II. Interpretation: Normal QT Duration

  1. Always confirm Prolonged QT with a manual measurement (QTc >500 ms)
    1. Computer read EKG can over-measure QTc (e.g. in presence of U Waves)
  2. QT modifying factors
    1. Normal QT decreases with increasing Heart Rate
    2. QT is longer in leads V2 and v3
  3. Calculation of QTc or corrected QT (Bazett's Formula)
    1. QTc = QT/(sqrt RR Interval)
    2. QTc is normally <0.44
  4. 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
  5. 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

IV. Causes: Shortened QT

V. Complications: QT Prolongation resulting in Torsades de Pointes

  1. See Torsades de Pointes for management
  2. Exercise caution in patients with QT Prolongation when prescribing new medications
    1. See Prolonged QT Interval due to Medication
    2. Select medications that do not prolong the QT Interval further
  3. Not all Prolonged QT intervals significantly predispose to Torsades
    1. Medication specific effects (e.g. some medications are more likely to result in Torsades)
    2. Degree of QT Prolongation does not correlate with risk of Torsades
  4. Evaluate context of QT Prolongation
    1. QT Prolongation and reassuring findings that require no change in regimen
      1. Chronic, stable, asymptomatic use of a medication associated with QT Prolongation
    2. QT Prolongation and red flag findings that suggest further evaluation or regimen change
      1. Family History of Sudden Cardiac Death
      2. Symptoms (high risk group that requires thorough evaluation)
        1. Syncope
        2. Palpitations
        3. Spells or Seizures (presenting event in 10% of pediatric cases)

VI. References

  1. Rollings (1984) Facts and Formulas, p. 64
  2. Mirvis in Braunwald (2001) Cardiovascular, p. 92-3

Images: Related links to external sites (from Google)

Ontology: Prolonged QT interval (C0151878)

Definition (NCI) A QT interval that is prolonged. The length of the QT interval varies with the heart rate.
Definition (NCI) A QT interval that is prolonged. The length of the QT interval varies with the heart rate. (NCI)
Concepts Finding (T033)
SnomedCT 123332001, 142130001, 111975006
English QT INTERVAL PROLONGED, LONG QT, Q-T interval, increased, QT INTERVAL INCREASED, ELECTROCARDIOGRAPHIC, QT PROLONGED, QT INCREASED, Increased Q-T interval (finding), Prolonged QT Interval, PROLONGED QT, Prolonged QT, Electrocardiogram QT prolonged, QT increased, QT interval prolonged, Long QT, QT prolonged, interval prolong qt, interval prolonged qt, intervals prolong qt, long qt, increased qt, prolong qt, qt prolonged, intervals prolonged qt, prolonged qt interval, prolonged qt, qt interval prolonged, Prolonged QT interval, Increased QT interval, Prolonged QT interval (finding), Increased Q-T interval, Increased Q-T interval -RETIRED-
Italian Intervallo QT dell'elettrocardiogramma, prolungato, QT prolungato, Intervallo QT prolungato, QT lungo, QT aumentato
Dutch QT verhoogd, lang QT, QT-interval verlengd, QT verlengd, elektrocardiogram QT verlengd
French QT long, Intervalle QT prolongé, QT augmenté, QT prolongé, ALLONGEMENT DU QT, Intervalle QT prolongé à l'électrocardiogramme
German QT verlaengert, QT-Intervall verlaengert, langes QT, QT erhoeht, QT-VERLAENGERUNG, Elektrokardiogramm QT verlaengert
Portuguese QT longo, Intervalo QT prolongado, QT sumentado, QT prolongado, QT PROLONGADO, Intervalo QT prolongado (ECG)
Spanish QT largo, Intervalo QT alargado, QT aumentado, QT prolongado, Prolonged QT interval, Intervalo QT del electrocardiograma prolongado, QT ALARGADO, aumento del intervalo QT, intervalo Q - T aumentado, intervalo Q - T prolongado - RETIRADO - (concepto no activo), intervalo Q - T prolongado - RETIRADO -, intervalo QT prolongado (hallazgo), intervalo QT prolongado
Japanese QT間隔延長, 心電図QT延長, QT延長, QTエンチョウ, QTカンカクエンチョウ, シンデンズQTエンチョウ
Czech QT interval prodloužený, QT prodloužení, Prodloužený QT interval na elektrokardiogramu, Dlouhý QT interval, Prodloužení QT
Hungarian Hosszú QT, QT megnyúlt, Elektrokardiogram QT megnyúlt, QT idő megnyúlt, QT emelkedett