Side Effect: QT Interval Prolongation

What is the QT interval?

The QT interval is measured using a painless procedure called an electrocardiogram. This is often abbreviated as an ECG, or EKG. Electrode pads are placed on the chest and measure the electrical conductivity through the heart. There are different phases of a heart beat, with the QT-phase being one of them.

A normal QTc interval is:

• Between 390 - 460 milliseconds for a woman
  • Some references use an upper limit of normal of 480 msec
• Between 390 - 450 milliseconds for a man
  • Some references use an upper limit of normal of 470 msec

Importantly, up to 20% of healthy people have a QTc interval outside of this range

What does QT Interval Prolongation look like?

QT prolongation is when the QT interval takes longer to complete that usual. When determining if the QT interval is prolonged due to medications, a corrected value is used, known as the QTc interval.

• Although one can often feel when the heart beats very fast, or if it skips a beat, usually people cannot feel if the time it takes for their heart to beat is increasing. This is because it the prolongation takes place in only a fraction of a second.

Who gets QT Interval Prolongation?

Several causes of QT prolongation exist. These include:

• Genetic causes
• Medications
  • Certain antibiotics
  • Certain antiarrhythmics
  • Certain antipsychotics
  • Certain antinausea drugs
• Low blood potassium
• Low blood magnesium

How to prevent QT Interval Prolongation

QTc prolongation can sometimes be minimized by having a doctor or pharmacist look over your medication list to see if any of your medications may be contributing.

If you have been told that your QTc is close to the upper limit of the normal range, you should make sure that any new medications are screened for the possibility of further prolonging the QTc interval before you take them. This will reduce the risk of developing dangerous changes in heart rhythm, such as torsades de pointe.