Following notes will give you an idea how to calculate QT interval
LQTS: Calculating the corrected QTc
QTc= QT/square root of the RR interval
This will correct QT for the heart rate- there is normally an inverse relation between heart rate and QTinterval,as one goes up/the other goes down and vice versa
Long QT by definition QTc in - males >470 ms
- females of > 480 ms
Borderline prolonged QTc is present if the corrected QTc is 450-470 ms
Average QTc for someone with the LQTS is 490 ms
Diagnostic Criteria for LQTS
Certain points are given to each criteria
ECG findings
QTc
>480 3
460-470 2
450 (male) 1
Torsdade De Pointes 2
T-wave alternans 1
Notched T wave in 3 leads 1
Low heart rate for age 0.5
Clinical History
Syncope with stress 2
without stress 1
Congenital deafness 0.5
Family history
Definite LQTS 1
Unexplained SCD in immediate family member that is less than 30 years of age 0.5
<1 points low probability of QT prolongation
2-3 points intermediate probability
>4 points high probability
