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QTc: how long is too long?
  1. J N Johnson1,
  2. M J Ackerman1,2,3
  1. 1
    Department of Pediatrics/Division of Pediatric Cardiology, Mayo Clinic, Rochester, Minnesota, USA
  2. 2
    Department of Medicine/Division of Cardiovascular Diseases, Mayo Clinic, Rochester, Minnesota, USA
  3. 3
    Department of Molecular Pharmacology & Experimental Therapeutics, Mayo Clinic, Rochester, Minnesota, USA
  1. Correspondence to Dr M J Ackerman, Long QT Syndrome Clinic and the Mayo Clinic Windland Smith Rice Sudden Death Genomics Laboratory, Mayo Clinic, Guggenheim 501, 200 First Street SW, Rochester, MN 55905, USA; ackerman.michael{at}


Congenital long QT syndrome (LQTS) affects an estimated 1 in 2500 people and typically presents with syncope, seizures or sudden death. Whereas someone exhibiting marked prolongation of the QT interval with QTc exceeding 500 ms who was just externally defibrillated from torsades de pointes while swimming poses negligible diagnostic challenge as to the unequivocal probability of LQTS, the certainty is considerably less for the otherwise asymptomatic person who happens to host a QTc value coined “borderline” (QTc ⩾440 ms). Although a normal QT interval imparts a much lower risk of life-threatening events, it does not preclude a patient from nevertheless harbouring a potentially lethal LQTS-causing genetic mutation. Indeed, genetic testing exerts significant diagnostic, prognostic and therapeutic implications. However, the 12-lead ECG remains the universal initial diagnostic test in the evaluation of LQTS and is subject to miscalculation, misinterpretation and mishandling. This review discusses the components of accurate QTc measurement and diagnosis, re-examines what is known about factors affecting QT interval measurement, and clarifies current recommendations regarding diagnosis of so-called “borderline” QT interval prolongation. The current guideline recommendations for the athlete with LQTS are also summarised.

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  • Competing interests MA is a consultant for PGxHealth, Medtronic, Pfizer, Mayo Clinic Health Solutions, the Mayo Clinic Windland Smith Rice Sudden Death Genomics Laboratory and receives royalties from LQTS and CPVT genetic testing (FAMILION) by PGxHealth.

  • Provenance and peer review Commissioned; not externally peer reviewed.