QT/QTc- Interval

•	Start of Q-wave to end of the T-wave (time of ventricular depolarization + repolarization).

•	Life threatening risk of prolonged QTc >500ms = Torsades de pointes (TdP)

•	Prolonged QT/QTc interval may be a clue to electrolyte disturbances (hypocalcemia or hypokalemia), drug effects (quinidine, procainamide, amiodarone, or sotalol), or myocardial ischemia (usually with prominent T wave inversions). 

•	Shortened QT intervals are seen with hypercalcemia and digitalis effect. 

•	Each 10-millisecond increase in QTc contributes approx a 5% to 7% additional increase in risk for TdP.

•	QTc of 540 milliseconds has a 63% to 97% higher risk of developing TdP than a patient with QTc of 440 milliseconds.



How do you measure it:

•	Find a lead with the tallest T wave and count the little boxes from the start of the QRS complex to the point where the T wave comes back down to the isoelectric line. 

•	Multiply the number of little boxes by 0.04 seconds. 

•	Example if you counted 8 boxes then QT interval is 8 x 0.04 = 0.32 seconds (320 milliseconds).

•	QT interval should be less than half the preceding R-R interval (Works for regular rates between 65-90).



Methods of calculating the QTc:

•	Bazett formula, QTc = QT / √RR.  

•	Fridericia formula (QTc = QT / RR1/3)

•	Hodges [QTc = QT + 0.00175 x (HR - 60)]

•	Framingham linear regression analysis {QTc = QT + 0.154 x (1 - RR)}

•	Karjalainen et al. [QT nomogram] 

•	Rautaharju formula, QTc = QT x (120 + HR) / 180



#QTInterval #QTc #Torsadesdepointes #TdP #Bazettformula #Fridericiaformula 

#Hodgesformula #Framinghamanalysis #Karjalainen #QTnomogram #Rautaharjuformula #QT