QT Interval

Aka: QT Interval, QT Duration, QT Prolongation, Prolonged QT

1. Interpretation: Normal QT Duration
   1. QT modifying factors
      1. Normal QT decreases with increasing Heart Rate
      2. QT is longer in leads V2 and v3
   2. Calculation of QTc or corrected QT (Bazett's Formula)
      1. QTc = QT/(sqrt RR Interval)
      2. QTc is normally <0.44
   3. Approximation of normal QT
      1. QT Interval shortens with decreasing RR Interval
      2. QT = 0.5 x preceding RR Interval (if normal rate)
      3. Approximate normal QT Interval
         1. QT <= 0.38 if Heart Rate 80 bpm or greater
         2. Add 0.02 sec for every 10 bpm below 80
         3. Example: Normal QT <= 0.46 if Heart Rate 40-50
   4. Heart Rate determined QT
      1. 115 - 84 bpm: QT 0.30 to 0.37 seconds
      2. 83 - 72 bpm: QT 0.32 to 0.40 seconds
      3. 71 - 63 bpm: QT 0.34 to 0.42 seconds
      4. 62 - 56 bpm: QT 0.36 to 0.43 seconds
      5. 55 - 45 bpm: QT 0.39 to 0.46 seconds
2. Causes: Prolonged QT
   1. See Prolonged QT Interval due to Medication
   2. Familial long QT Syndrome
   3. Congestive Heart Failure
   4. Myocardial Infarction
   5. Hypocalcemia
   6. Hypomagnesemia
   7. Type I Antiarrhythmic drugs
   8. Rheumatic Fever
   9. Myocarditis
   10. Congenital Heart Disease
3. Causes: Shortened QT
   1. Digoxin (Digitalis)
   2. Hypercalcemia
   3. Hyperkalemia
   4. Phenothiazines
4. References
   1. Rollings (1984) Facts and Formulas, p. 64
   2. Mirvis in Braunwald (2001) Cardiovascular, p. 92-3