• Action Potential and QT Interval
• Channelopathy
• ECG and Long QT Syndrome
• Congenital Long QT Syndrome
• Associated LQT Syndromes
• Acquired Long QT Syndrome

Action Potential and QT Interval

• During the action potential
• Ions pass through ion channels in the cardiomyocyte membrane
• Electrical potential changes
• A action potential curve is generated
  
  
• Ventricular action potential creates on ECG:
• QRS complex - ventricular depolarization
• T wave - ventricular repolarization
  
  
• With intact ion channels, the
• QT interval is 0.35 - 0.44s
  
  
• With ion channel dysfunction
• Action potential changes and subsequently, the ECG curve changes
• QT interval may prolong or shorten

ECG and Long QT Syndrome

• It is an extended corrected QT (QT_C) > 440ms
  
  
• QT interval shortens in tachycardia and lengthens in bradycardia
• Therefore, corrected QT (QT_C) is calculated
• This does not apply to short QT syndrome (SQTS)
• SQTS has a QT interval independent of heart rate
  
  
• Long QT syndrome is divided into:
• Congenital long QT syndrome
• Acquired long QT syndrome

Prolonged QT Interval

• QT_C = 0.58s (QT_C > 440ms)

Congenital Long QT Syndrome

• It is a congenital channelopathy where the QT_C > 440ms
• With a prolonged QT interval, regardless of the cause, malignant arrhythmia (such as Torsades de Pointes) is a risk
• Arrhythmia triggers: physical exertion, emotional stress, sleep...
• T wave alternans worsens prognosis
  
  
• There are 13 genetic mutations that prolong the QT interval
• Long QT Syndrome has 13 types (LQTS 1-13), each type has:
• Different mutated channels during repolarization (for potassium, calcium, sodium)
• Different action potential curve
• Prolonged QT interval
• Different shape of T waves (repolarization)
• Normal QT interval does not exclude LQTS
• QT can be prolonged with stress or exertion
• The most common are the first 3 types (LQTS 1-3)
• They account for 95% of all LQT syndromes
• LQTS 4-13 are very rare

Associated LQT Syndromes

• Some genotypes of long QT syndrome are part of other syndromes
• Jervell and Lange-Nielsen Syndrome (LQTS5 + Congenital Deafness)
• Sometimes, any LQTS with deafness is generally referred to as Jervell and Lange-Nielsen Syndrome
• Romano-Ward Syndrome (LQTS1 without Deafness)
• Sometimes, any LQTS without deafness is generally referred to as Romano-Ward Syndrome
• Andersen-Tawil Syndrome (LQTS7 + Skeletal Deformities)
• Timothy Syndrome (LQTS8 + Autism + Immunodeficiency)
• In practice, these syndrome names are rarely used
• Only the term Long QT Syndrome is commonly used

Congenital Long QT Syndrome (LQTS 1)

• Heart Rate: 65/min.
• Corrected QT (QT_C): 520ms
• QT Interval: 0.5s
• The patient has been ruled out for acquired long QT syndrome

Congenital Long QT Syndrome (LQTS 3)

• Heart Rate: 90/min.
• Corrected QT (QT_C): 490ms
• QT Interval: 0.4s
• The patient has been ruled out for acquired long QT syndrome

Acquired Long QT Syndrome

• Heart Rate: 110/min.
• Corrected QT (QT_C): 515ms
• QT Interval: 0.38s
• The patient had hypomagnesemia

Acquired Long QT Interval Syndrome

• Corrected (QT_C): 630ms
• QT interval 0.78s
• Patient was hypothermic
• Sinus bradycardia: 40/min.
• Osborn wave (V2)
• AV block I degree (PQ interval 0.42s in lead II)

Acquired Long QT Interval Syndrome

• Frequency: 95/min.
• Corrected (QT_C): 620ms
• QT interval 0.55s
• Patient had Intracranial Hypertension due to subarachnoid hemorrhage
• Cerebral (deep) T waves (V3-V6, II, III, aVF)

Acquired Long QT Interval Syndrome and Torsades de Pointes

• Frequency: 52/min.
• Corrected (QT_C): 559ms
• QT interval 0.6s
• Patient was taking sotalol (antiarrhythmic)
• Torsades de Pointes (second half of the ECG)
• Prolonged QT interval, regardless of the cause, represents a risk of Torsades de Pointes
• Torsades de Pointes often starts with a short-long-short RR sequence