning of ventricular depolarization (the Q-wave or start of the QRS) to the end of ventricular repolarization (the end of the T-wave). A prolonged QT interval creates a longer window of electrical vulnerability during which an early beat (typically a PVC) can trigger a dangerous arrhythmia called torsades de pointes (TdP), which can degenerate into ventricular fibrillation and cardiac arrest.

QTc interval: Because the QT interval shortens at faster heart rates and lengthens at slower rates, the measurement is corrected for heart rate to produce the QTc (corrected QT interval). The most commonly used correction formula is Bazett’s correction: QTc = QT / sqrt(RR interval in seconds). Normal QTc is below 440 ms in men and below 460 ms in women. A QTc above 480 ms is abnormal; above 500 ms carries a substantially increased risk of TdP. The QTc varies by up to 10-20 ms within and between measurements; a single borderline QTc should be confirmed on repeat ECGs.

Inherited LQTS arises from mutations in genes encoding cardiac ion channels or channel-associated proteins, present from birth. The three most common types:

• LQTS Type 1 (KCNQ1): Deficiency of the slow potassium current (I-Ks), which is critical for rate adaptation during exercise. Events in LQTS1 are characteristically triggered by exercise and swimming, when the failure of I-Ks to adapt at faster heart rates prolongs the QT disproportionately. Beta-blockers are highly protective in LQTS1.

• LQTS Type 2 (KCNH2/HERG): Deficiency of the rapid potassium current (I-Kr). Events are characteristically triggered by auditory stimuli (alarm clocks, phone ringing, sudden loud noise) and by emotion. Women with LQTS2 have a higher event rate than men, particularly postpartum. Beta-blockers are moderately protective; avoidance of QT-prolonging drugs is critical.

• LQTS Type 3 (SCN5A): Gain-of-function mutation in the sodium channel (I-Na), causing persistent sodium influx during repolarization, which prolongs the action potential duration and QT interval. Events in LQTS3 occur at rest or during sleep (low heart rate), when the sodium leak is most prominent relative to compensatory currents. Beta-blockers are less effective than in LQTS1 or LQTS2; sodium channel blockers (mexiletine) are genotype-directed options.

Acquired LQTS is not inherited but arises from drug effects on cardiac ion channels or from metabolic conditions (hypokalemia, hypomagnesemia, hypothyroidism) that prolong repolarization in otherwise normal hearts. Drug-induced QT prolongation is the most common cause of acquired TdP and represents a major medication safety concern.

The Mechanism

Ventricular Repolarization and the QT Interval

Normal ventricular repolarization depends on a precisely timed sequence of ion channel openings and closings. After depolarization (the QRS complex), the ventricle returns to its resting state through the coordinated actions of outward potassium currents (I-Ks, I-Kr, I-K1) that bring the membrane potential back to baseline. Any disruption of these currents extends the duration of the action potential and lengthens the QT interval.

Different channels contribute at different phases:

• I-Kr (the rapidly activating delayed rectifier, encoded by KCNH2/HERG) is the dominant repolarizing current and is the target of most QT-prolonging drugs. It has a unique structural feature: the channel pore has a high-affinity binding site for lipophilic drugs that enter the cell during repolarization, allowing many structurally unrelated drugs to inadvertently block it.
• I-Ks (the slowly activating delayed rectifier, encoded by KCNQ1) provides critical reserve repolarizing capacity, particularly during exercise when faster heart rates demand rapid repolarization. LQTS1 patients lose this reserve.

Torsades de Pointes: The Dangerous Arrhythmia

Torsades de pointes (French: “twisting of the points”) describes a polymorphic VT in which the QRS axis rotates cyclically, producing the characteristic spindle-shaped ECG appearance. TdP is initiated by the “R-on-T” mechanism: a PVC falls during the prolonged vulnerable period of the repolarization (the T-wave), triggering a reentrant circuit that uses the heterogeneously recovered ventricular myocardium.

TdP episodes are often self-terminating (producing palpitations or presyncope without syncope). Prolonged TdP can degenerate into VFib and cardiac arrest. The unpredictability of this transition is what makes LQTS dangerous: a patient can have dozens of brief self-terminating TdP runs without knowing it, and then the next one does not terminate.

Short-long-short sequence: TdP in drug-induced LQTS is classically preceded by a characteristic sequence: a short coupling interval (ectopic beat), followed by a long pause, followed by the initiating beat of TdP. This sequence corresponds to the R-on-T PVC hitting the maximally prolonged QT during the pause. This is why bradycardia and pauses increase TdP risk: the longer the pause, the longer the QT, the more vulnerable the ventricle.

How We Diagnose

ECG Measurement

QTc measurement requires care: measure the QT interval on multiple leads (the longest QT in any lead is used), calculate the RR interval from the preceding P-P interval (not the P-R), apply Bazett’s correction. At very fast or very slow heart rates, Bazett’s correction introduces substantial error; the Fridericia correction (QTc = QT / cuberoot(RR)) is more accurate at extremes of rate.

T-wave morphology provides additional diagnostic information:

• LQTS1: broad-based T-waves
• LQTS2: low-amplitude, notched T-waves
• LQTS3: late-peaking T-waves with a long isoelectric segment before them

These patterns are not diagnostic alone, but in combination with the QTc and clinical history, they guide genetic testing.

The Schwartz Score

The Schwartz score (Schwartz PJ, et al. Circulation. 1993; DOI: 10.1161/01.CIR.88.2.782) provides a clinical probability framework for LQTS diagnosis based on ECG findings, clinical history, and family history. Points are assigned for QTc prolongation, T-wave notching, bradycardia, syncope on exertion or stress, family history of LQTS, and unexplained sudden cardiac death in a first-degree relative under age 30. Score above 3.5: high probability; 1-3: intermediate; below 1: low. The Schwartz score guides genetic testing decisions and clinical management 5 / Solid .

Genetic Testing

Genetic testing for LQTS has a detection rate of approximately 70-75% in patients with a clinically confirmed LQTS diagnosis (QTc above 480 ms with an appropriate clinical history). In patients with borderline QTc (440-480 ms) and a positive family history, the yield is lower. The most commonly mutated genes are KCNQ1, KCNH2, and SCN5A (LQT1, 2, and 3 together account for approximately 75% of genotype-positive cases). A positive genetic test confirms the diagnosis and enables family screening. A negative genetic test does not exclude LQTS in a patient with clear clinical criteria.

The Evidence

Beta-Blockers: The Cornerstone of Therapy

Beta-blockers are the primary pharmacologic treatment for inherited LQTS and are particularly effective for LQTS1 and LQTS2.

LQTS1: The protective mechanism of beta-blockers in LQTS1 is specific and elegant: by reducing heart rate during exercise, beta-blockers reduce the demand on I-Ks (the deficient current), lowering the probability that repolarization will be inadequate during exertion. In large multicenter registries, beta-blocker therapy in LQTS1 reduces the risk of cardiac events (syncope, aborted arrest, or sudden death) by approximately 64% 5 / Solid . Nadolol (non-selective, long-acting) is preferred over metoprolol; atenolol is considered inferior. Propranolol is also used, though adherence is lower due to its multiple daily dosing.

LQTS2: Beta-blockers reduce events in LQTS2 but are moderately less protective than in LQTS1. LQTS2 patients also benefit critically from avoidance of all drugs that prolong the QT interval and from measures to prevent auditory startle (alarm clocks that vibrate rather than ring, gradual alarm tones).

LQTS3: Beta-blockers provide limited benefit and the risk of bradycardia-induced QT prolongation must be monitored. Mexiletine (a late sodium channel blocker) shortens the QTc in LQTS3 patients by approximately 30-50 ms and is used as a genotype-directed adjunct 4 / Promising .

ICD Decisions in LQTS

The ICD in LQTS serves as a safety net for patients at highest risk. Current indications 5 / Solid :

Class I: Secondary prevention: any LQTS patient who has survived a cardiac arrest or sustained VT/VFib.

Class IIa: Primary prevention in LQTS patients with:

• Syncope and/or VT while receiving beta-blocker therapy
• LQTS2 or LQTS3 with QTc above 500 ms despite maximum tolerated therapy
• LQTS3 patients who are intolerant of beta-blockers

Class IIb: Asymptomatic LQTS with QTc above 500 ms on beta-blocker therapy.

The decision to implant an ICD in an asymptomatic young patient with LQTS is not straightforward. The device prevents sudden death but carries its own risks: inappropriate shocks (approximately 10-20% over 5 years), lead complications, infection, and the psychological burden of carrying a device that may fire. Every ICD decision in LQTS should involve a shared decision-making discussion and ideally consultation with an inherited arrhythmia specialist.

The Drug-Induced QT Problem: A Medication Safety Crisis

Drug-induced QT prolongation causing TdP is responsible for numerous post-market drug withdrawals and is one of the leading reasons a drug is withdrawn from clinical use. The QT-prolonging drug list is extensive:

Antiarrhythmics: Sotalol, dofetilide, and quinidine prolong QT as part of their mechanism (Class III effect). These drugs are contraindicated in patients with baseline QTc above 450 ms.

Antibiotics: Azithromycin (Z-pack), ciprofloxacin, and moxifloxacin prolong QT. In patients with known LQTS or other QT risk factors, these antibiotics should be used only when clinically necessary, with ECG monitoring 5 / Solid .

Antipsychotics: Haloperidol, quetiapine, risperidone, and ziprasidone all prolong QT. The risk is higher at higher doses and with IV administration.

Antiemetics: Ondansetron (Zofran) prolongs QT at high doses (IV 32 mg, which was withdrawn by the FDA for this reason in 2012). Oral 4-8 mg doses are lower risk but warrant attention in LQTS patients.

Antifungals: Fluconazole and voriconazole prolong QT.

The clinical implication: every patient with known LQTS must carry a list of prohibited and caution-required medications. The CredibleMeds QTDrugs database (crediblemeds.org) maintains a current, categorized list of QT-prolonging drugs updated regularly. Patients and prescribers should consult it before any new prescription.

The risk of drug-induced TdP is multiplied by:

• Baseline QTc prolongation (any cause)
• Hypokalemia (enhances QT prolongation from any drug)
• Hypomagnesemia (reduces threshold for TdP)
• Bradycardia (pauses potentiate TdP)
• Female sex (higher baseline QTc and greater susceptibility to drug-induced prolongation)
• Combinations of QT-prolonging drugs (additive effect)

Sex Differences

Women have a baseline QTc approximately 10-15 ms longer than men after puberty, attributable to sex hormonal modulation of cardiac ion channels (estrogen reduces I-Kr) 5 / Solid . This creates an inherently higher baseline vulnerability to repolarization prolongation. Consequences:

• Drug-induced TdP is 2-3 times more common in women than in men at equivalent drug concentrations
• LQTS2 events are more common in women, particularly postpartum (a period of rapidly falling estrogen and rising adrenergic activation)
• Women are more likely to have subclinical LQTS that becomes manifest only with drug challenge

The practical implication: when prescribing any QT-prolonging medication to a woman, the baseline QTc should be checked first. A QTc of 470 ms in a woman before starting azithromycin may warrant an alternative antibiotic.

The Patient Experience

Living with LQTS

The daily reality of inherited LQTS involves two simultaneous challenges: the biological one (managing risk) and the psychological one (living under the awareness of a risk that is largely invisible and unpredictable).

Most patients with LQTS will never have a cardiac arrest. Beta-blockers substantially reduce event risk. The QTc on the ECG is a number, not a certainty. But the patient who has been told they carry a pathogenic KCNQ1 variant lives with the awareness that swimming, competitive athletics, or a morning alarm clock carries a risk that a friend without the variant does not.

What Your Doctor Will Not Have Time to Explain

1. Your medication list now matters more than it ever has. Every new prescription requires QT review. Keep a copy of your CredibleMeds list and show it to every prescriber (including dentists who may give you antibiotics). Do not start a new medication without checking it against this list.

2. Beta-blockers for LQTS must not be stopped abruptly. Stopping a beta-blocker suddenly increases sympathetic rebound and substantially raises short-term event risk. If you need to stop your beta-blocker for any reason (surgery, side effects), discuss the taper with your cardiologist first.

3. Family members need ECGs. LQTS is autosomal dominant (one copy of the gene is sufficient to produce the condition). A first-degree relative (parent, sibling, child) of an LQTS patient has a 50% probability of carrying the same variant. An ECG and, if the family variant is known, targeted genetic testing should be offered to all first-degree relatives. Identifying an asymptomatic carrier before the first event is the goal.

4. Exercise restrictions are genotype-specific. LQTS1 patients have the highest risk during exercise and competitive athletics. Current guidelines recommend against competitive sports for untreated LQTS1. Treated LQTS1 patients may engage in recreational (non-competitive) exercise at moderate intensity with appropriate monitoring. LQTS3 patients have their highest risk at rest or during sleep, and the exercise restriction rationale is different.

Sex-Specific Patient Considerations

Women with LQTS should discuss the postpartum period specifically with their cardiologist before pregnancy. The first 9 months postpartum represent a period of increased LQTS2 event risk, attributed to falling estrogen levels and rising adrenergic tone during breastfeeding. Beta-blocker continuation through pregnancy and the postpartum period is safe for most women and is specifically recommended for LQTS2 patients during this window 5 / Solid .

Decisions and Trade-Offs

Beta-Blocker for Life

For virtually all patients with confirmed pathogenic LQTS1 or LQTS2 variants, nadolol or propranolol is recommended for life. The decision to start is straightforward given the protective evidence. The decisions that require nuance:

• Which beta-blocker: Nadolol (preferred, once daily, non-selective) versus propranolol (three times daily). Atenolol should be avoided; its protection is inferior in registry data. Metoprolol’s non-selectivity limitations make it a second-line option.
• Dose optimization: The target is a resting heart rate of 50-60 bpm and an adequate rate response to exercise (verified on exercise stress test). Underdosing provides inadequate protection; overdosing causes fatigue and bradycardia.
• Adherence: Beta-blocker non-adherence in LQTS is a significant cause of preventable sudden death. The event risk in patients with LQTS who discontinue beta-blockers without medical supervision is substantially higher than in those on therapy.

ICD vs. Beta-Blocker Alone

For high-risk patients (prior cardiac arrest, syncope on beta-blocker therapy, very prolonged QTc above 500 ms), the ICD adds protection that beta-blockers alone cannot provide. The shared decision-making conversation for an 18-year-old with LQTS2 and a QTc of 510 ms who has had one syncopal episode on nadolol requires careful weighing of ICD benefit (prevents the next VFib) against ICD burden (inappropriate shocks, device revisions, 40+ years of device management for a patient implanted in adolescence).

Subcutaneous ICD (S-ICD) is increasingly preferred in young LQTS patients because it avoids transvenous leads and their long-term lead complications; however, the S-ICD cannot deliver pacing (relevant for patients who develop pauses or who need anti-tachycardia pacing). The PRAETORIAN trial included LQTS patients; S-ICD was non-inferior to transvenous ICD for shock efficacy 5 / Solid .

Three Questions to Ask Your Cardiologist

1. “What type of LQTS do I have (Type 1, 2, or 3), and does the genotype change my specific exercise restrictions, my most dangerous triggers, and my medication management?”
2. “Should my family members be screened? If the variant is known, can we offer targeted genetic testing to my children and siblings before they develop symptoms?”
3. “What is my current risk classification based on my QTc, genotype, and symptom history, and does that change the recommendation about whether an ICD adds net benefit over beta-blockers alone for me specifically?”

The SDE Synthesis

Long QT syndrome exists at the intersection of inherited predisposition and the medical system’s obligation to prevent a death that is preventable. Most patients with LQTS who die of sudden cardiac arrest did not know they had it. The family member who was never screened. The athlete whose syncope was attributed to dehydration. The teenager prescribed azithromycin by a physician who did not know she had a borderline QTc.

The SDE framework engages with inherited cardiac risk systematically: genetic counseling, family cascade screening, and the medication review that LQTS patients need every time they interact with the healthcare system. An SDE Audit for any patient with unexplained syncope, family history of sudden cardiac death at a young age, or borderline QTc on a routine ECG includes the clinical pathway to establish or exclude LQTS before the first preventable event.

Cross-links within the SDE system: The Foundations article on Brugada Syndrome (SDE-F-RHTM-011) covers the other major inherited sodium channel channelopathy. The Foundations article on WPW (SDE-F-RHTM-012) covers the accessory pathway disorder that can produce VFib by a different mechanism. The Foundations article on the ICD (SDE-F-DEVI-014) and the S-ICD (SDE-F-DEVI-015) provide device context for the ICD decision in LQTS.