Long QT Syndrome

• Prolonged QT interval
• "The 99th percentile for QTc in adult males is 470 ms and in adult females 480 ms, with significant overlap between the normal spectrum and genetically affected individuals with no or only mild QT prolongation" (Circulation;129:1524)
  
• Predisposes to ventricular tachyarrhythmias (PVT, VT, Torsades, VF) which can lead to syncope, cardiac arrest, or sudden death
• Congenital - rare
• Medication induced - common
• Blockade of cardiac potassium efflux
• Risks
• Female sex
• Electrolyte disturbances (hypoK+ and hypoMg2+ and hypoCa2+)
• Hypothermia
• Abnormal thyroid function
• Structural heart disease
• Bradycardia
• Medications
  

Patients with Long QT Syndrome should AVOID

• Competitive sports, strenuous exercise, and stress-related emotions
• Medications
  

Torsades

• Diagnosed only in setting of prolonged QT interval
• If no underlying Long QT then it is PVT
• IV Magnesium
• Overdrive Pacing
  

How to measure the QT

• The QT interval is measured from the beginning of the QRS to the end of the T-wave
• The QT represents ventricular depolarization and subsequent repolarization
• Generally look at lead II or V5/V6
• Measure multiple beats using longest interval
• Use the maximum slope intersection with the baseline to define the end of the T-wave

Diagnosis

• QT interval varies with ventricular rate
• The QT shortens at faster heart rates
• The QT lengthens at slower heart rates
  
• QTc = QT corrected
• QTc = QT / [SQRT (R-R Interval)]
• QTc should be less than 0.450 seconds
• QTc > 0.5 seconds increases risk of tachyarrhythmias

Reference: http://emedicine.medscape.com/article/157826-overview
Reference: http://img.medscape.com/pi/emed/ckb/cardiology/150072-157826-1861.jpg
Reference: http://lifeinthefastlane.com/ecg-library/tdp/
Reference: http://lifeinthefastlane.com/ecg-library/basics/qt_interval/
Reference: http://3.bp.blogspot.com/-CdiiruGfA-c/U44onUQp0WI/AAAAAAAAEHk/tJJjV2_tk-w/s1600/Measurement+of+QT.png
Reference: http://semj.sums.ac.ir/vol11/apr2010/88044.htm
Reference: Top Ten (or 11) EKG Killers. Micelle Haydel, MD. LSUHSC New Orleans