QT-interval

ECG: Normal QT-interval (380 ms) with normal corrected QT-interval regardless of the formula used;

Comments: The analysis of an electrocardiogram includes the systematic measurement of the QT-interval from the beginning of the QRS-complex to the end of the T-wave. The QT-interval therefore corresponds to the cumulative duration of ventricular depolarization and repolarization, and is an essential marker of sudden death; indeed, the prolongation of the QT-interval is significantly associated with an increased risk of polymorphic ventricular arrhythmias (torsade de pointes). The normal values ​​of the QT-interval are dependent on gender (longer in women), age and essentially heart rate; the duration of the QT-interval is shortened physiologically for a faster heart rate and conversely lengthens for a slower heart rate. The measured value of the QT must therefore be corrected according to heart rate. Measurement of the QT-interval is probably one of the most difficult moments in the interpretation of an electrocardiogram due to the lack of standardization and the limits of automatic measurements performed by the device which are often correct but with a risk of over- or underestimation. A manual measurement is therefore probably advisable although poses a number of questions:

• In which lead should the QT-interval be measured?

The duration of the QT-interval varies according to the studied lead (50 ms on average between the shortest QT and the longest QT), due to a different projection of the depolarization and repolarization vectors. Historically, QT measurement was performed in lead II and all standards were defined from this lead. Indeed, the axis of the main vector of the QRS-complex and of the T-wave is directed downwards and to the left hence in the direction of lead II, which explains why the deflections are maximal and therefore more easily identifiable and measurable in this lead. In addition, it appears that the QT-interval is often the longest in lead II in a certain proportion of patients with congenital long QT. When the measurement in lead II is not optimal, it is generally carried out in V5 or V6 or in the lead where the QT-interval appears to be the longest (very often V2 or V3) and the end of the T-wave that is the most individualized, even though the measurement of the QT-interval in each of the twelve leads is difficult to achieve in daily clinical practice.

• What chart speed should be used?

A standard chart speed of 25 mm/s with an amplification of 10 mm/mV is generally used. Indeed, a faster speed (50 mm/s) can lead to a distortion of the signals of low amplitude (T-wave or U-wave) which alters the quality of the measurement.

• Where does the T-wave end?

Determining the end of the T-wave constitutes the main difficulty of this measurement. Indeed, the T-wave is a low frequency electrical signal and the return to the isoelectric line is difficult to determine with accuracy. The tangent method is generally used in clinical practice and consists in drawing a tangent to the steepest slope of the T-wave (descending portion in the case of a positive T-wave or ascending portion in the case of a negative T-wave) and to take its intersection with the isoelectric line to determine the end of the QT-interval. When the T-wave is bifid (use of the tangent of the second hump), or in the presence of a U-wave (use of the tangent of the largest T-wave between the T-wave and the U-wave) or when the end of the T-wave is poorly defined due to an undulation of the baseline, the measurement becomes even more difficult.

• Which formula to use to correct the QT-interval value?

The duration of repolarization adapts to the changes in heart rate, unlike the depolarization and the duration of the QRS-complex which are fixed. The ventricular myocardium thus remains excitable, the repolarization being fully completed, even when the heart rate becomes rapid. In some patients with long QT syndrome, this adaptation of the QT-interval does not occur normally, which increases the risk of ventricular rhythm disorders during exertion. Several correction formulas (linear, logarithmic or exponential) have been proposed to determine whether the QT value is prolonged compared to its expected value for a heart rate at 60 bpm (1 second between the QRS). There are a number of controversies regarding the formula to use to correct the QT-interval as a function of heart rate. Bazett’s formula is probably the oldest formula (since 1920) and to this day remains the most commonly used. It would appear that this formula is suitable for cardiac frequencies between 60 and 100 bpm, slightly less when moving away from these values; indeed, this formula has the inconvenience of overestimating the QTc interval at high cardiac rates and underestimating this interval at low cardiac rates. Fredericia’s formula, which is also very old, is perhaps more suitable for fast heart rates. The Framingham linear formula, determined from the values of the famous Framingham cohort, can also be used. There are a number of other formulas in the literature, with Bazett’s formula remaining by far the most widely used in clinical practice.

• Bazett’s formula: QTc = QT / √ RR
• Fredericia’s formula: QTc = QT / RR 1/3
• Framingham formula: QTc = QT + 0.154 (1 – RR)
• Hodges formula: QTc = QT + 1.75 (heart rate – 60)

• How many cycles must be averaged in the presence of an arrhythmia?

When the heart rate is regular, the duration of the QT-interval is more homogeneous and must be averaged over at least 3 cycles. In the case of atrial arrhythmia and of irregular RR cycles or of frequent extrasystoles, it is recommended to average a greater number of cycles (ten or so), choosing the most regular cycles possible (the duration of the repolarization does not immediately adjust to the heart rate, several cycles may be necessary). The extrasystole and the ensuing cycle should be excluded from the measurement. On the other hand, the cycle following the extrasystole can provide decisive information in certain patients, the sudden variation in heart rate potentially generating a considerable prolongation of the QT-interval with a giant negative T-wave indicative of a major risk of ventricular arrhythmia.

•  what are the normal values of the QT-interval?

The duration of the QT-interval is shorter in men, especially in younger patients. The differences between men and women decrease with age. It is clear that there is no single QT-interval threshold value enabling to differentiate between healthy patients and patients at risk of sudden death. Bazett’s formula is generally used to determine the QT-interval norm. A corrected QT-interval is considered long when it exceeds 450 ms in men or 470 ms in women. Values between 430 and 450 ms in men and between 450 and 470 ms in women are considered as limit. A corrected QT-interval above 500 ms in a patient with congenital long QT is associated with a significantly increased risk of sudden death. A corrected QT-interval is considered short if the duration is less than 350 to 320 ms (regardless of gender).

Take-home message: The measurement of the QT-interval is generally performed in lead II, at a chart speed of 25 mm/s, by averaging several cycles, using the tangent method to determine the end of the T-wave and by correcting the value using Bazett’s formula.