Reviewed By: Pramod Kerkar, MD, FFARCSI

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.

QT Interval

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.

How Long Is Too Long For A QTc Interval?

How Long Is Too Long For A QTc Interval?

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

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.

Causes of Long QT Syndrome

  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

Why Are We Worried About Prolong QTc?

Prolong QTc may lead to:

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

Summary

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:

Pramod Kerkar

Written, Edited or Reviewed By:

, MD,FFARCSI

Pain Assist Inc.

Last Modified On: October 15, 2018

This article does not provide medical advice. See disclaimer

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