Long QT

Prolonged duration of ventricular repolarisation which ↑ risk of polymorphic ventricular tachycardia. Polymorphic VT:

The QT interval varies with and must be corrected for the heart rate.

Bazett’s Formula suggests:

\[QTc = {QT \over \sqrt{R \textendash R \ Interval}}\]

• May:
  • Resolve spontaneously
  • Degenerate to VF
• Be triggered by:
  • ↑ Sympathetic tone
    • Exercise
    • Sudden loud noises
    • Emotional stress
    • Pain
• Occurs with ↑ risk with ↑ QTc
  
  • Borderline 440-470ms
  • High with QTc >500ms

Epidemiology and Risk Factors

Pathophysiology

Myocardial repolarisation:

• Occurs due to a combination of:
  • Outward K⁺ flow
  • Inward Ca⁺ and Na⁺ flow
    Partially counters K⁺ current.
• Is not homogenous between epicardium, mid-myocardium, and endocardial cells
  • Mid-myocardium is slower due to reduced K⁺ and Na⁺ channel density
    The tail of the T-wave tends to represent repolarisation of mid-myocardial cells.
• ↓ mid-myocardial K⁺ or ↑ Na⁺ channel function in the mid-myocardium will therefore prolong the action potential
  • This may lead to late calcium inflow, and early after-depolarisations
    • If these EADs reach a threshold amplitude, VT may result

Aetiology

May be:

• Congenital
  1/2,500-5,000. Multiple major genotypes have been developed, which include:
  • Jervell–Lange-Nielsen
    Autosomal recessive.
  • Romano–Ward
    Autosomal dominant.
• Acquired
  Likely associated with some genetic abnormality. Include:
  • Drugs
  • Electrolyte abnormalities
  • Malnutrition
    In addition to any risk due to neurological abnormalities.
  • Neurological injury

Clinical Manifestations

History:

• Sudden syncope
  30% of congenital present with syncope or aborted sudden death.
• Pseudo-seizures during exercise or stress
• Family history of sudden death

Diagnostic Approach and DDx

Investigations

ECG:

• QTc >440ms
  May only be revealed in situations of physiological stress (e.g. a stress ECG.

Management

Resuscitation:

1g MgSO₄ is ~4mmoL of Mg²⁺.

• C
  • Polymorphic VT/VF
    • DC cardioversion
    • Mg²⁺
      Reduces EADs by ↑ encouraging resting repolarisation.
      • Given as 2g MgSO₄ over 2-3 minutes
        May be repeated with another 2g after 15 minutes.
      • Consider infusion of 3-20mg/min if arrhythmias persist

Specific therapy:

• Pharmacological
  • Antiarrhythmic
    • β-blockade
• Procedural
  • Pacing
    ↑ HR and reduces QTc.
  • AICD

Supportive care:

Disposition:

Medical management:

• β-blockade
• Pacing
  ↑ HR and reduces QTc.
• AICD

Potential Precipitants

Drugs associated with development of polymorphic VT:

• Anti-arrhythmics
  • Sotalol
  • Amiodarone
  • Procainamide
  • Flecainide
  • Disopyramide
• Antibiotics
  • Erythromycin
  • Clarithromycin
  • Fluconazole
• Anti-psychotics
  • Droperidol
  • Haloperidol
• Anti-depressants
  • Fluoxetine
• Sympathomimetics
  • Ketamine

Anaesthetic Considerations

• B
  • Maintain normocarbia
• C
  • Malignant arrhythmia
    • High risk of intraoperative arrhythmia
    • May be precipitated by ↑ sympathetic tone
      • Intraoperative sympatholysis is crucial
    • Cardiology review pre-operatively is ideal
  • Avoid bradycardia
  • Avoid hypotension
• E
  • Avoid reversal of neuromuscular block
    QTc is prolonged with neostigmine-glycopyrrolate.

Marginal and Ineffective Therapies

Complications

Prognosis

Key Studies

---

References

1. Hunter JD, Sharma P, Rathi S. Long QT syndrome. Contin Educ Anaesth Crit Care Pain. 2008 Apr 1;8(2):67–70.