#### 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. 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
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)

#### 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