• Action Potential and QT Interval
• Channelopathy
• Short QT Syndrome (SQTS)
• Acquired Short QT Interval
• ECG and Short QT Syndrome

Action Potential and QT Interval

• During action potential
• Ion channels in the cardiomyocyte membrane allow ions to pass through
• Electrical potential changes
• Action potential curve is generated
  
  
• Ventricular action potential creates the following on the ECG:
• QRS complex - ventricular depolarization
• T wave - ventricular repolarization
  
  
• With intact ion channels:
• QT interval 0.35 - 0.44s
  
  
• With disruption of ion channels:
• Action potential changes and consequently the ECG curve changes
• QT interval prolongs or shortens

Short QT Syndrome (SQTS)

• It is a congenital channelopathy where the QT < 340ms
• Not divided into acquired and congenital like Long QT Syndrome
• SQTS has a QT interval independent of heart rate
• Thus, corrected QT (QT_C) does not need to be calculated
• "Pseudonormalization" of the QT interval
• During tachycardia, QT_C may be normal
• Therefore, QT interval in SQTS should be evaluated at a rate of 60/min.
• There are 5 genetic mutations that shorten the QT interval
  
  
• Short QT Syndrome has 5 types (SQTS 1-5), each type has:
• Different mutated channels during repolarization (for potassium, calcium)
• A different curve of action potential
• Shortened QT interval
• Different shape of T wave (repolarization)
• The association of ion channel mutation with QT interval length:
• Mutation of potassium channels (SQTS 1-3) results in QT < 320ms
• Mutation of calcium channels (SQTS 4-5) results in QT < 340ms
• With a shortened QT interval, regardless of the cause, the risks include:
• Atrial Fibrillation
• Ventricular Fibrillation
• Sudden Cardiac Death

ECG and Short QT Syndrome

• Short QT < 340ms
• QT < 320ms is associated with a defect in (K⁺) channels (SQTS 1-3)
• QT < 340ms is associated with a defect in (Ca⁺) channels (SQTS 4-5)
• Short QT interval does not change with heart rate
• Absent ST segment (or very short ST segment)
• Spiked T waves in precordial leads (V1-V6)

Short QT and Ventricular Fibrillation

• The patient had SQTS 1
• Short QT interval 0.24s
• With every patient with a short QT, the risks include:
• Atrial Fibrillation
• Ventricular Fibrillation
• Sudden Cardiac Death

Short QT < 340ms

Short QT Syndrome (SQTS 1)

• Heart rate: 60/min.
• QT interval in the patient did not change with heart rate
• Short QT interval: 240ms

Short QT Syndrome (SQTS 1)

• Heart rate: 60/min.
• QT interval in the patient did not change with heart rate
• Short QT interval: 300ms

Acquired Short QT Interval

• Heart rate: 65/min.
• The patient had hyperkalemia (7.1 mmol/l)
• Short QT interval: 0.34s
• Corrected QT (QT_C) 0.35s (short QT has a threshold of approximately < 0.34s)
• Acquired short QT interval changes with heart rate, so corrected QT (QT_C) is calculated
• Peaked T waves in the precordial leads (V1-V6)

Acquired Short QT Interval

• Heart rate: 90/min.
• Atrial Fibrillation
• Heart rhythm is irregularly irregular
• The patient was on digoxin and was not overdosed. Digoxin can cause ECG changes even without overdose:
• Short QT interval: 260ms
• Corrected QT (QT_C) 0.31s
• Acquired short QT interval changes with heart rate, so corrected QT (QT_C) is calculated
• Descending ST depressions (II, aVF, V1-6)
• Negative T waves (V1-3), biphasic T waves (V4-6)