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. Complications: QT Prolongation resulting in Torsades de Pointes

  1. See Torsades de Pointes for management
  2. QT Prolongation risks R-On-T Phenomenon (PVC on T Wave) provoking polymorphic VT (Torsades)
    1. Consider baseline EKG prior to administering medications that may increase QTc
  3. Caution in patients with pre-existing QT Prolongation (QTc >450 ms) when prescribing new medications
    1. See Prolonged QT Interval due to Medication
    2. Select medications that do not prolong the QT Interval further
    3. Stop medication causes of Prolonged QT when QTc rises above 500 ms
    4. Avoid Beta Blockers, due to Syncope risk, precipitating Torsades (3.6 fold increased risk)
      1. Jons (2010) J Am Coll Cardiol 55(8): 783-8 [PubMed]
  4. Not all Prolonged QT intervals significantly predispose to Torsades
    1. Degree of QT Prolongation does not correlate with risk of Torsades
    2. Medication specific effects (e.g. some medications are more likely to result in Torsades)
      1. See Prolonged QT Interval due to Medication for likelihood of Torsades
      2. Avoid combining multiple medications each of which increase QT Interval
  5. Coexisting conditions which make Drug-Induced Torsades de Pointes more likely
    1. Baseline QT Prolongation
    2. Bradycardia
    3. Acute Myocardial Infarction
    4. Low Ejection Fraction
    5. Hypokalemia
    6. Hypomagnesemia
    7. Volume depletion (Vomiting, Diuretics)
    8. Female gender (long QT Interval at baseline)
  6. 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)

V. Causes: Shortened QT (<330 ms)

  1. Digoxin (Digitalis)
  2. Hypercalcemia
  3. Hyperkalemia
  4. Phenothiazines
  5. Sympathomimetics
  6. Familial (cardiac ion channel abnormalities)

VI. Complications: Shortened QT (<330 ms)

VII. References

  1. Joshi and Dermark (2016) Crit Dec Emerg Med 30(8):3-12
  2. Mirvis in Braunwald (2001) Cardiovascular, p. 92-3
  3. Rollings (1984) Facts and Formulas, p. 64

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Ontology: Prolonged QT interval (C0151878)

Definition (NCI_CDISC) An electrocardiographic finding in which the QT interval not corrected for heart rate is prolonged. Thresholds for different age, gender, and patient populations exist.
Definition (NCI) An electrocardiographic finding in which the QT interval not corrected for heart rate is prolonged. Thresholds for different age, gender, and patient populations exist. (CDISC)
Concepts Finding (T033)
SnomedCT 123332001, 142130001, 111975006
English QT PROLONGED, 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, Increased Q-T interval, Increased Q-T interval (finding), PROLONGED QT, Prolonged QT, Prolonged QT Interval by EKG Finding, Prolonged QT Interval, Prolonged QT Interval by ECG Finding, Prolonged QT interval, Increased QT interval, Prolonged QT interval (finding), 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, Intervalo QT del electrocardiograma prolongado, QT ALARGADO, aumento del intervalo QT, intervalo Q - T prolongado - RETIRADO -, intervalo Q - T aumentado, intervalo Q - T prolongado - RETIRADO - (concepto no activo), 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