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How will you identify Long QT Syndrome ?

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
>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