Exam

QT Interval

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QT Interval, QT Duration, Electrocardiogram QT corrected Interval, QTc Interval, QTc, QT Prolongation, Prolonged QT, Long QT, Pronged Qtc, QTc Prolongation

  • Definitions
  1. QT Interval
    1. EKG interval between the begining of the QRS interval and the end of the T Wave
    2. Represents ventricular electrical activity (from ventricular depolarization to repolarization)
  • 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 <0.45 in men (<0.46 in women and children)
    3. Prolonged QTc (esp. >0.5) is at risk of Torsades de Pointes (see below)
  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
  • Causes
  • Prolonged QT (QTc > 500 ms)
  1. See Prolonged QT Interval due to Medication
  2. Familial Long QT Syndrome (congenital Prolonged QT, cardiac ion channel abnormalities)
    1. Jervall-Lange-Nielsen Syndrome (associated with Deafness)
    2. Romano-Ward Syndrome (not associated with Deafness)
  3. Congestive Heart Failure
  4. Cardiomyopathy
  5. Myocardial Infarction
  6. Hypocalcemia
  7. Hypokalemia
  8. Hypomagnesemia
  9. Type I Antiarrhythmic drugs (Sodium channel blocking drugs)
  10. Rheumatic Fever
  11. Myocarditis
  12. Congenital Heart Disease
  13. Hypothermia
  14. Increased Intracranial Pressure
  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)
  • Causes
  • Shortened QT (<330 ms)
  1. Digoxin (Digitalis)
  2. Hypercalcemia
  3. Hyperkalemia
  4. Phenothiazines
  5. Sympathomimetics
  6. Familial (cardiac ion channel abnormalities)
  • Complications
  • Shortened QT (<330 ms)
  • 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