QTc interval

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The QTc interval represents the QT interval at a standardized heart rate of 60/minute. It can be calculated by various correction formulae. However while in populations there is a relationship between the RR interval and the QT interval regrettably for a human individual none of these are likely to be correct[1]

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  • Your ECG machine may calculate the QTc for you. You then have a major issue as you probably don't know the formula used by the machine and you might not know how to interpret an abnormal result fully in the patient context...but such interpretation can be very important
  • Calculate QTc manually from either lead V3 or V4, the others will introduce a 10% or more error either way [2]

Correction formula in common use include:

  1. Bazett’s correction: QTc =QT/RR0.5 (=QT/√RR)
    • Bazett’s correction is frequently used in clinical practice and in the medical literature. It overcorrects at elevated heart rates above about 110/minute and under corrects at heart rates below 60 bpm. It is also the least accurate within the range 60 to 110/minute.[3] These problems have been known since at least 1961. In clinical practice this might not matter as on fact with a cut off of 440msec it may be as good as any other at making you suspect a long QT syndrome[4].
  2. Fridericia’s correction: QTc = QT/RR0.33
    • Better than Bazett's and by internal consensus should be measured as well as Bazett's correction when evaluating drugs[5]
  3. Framingham correction QTc = QT + 0.154(1-RR)
  4. Hodges correction QTc = QT + 1.75 (heart rate - 60)
    • Does not seem too bad but Bazett's won clinical hearts and minds by many years.
  5. Nomogram formulae.
    • The most accurate, using a correction factor for each heart rate but impracticable in clinical practice. It should be used in experimental individuals and critical studies.[6]

Matters get even more complex when using exercise ECGs and the various formulas[7]. In drug development you will often see the term QT/QTc interval to cover everything discussed above. QT dispersion is also a useful measure.

External links

References

  1. Malik M, Färbom P, Batchvarov V, Hnatkova K, Camm AJ. Relation between QT and RR intervals is highly individual among healthy subjects: implications for heart rate correction of the QT interval. Heart (British Cardiac Society) 2002;87(3):220-8.
  2. Sadanaga T, Sadanaga F, Yao H, Fujishima M. An evaluation of ECG leads used to assess QT prolongation. Cardiology 2006;105:149-54. (Direct link – subscription may be required.)
  3. Dogan A, Tunc E, Varol E, Ozaydin M, Ozturk M. Comparison of the four formulas of adjusting QT interval for the heart rate in the middle-aged healthy Turkish men. Annals of noninvasive electrocardiology : the official journal of the International Society for Holter and Noninvasive Electrocardiology, Inc 2005;10:134-41. (Direct link – subscription may be required.)
  4. Brouwer J, Van Den Berg MP, Grobbee DE, Haaksma J, Wilde AA. Diagnostic performance of various QTc interval formulas in a large family with long QT syndrome type 3: Bazett's formula not so bad after all... Annals of noninvasive electrocardiology : the official journal of the International Society for Holter and Noninvasive Electrocardiology, Inc 2003;8:269-74.
  5. International Conference on Harmonisation E14 Guideline: The clinical evaluation of QT/QTc interval prolongation and proarrhythmic potential for non-antiarrhythmic drugs. 2005.
  6. Malik M. Problems of heart rate correction in assessment of drug-induced QT interval prolongation. Journal of cardiovascular electrophysiology 2001;12:411-20.
  7. Benatar A, Decraene T. Comparison of formulae for heart rate correction of QT interval in exercise ECGs from healthy children. Heart (British Cardiac Society) 2001;86:199-202.
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