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QT Interval

QT interval

Corrected QT Interval (QT_c)

• QT interval changes with heart rate
• QT shortens with increasing frequency
• QT lengthens with decreasing frequency
• Only short QT syndrome has a QT interval independent of frequency
• Normal QT interval is 0.35s - 0.44s.
• This applies only to a rate of 60/min.
• Corrected QT interval (QT_c)
• Is calculated for frequencies other than 60/min.
• QT_c is the QT interval corrected to a rate of 60/min.
• Calculator for (QT_c)
  
  
• The following patient has a QT_c of 0.4s (neither prolonged nor shortened)
• Heart rate: 40/min., QT: 0.49s, QT_c: 0.4s
• Heart rate: 60/min., QT: 0.40s, QT_c: 0.4s
• Heart rate: 80/min., QT: 0.35s, QT_c: 0.4s
• Heart rate: 100/min., QT: 0.31s, QT_c: 0.4s

ECG and QT Interval

• It is best visible in leads: II, V5, V6
• It is the distance from the beginning of the Q wave to the end of the T wave
• It ranges from 0.35s to 0.44s.
• This applies to a heart rate of 60/min.
  
  
• QU interval = QT interval
• If the T and U waves are merged, then we measure the QU interval
  
  
• JT interval = QT interval
• If there are wide QRS complexes (ventricular rhythm), QT is measured:
• From the J point to the end of the T wave
• JT_C > 330ms (analogous to QT_C > 440ms)
  
  
• For assessing the QT interval, we always use QT_C
• QT_C is prolonged if it is:
• in men > 440ms
• in women > 460ms
• QT_C > 500ms (risk of Torsades de Pointes)
• QT_C is shortened < 350ms
• (risk of ventricular fibrillation and atrial fibrillation)

Prolonged QT_C > 440ms

• The QT interval is prolonged if it is:
• QT_C in men > 440ms
• QT_C in women > 460ms
• For QT_C > 500ms (risk of Torsades de Pointes)
  
  
• Most common causes of prolonged QT interval
• Hypokalemia
• Hypomagnesemia
• Hypocalcemia
• Hypothermia
• Myocardial ischemia
• Intracranial hypertension
• Long QT Syndrome
• Post-cardiac arrest syndrome
• There are more than 100 medications that prolong the QT interval

Prolonged QT Interval

• QT_C = 0.58s

Short QT_C < 350ms

• QT interval is shortened if
• QT_C < 350ms
• Can lead to ventricular fibrillation
  
  
• Most common causes of a shortened QT interval
• Hypercalcemia
• Short QT Syndrome
• Digoxin

Short QT Interval

• QT_C = 0.32s

QT Interval (0.4s), QT_C (0.4s)

• Frequency 60/min.
• No need to calculate QT_C (at 60/min frequency, QT = QT_C)
• QT_C interval (0.4s) is not prolonged (0.35s - 0.44s)
• QT is less than half of the RR interval
• QT (0.4s) < 1/2 RR (0.49s)
• This rule applies only at frequencies of 60-100/min.

QT Interval (0.3s), QT_C (0.45s)

• Frequency 136/min.
• QT Interval (0.3s)
• QT_C interval (0.45s) is prolonged (>0.44s)
• QT_C calculation

Wide QRS Complexes and QT Interval (JT Interval)

• In ventricular rhythm, QRS complexes are widened (>0.12s)
• As a secondary effect, QT interval also prolongs
• QT prolongs due to the widened QRS complex (depolarization)
• QT does not prolong due to widened T wave (repolarization)
• Therefore, widened QT interval in wide QRS complexes is irrelevant
• It does not indicate the risk of Torsades de Pointes
• In wide QRS complexes, measure the JT interval and corrected (JT_C) interval
• Calculated exactly like QT and QT_C
• J point is the point where the ST segment deviates from the QRS complex
• JT = 0.24s
• JT_C = 0.31s
• JT_C > 0.33s is considered prolonged
• It is analogous to QT_C > 0.44s

QT Interval and U Wave

• If the U wave is separated from the QT interval but is higher than 50% of the T wave
• There is no academic consensus on whether it should or should not be included in the length of the QT interval

Hyperkalemia and Shortened QT_C (0.32s)

• Hyperkalemia shortens the QT interval
• QT_C = 0.32s (< 0.35s)
• The patient has an increased risk of ventricular fibrillation