The q t interval

ECG QT Interval

1. The Q-T Interval Jast Arnaldo T. Tejada III MD Cardiology Fellow Mediatrix Heart Institute

2. Objectives To discuss important concepts & how to interpret Q-T Interval on ECG To discuss important concepts of the causes & etiology of Q-T prolongation To provide ECG samples of Abnormal Q-T intervals for different pathology.

3. QT interval QT interval includes the QRS complex , ST segment ,T wave Corresponding to phases 0 to 3 of the action potential Beginning of the QRS complex and end of T wave

4. QT interval Presence of a U wave is not included in the measurement Multiple leads should be selected QT interval is the longest QT in ECG

5. QTc: QT interval Affected by heart rate. Longer when the heart rate is slower and shorter Should always be corrected for heart rate

6. QTc: QT interval

7. QTc: Bazett Formula Fridericia formula: QT C  = QT / RR  1/3 Framingham formula:  QT C  = QT + 0.154 (1 – RR) Hodges formula:  QT C  = QT + 1.75 (heart rate – 60)

8. QTc: Bazett Formula (Normal) <0.42 seconds in men (Normal) <0.44 seconds in women (Prolonged) >0.44 seconds in men (Prolonged) >0.46 seconds in women and children

9. QTc: Bazett Formula

10. QTc: Bazett Formula Step1: Measure the QT interval: The QT interval measures 10 small blocks. This is equivalent to 0.40 seconds Step2: Measure the R-R interval: The R-R interval measures 14 small blocks, which is equivalent to 0.56 sec. The square root of 0.56 seconds is 0.75 sec Step3: Calculate the QTc: Using the Bazett formula as shown below: QTc = 0.40 ÷ 4 0.75 = 0.53 seconds. The QTc is prolonged.

11. Importance of the QT Interval Prolonged QT - increased risk for Ventricular Arrythmias especially Torsades de Pointes Congenital short QT – associated with paroxysmal atrial & ventricular fibrillation & Sudden cardiac death

12. Causes of prolonged QTc (>440ms) Hypokalemia

13. Causes of prolonged QTc (>440ms) Hypomagnesaemia

14. Causes of prolonged QTc (>440ms) Hypocalcemia

15. Causes of prolonged QTc (>440ms) Hypothermia

16. Causes of prolonged QTc (>440ms) Myocardial Ischemia

17. Causes of prolonged QTc (>440ms) Raised ICP

18. Causes of prolonged QTc (>440ms) Congenital Long QT Syndrome

19. Causes of short QTc (<350ms) Hypercalcemia

20. Causes of short QTc (<350ms) Congenital short QT syndrome

21. Causes of short QTc (<350ms) Digoxin

22. THANK YOU!