Long QT and Channelopathies — SAQ

Stem

A candidate is asked to manage a classic presentation of Long QT and Channelopathies in an exam setting. Use precise definitions, scores, doses, and decision thresholds.

Core knowledge (model answer backbone)

Long QT syndrome and the related cardiac channelopathies (Brugada, CPVT, short QT, early repolarisation) are inherited arrhythmia syndromes caused by mutations in cardiac ion-channel genes that predispose young, structurally normal hearts to syncope, torsades de pointes, ventricular fibrillation and sudden cardiac death.^[1] The dominant therapy is beta-blockade (nadolol or propranolol for LQTS and CPVT), avoidance of QT-prolonging drugs, lifestyle modification, and ICD implantation for secondary prevention or high-risk primary prevention.^[1] Acute torsades de pointes is treated with a slow 2 g IV magnesium push, defibrillation if pulseless or sustained, and rate acceleration by overdrive pacing at 90 to 110 beats/min for pause-dependent forms — isoprenaline (10 to 20 microgram IV push, or an infusion titrated to about 100 beats/min) is reserved for acquired long QT, because it is contraindicated in congenital LQTS where it can paradoxically lengthen the QT.^[2]

Red flags^[1][2]

• Syncope during exercise, swimming, auditory stimulation, or emotional stress in a young patient - think LQT1, LQT2 or CPVT - exercise restriction, beta-blocker, urgent cardiology referral
• Torsades de pointes in a patient on a QT-prolonging drug (macrolide, fluoroquinolone, antipsychotic, methadone, amiodarone, sotalol, ondansetron, haloperidol) - stop the drug, IV magnesium 2 g, correct K+ and Mg2+, defibrillation if sustained
• Brugada type 1 pattern (coved ST elevation in V1-V2) with syncope or VF - ICD is the only proven therapy; avoid sodium-channel blockers; treat fever aggressively
• Family history of sudden cardiac death in a relative under 40, unexplained drowning, or single-vehicle crash - cascade screening of first-degree relatives with ECG, exercise test, and genetic testing

High-yield structure examiners expect

Cover: Overview & Definition, Classification, Epidemiology & Risk Factors, Pathophysiology, Clinical Presentation, Differential Diagnosis.

Key doses / thresholds (sourced)

• Magnesium sulfate 2 g by slow IV push is first-line in torsades, then an infusion of 1 to 4 g per hour to keep serum magnesium above 2 mmol/L, stopping once it exceeds 3 mmol/L — a supratherapeutic target, so check levels: toxicity above 3.5 mmol/L causes confusion, respiratory depression, coma and cardiac arrest^[2]
• Replace potassium to a target of 4.5 to 5 mmol/L while treating torsades^[2]
• Unstable torsades with a pulse: synchronised cardioversion (100 J monophasic, 50 J biphasic); pulseless torsades: defibrillate^[2]
• Pause-dependent torsades: overdrive pacing at 90 to 110 beats/min. Isoproterenol (10 to 20 microgram IV push, or an infusion titrated to a rate of about 100 beats/min) is an option in acquired long QT only — it is contraindicated in congenital LQTS because it can paradoxically lengthen the QT^[2]
• LQTS diagnostic thresholds (2013 HRS/EHRA/APHRS): QTc 500 ms or more on repeated ECGs with secondary causes excluded, a Schwartz score of 3.5 or more, or a pathogenic LQTS variant at any QTc^[1][3]
• Mexiletine 6 to 8 mg/kg/day is the gene-specific add-on in LQT3^[1]

Questions

a) Define the condition and give the most important classification or severity framework used in exams. (3 marks)

• Clear one-line definition matching standard teaching.
• Named classification / stages / types with discriminating features.
• One sentence on why classification changes management.

b) Outline pathophysiology in a mechanism chain that explains the main clinical features. (3 marks)

• Initiating insult → intermediate pathway → end-organ effect.
• Link at least two symptoms/signs to mechanism.
• Mention one complication pathway (e.g. shock, perforation, herniation, arrhythmia).

c) List discriminating clinical features and bedside assessment. (3 marks)

• Classic presentation plus one atypical group (elderly, pregnancy, child, immunocompromised).
• Named signs/manoeuvres if relevant.
• What must never be missed on exam/bedside (pregnancy test, airway, glucose, etc.).

d) Investigations with thresholds and one named score if applicable. (3 marks)

• First-line tests and what positive findings mean.
• Gold-standard or definitive investigation when needed.
• Score components reproduced exactly if a named score is standard for this topic.

e) Immediate resuscitation and definitive management with doses where standard. (3 marks)

• ABC / time-critical steps first.
• First-line drug(s) with agent + dose + route (or procedure steps).
• Escalation triggers (theatre, ICU, thrombolysis window, antidote, etc.).
• Disposition and safety-netting.

Marking tips

Full marks require specificity (numbers, names, doses) not generic "give antibiotics/fluids." Regional practice (ICMR / NICE / AHA) may be cited as alternative where relevant.

References3ShowHide

1. [1]Priori SG, Wilde AA, Horie M, et al. HRS/EHRA/APHRS expert consensus statement on the diagnosis and management of patients with inherited primary arrhythmia syndromes: document endorsed by HRS, EHRA, and APHRS in May 2013 and by ACCF, AHA, PACES, and AEPC in June 2013 Heart Rhythm, 2013.PMID 24011539
2. [2]Cohagan B, Brandis D Torsade de Pointes StatPearls, 2026.PMID 29083738
3. [3]Schwartz PJ, Crotti L. QTc behavior during exercise and genetic testing for the long-QT syndrome Circulation, 2011.PMID 22083145