Eight years old female presented to the pediatric cardiology ward due to palpitations. Paroxysmal palpitations, no syncope thus far. Family history unremarkable.
The ECG showed the following:
Sinus rhythm, HR 133 bpm, QT interval 350ms. R-R interval 450ms. QTc according to Bazett´s formula 522ms. Now, since Bazett´s formula is known to under-correct at high heart rates, sometimes Fridericia´s formula is used to correct for HR. Still, with Fridericia´s formula the QTc is 457ms.
QTc is considered normal up to 440ms in boys/men and 450ms in girls/ 460ms in women. Reference cut-offs vary a little according to source.
A QTc >500ms (Bazett) is associated with a higher risk of arrhythmias. Therapy includes Beta-blockers (Nadolol or Propranolol) to shorten QTc and decrease arrhythmic risk.
Long QT syndrome is an inherited channelopathy, based on genetic mutations. Currently, LQTS can be classified into more than 10 subtypes, of which LQTS 1, LQTS 2, and LQTS 3 are most common. It can be associated with congenital deafness, as in Jerwell-Lange-Nielsen Syndrome.
A specific arrhythmia associated with LQTS is Torsade-de-pointes tachycardia, which shows a typical spindle-shaped QRS morphology. An example of this arrhythmia is shown in the blog entry from March 28, 2022 "A perfect storm".
Further useful information on LQTS can be read in an article by Peter Schwartz et al. https://www.ahajournals.org/doi/10.1161/circep.111.962019