QT Interval
The QT interval seen in the ECG is measured from the beginning of the QRS complex (starting point of the Q wave) to the end of the T wave as it returns to the baseline and usually measured using either lead II or lead V5 of the 12-lead ECG. The QT interval varies with the heart rate therefore corrected QT (QTc) is the measurement taken into account. The QT interval measurement is used to calculate the QTc
What Is Normal QT QTc On ECG?
- The normal QTc interval is taken as 0.36-0.44s.
- The normal QT interval in women is slightly higher than in men.
- Normal QTc for males 0.36 – 0.44s.
- Normal QTc for females 0.36 – 0.46s.
The heart rate has a direct impact on the QT interval. When the heart is lower the QT interval is longer and when the heart rate is higher the QT interval is shorter. In order to correct, this corrected QT interval (QTc) is calculated by using a formula.
QTc estimate the QT interval at a heart rate of 60 bpm. This allows comparison of QT values over time at different heart rates and improves detection of patients at increased risk of arrhythmias. There are several formulas but the common used formulas are Bazzet’s formula and Fredericia’s formula.
Out of these two Bazzet’s formula is used commonly due to its simplicity. It provides an adequate correction for heart rates ranging from 60 – 100 bpm but over-corrects at heart rates > 100 bpm and under-corrects at heart rates < 60 bpm. At heart rates outside of the 60 – 100 bpm range, the Fredericia corrections are more accurate and should be used instead.
- Bazett’s formula – QTcB = QT/√2(R − R)
- Fredericia’s formula – QTcF = QT/√3(R − R)
- R-R interval = 60/ heart rate (it can also be measured by measuring the distance from one R wave to the next R wave manually in an ECG).
- All measurement are taken in seconds.
- E.g.: QT interval is 40s, Heart rate is 80 bpm
- R-R = 60/ 80
- = 0.75s
- QTcB = 40/ /√2 (0.75)
- = 46s
Difficulties Faced When Measuring QT Interval
The QT interval measurement in an ECG can differ from one interpreter to another. This is a usual human error and it’s difficult to correct this error. Therefore automated QT interval measurement technology has been introduced. However even with automated QT interval measurement several problems have been identified which gives an inaccurate measurement.
- A wide QRS complex – may give an impression that the QT interval is prolong.
- Interpretation of the T wave end point is one of the main issues faced when interpreting the QT interval either manually or automatically.
- The problems occur due to
- Amplitude of the T wave
- T wave merging with U wave or P wave
- The morphology of the T wave
- Irregular rhythm – this makes it difficult to get a consistent QT measurement.
- When the heart rate is lower than 50 bpm or higher than 120 bpm longer – it may affect the QTc, so longer ECG recordings and several measurements should be considered.
- In athletes and children sometimes the R-R interval shows beat-to-beat variability – it may affect the QTc, so longer ECG recordings and several measurements should be considered.
Summary
The QT interval is measured from the beginning of the Q wave to the end point at the baseline of the T wave and usually measured using either lead II or lead V5 of the 12-lead ECG. The heart rate has a direct impact on the QT interval. When the heart rate is slower he QT interval is longer and when the heart rate is higher the QT interval is shorter. To overcome this calculated QT (QTc) is calculated by using a formula. The common two formulas are Bazzet’s formula and Fredericia’s formula. Normal QTc range is 0.36-0.44s. Normal QTc for males 0.36 – 0.44s, for females 0.36 – 0.46s.
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