How Long Is Too Long For A QTc Interval?

Most of you must have seen an ECG strip; in that you have seen there is a wave pattern. The ECG shows the electrical impulses generated by your heart that are being transferred form electrodes which are attached to your skin. The electrical activity of the heart represent as the PQRST wave.

The QT interval seen in the ECG is measured from the beginning of the QRS complex (starting point of the Q wave) to the end of the T wave. It represents the amount of time the ventricles (lower heart chambers) takes to contract and relax.

The normal QT interval varies with age, gender and heart rate. The QT interval is longer when the heart rate is slow and the QT interval is shorter when the heart rate is fast. So to overcome this corrected QT (QTc) is calculated by using the Bazett formula or Fridericia’s formula.

QTcB = QT/√2(R − R) Bazett’s square root formula

QTcF = QT/√3(R − R) Fridericia’s cube root formula

• R-R interval is the distance from one R wave to the next R wave. It can be measured directly from the ECG if not can be measured by dividing 60 from the patient’s heart rate.
• A useful rule of thumb is that a normal QT is less than half the preceding RR interval.
  • The normal QTc interval is taken 0.36 – 0.44s.
  • Prolong QTc interval in males >0.47s
  • Prolong QTc interval in females >0.48s
  • If the QT interval is more than 0.50s it’s considers as a dangerous level for any age or gender.

Long QT syndrome (LQTS) is a disease which affects the repolarization (relaxation) of the heart muscles. The symptoms of faintish attacks and palpitations can occur; some may be asymptomatic or have a family history of LQTS.

1. Congenital (75%)
   • Jervell–Lange–Nielsen (autosomal recessive)
   • Romano–Ward (autosomal dominant)
   • Single gene mutations LQT1 – LQT10
2. Acquired (25%)
   • Electrolyte abnormalities
     • Hypokalemia
     • Hypomagnesemia
     • Hypocalcemia
   • Drugs
     • Quinidine, disopyramide
     • Sotalol, amiodarone
     • Tricyclic antidepressants – amitriptyline
     • Phenothizine drugs – chlorpromazine
     • Antipsychotics -haloperidol, olanzapine
     • Macrolides – erythromycin
     • Quinolones – ciprofloxacin
     • Methadone
   • Poisons
     • Organophosphate insecticides
   • Miscellaneous
     • Bradycardia
     • Mitral valve prolapse
     • Acute myocardial infarction
     • Diabetes
     • Prolonged fasting state
     • Protein diets (long-term)
     • Central nervous system diseases – dystrophia myotonica

Prolong QTc may lead to:

• Ventricular arrhythmias (ventricle rhythm abnormalities)
• Cardiac arrest
• Sudden death

QT interval is measured form the beginning of the QRS complex to the end of the T wave. QT interval represents the ventricular contraction and relaxation. The normal QT interval varies with age, gender and heart rate. The QT interval is longer when the heart rate is slow and the QT interval is shorter when the heart rate is fast. So to overcome this corrected QT (QTc) is calculated by using the Bazett formula or Fridericia’s formula. The normal QTc range is 0.36-0.44s. Prolong QTc in adult males >0.44s and in adult females>0.46s. QTc more than 0.5s is considered a dangerous level for any age or gender. Long QT syndrome (LQTS) is a disease that affects the repolarization of the heart. There are several causes and 75% of the time the cause is a congenital problem. Acquired LQTS is by electrolyte imbalances, drugs and by other miscellaneous causes. LQTS may lead to ventricular arrhythmias, cardiac arrest and sudden death.

Also Read:

• Long QT Syndrome: Causes, Symptoms, Treatment
• How Serious Is Long QT Syndrome?
• Is Long QT Syndrome A Genetic Disorder?
• What Are The Home Remedies For Long QT Syndrome?
• What Are The Lifestyle Changes For Long QT Syndrome?
• What Drugs To Avoid If You Have Long QT Syndrome?
• How Many Types Of Long QT Syndrome Are There?