Satyendra Dhar, MD @DharSaty · 1 year ago
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QT/QTc- Interval • Start of Q-wave to end of the T-wave (time of ventricular depolarization + repolarization). • Life threatening
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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
Satyendra Dhar, MD @DharSaty · 1 year ago
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