Prolonged QT

A prolonged QT interval - which represents a lengthier time for the ventricles to depolarize and repolarize - increases the chance that a beat gets transmitted while the heart is still repolarizing. The biggest concern, the R-on-T phenomenon, results from an impulse occurring during the previous T wave while the ventriclar membrane is still in the process of recovering. This can transform the rhythm into one of the lethal arrhythmias like Torsades de Pointes.

When a patient has a prolonged QT, avoid starting new medications or increasing the dose of medications the patient is already taking that are known to prolong the QT interval. These drugs include:

  • Antiarrhythmics (e.g. amiodarone, procainamide)
  • Anti-psychotics (e.g. haloperidol, amitriptyline)
  • Certain antibiotics (e.g. fluoroquinolone, ciprofloxacin, azithromycin)
  • Methadone
  • Ondansetron

Corrected QT interval

Since the QT interval varies with heart rate, the corrected QT interval (QTc) attempts to standardize this value. Using QTc, a prolonged QT interval is generally defined as:

  • greater than 0.44 s in males, or
  • greater than 0.46 s in females

In both cases, a QTc greater than 0.5 s is generally seen as concerning with the potential to transform into Torsades de Pointes.

There are a number of ways to calculate QTc. The way that many clinician's were taught to determine the QTc was using Bazett's formula:

QTQTc

Unfortunately, Bazett's formula is unreliable at very low and very fast heart rates. There are a number of other QTc calculators out there that tries to take into account the extremes in heart rate, and many of them are available on the MDCalc QTc calculator page.