corrected QT Interval

(QTc)

The QT interval corrected for heart rate by Bazett, Fridericia, Framingham, or Hodges formulas.
It is inversely proportional to heart rate. The QT interval (QT) shortens at faster heart rates and the QT interval lengthens at slower heart rates.
If the T wave ends beyond the halfway point between the RR interval (RR), it is prolonged.
• RR = 60 / heart rate

(Formulas)
• Bazett formula: QTc = QT / square root of RR
• Fridericia formula: QTc = QT / cube root of RR
• Framingham formula: QTC = QT + 0.154 (1 - RR)
• Hodges formula: QTC = QT + 1.75 (heart rate - 60)

(Normal QTc interval)
• males: 350-450 ms
• females: 360-460 ms

(Causes of a prolonged QTc)
congenital long QT syndrome:
• altered DNA
acquired long QT syndrome:
• some medical conditions: E.g. intracranial hemorrhage, intracranial hypertension, myocardial ischemia, return of spontaneous circulation (ROSC) post-cardiac arrest.
• certain drugs (i.e. drug-induced long QT syndrome): E.g. erythromycin, azithromycin, certain antifungals for yeast infections, diuretics that induce hypokalemia, anti-arrhythmics, some antidepressants and antipsychotics, some antiemetics.
• mineral imbalances or electrolyte abnormalities: E.g. hypocalcemia, hypomagnesemia, and hypokalemia.

(Causes of a short QTc)
• hypercalcaemia
• congenital short QT syndrome
• digoxin effect: Digoxin produces a relative shortening of the QT interval, along with downward sloping ST segment depression in the lateral leads, widespread T-wave flattening and inversion, and a multitude of arrhythmias.

(Complications)
• Long QT syndrome is associated with an increased risk of ventricular arrhythmias (esp. Torsades de Pointes), ventricular fibrillation, and sudden death.
• Congenital short QT syndrome is associated with an increased risk of paroxysmal atrial, ventricular fibrillation, and sudden cardiac death.