1. Definition and background
The QT interval is the period from the start of the QRS complex to the end of the T wave.
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The sum complex (cumulative complex) results from averaging the normal beats of equal morphology of the last 10 seconds of the first section.
Figure 1 - Progress of a cardiac excitation
2. Diagnostic significance of the QT interval
The QT interval is the period of de- and repolarisation of the ventricles. QT prolongations indicate a disturbance of these processes. Basically, acquired and congenital QT prolongations can be distinguished.
Acquired QT prolongations are frequently caused by certain drugs such as antiarrhythmic agents (class I and II e.g. quinidine, sotalol and others), tricyclic antidepressants as well as antibiotics. They can prolong the QT interval and the repolarisation of the heart and increase the risk for dangerous cardiac arrhythmia.
Congenital QT prolongations can often be attributed to genetic causes. Some known diseases are the Jervell and Lange-Nielsen Syndrome (JLNS) or the Romano-Ward Syndrome (RWS).
Cardinal symptoms are hearing loss (JLNS), syncope with stress, effort or cold as well as a considerably prolonged QT interval (>500ms), frequently associated with tachy-arrhythmia (ventricular tachycardia, episodes of Torsades de Pointes (TdP) with polymorphic ventricular tachycardia and ventricular fibrillation), which may result in syncope or sudden death.
If the QT interval is prolonged, normally further risk factors are given such as heart diseases, drug interactions, a genetic predisposition or electrolyte imbalances.
3. Determination of the QT interval
- Determination of the start of a QRS complex per channel
On the basis of the maximum deflection of the complex search is made towards the left, until the ECG merges with the isoelectric line. The start of the QRS complex is defined as the point in the ECG that has the largest distance to a straight line defined by 2 points: the first point is located on the isoelectric line before the point in time when the ECG merges with the isoelectric line. The second point lies above the first positive or below the first negative deflection of the QRS complex.
- Determination of the end of the T wave for each channel
In analogy with the determination of the start of the QRS complex, search is made towards the right on the basis of the maximum or minimum of the T wave, until the ECG merges with the isoelectric line. The end of the T wave is defined as the point in the ECG that has the largest distance to a straight line defined by 2 points: the first point lies on the isoelectric line behind the point in time when the ECG merges with the isoelectric line. The second point lies above the T wave maximum or below the T wave minimum.
4. QTc -
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The heart rate-corrected QT interval
As the QT interval depends on the heart rate - with high heart rate the QT interval is shorter and vice versa - a heart rate-adjusted, standardised QT interval is used for diagnostics, the so-called QTc interval. The QTc value shows which value the measured QT interval would have if it had been determined at a heart rate of 60/min, i.e. a R-R interval of 1s.
In custo diagnostic, the formulas according to Bazett and Fridericia are used.
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With higher heart rates (> 80/min) in particular, the rate correction according to Fridericia is considered to be more suitable than the formula according to Bazett.
Further formulas for heart rate-adjusted, QT-dependent considerations can be found in literature (see e. g. [6]). The formulas according to Bazett and Fridericia were developed around 1920 / 1937 (Hegglin). Since 2012, the Framingham formula has become available which has been determined on the basis of a considerably larger patient collective.
The formula for calculating the heart rate-corrected QT interval (QTc) in milliseconds according to Framingham is: QTc [ms] = QT interval [ms] + (0.154 * (1000 - R-R interval [ms]).
5. QT dispersion
The duration of the QT interval varies from lead to lead and is the longest in the precordial chest leads. The difference between the longest and the shortest QT interval is called QT dispersion. People with a healthy heart have a QT dispersion around 50ms (+/- 18ms), values exceeding 80ms have to be considered as pathologic and indicate an electrical inhomogeneity of the ventricles with an increased risk for heart rhythm-related events, particularly the sudden cardiac death [2].
The measurement of the QT interval with prominent U wave can be difficult or even impossible (in case of T-U merges). From a pathophysiological point of view, it makes sense to consider and measure also the complete repolarisation, which means including the U wave. [2].
6. Normal ranges of the QT interval
QT intervals are measured in milliseconds. The normal range for QTc is 350ms to 430ms, the average value in women being significantly higher, with 421ms versus 409ms in men [2]. According to Gertsch (2008), slightly shortened or slightly prolonged QT intervals may also occur in people with a healthy heart, however the QTc interval should not exceed 460ms [3].
In custo diagnostic, the measured value table indicates a percentage value in addition to QTc interval. This is based on the formula by Hegglin and Holzmann (1937) for calculating a heart rate-corresponding QT target value.
The formula is: QT target value = 0.39 x (square root of R-R interval +/- 0.04 sec) [4].
The result is a heart rate-corrected QTc target value of 390ms (+/- 40ms). The percentage indication in custo diagnostic specifies the deviation from this (average) target value (390ms) in percent.
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Table 1 QTc limit values [5]
7. Measurement of the QT interval in custo diagnostic
First of all, the determination of the QT interval is explicitly regulated in the DIN EN 60601-2-51 standard, i.e. there is a clearly defined requirement that has to be observed.
The above-mentioned standard literally states the following:
"…The global intervals of P, QRS and T are physiologically defined as the earliest start in a LEAD and the latest end of another LEAD. (Due to the different dispersion of wave progressions, the starts and ends of the wave progressions are not necessarily visible in all LEADS simultaneously.) Figure FF.2 shows an example where P start is determined by LEAD II, P end by LEAD G I, QRS start by LEAD V1 and LEAD G V3, QRS end by LEAD G V5 and T end by LEAD G V2 and V3…"
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- in custo diagnostic, 5 the settings are as described in the previous section. An adaptation can be made as described in annex 2.
8. Summary
The QT interval in custo diagnostic is measured according to the normative requirements of DIN EN 60601-2-25 (VDE 0750-2-25):2016-08.
This standard exactly stipulates how start and end of a QT interval have to be determined and how the measurement over a 12-channel ECG has to be carried out. custo diagnostic strictly observes these regulations. For the calculation of the corrected QT interval, the QTc interval, various formulas can be used such as the Bazett or the Fridericia formula.
For the definition of the pathologic limits, custo diagnostic refers to the current scientific literature, currently Lewalter & Lüderitz (2010).
Furthermore, the limits can also be defined by the physician him- or herself - if this appears to be necessary. These modifications will be noted and are reserved for the medical expert (physician).
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Important notice:
The contents made available here have been generated to the best of our knowledge and belief. We do not assume any responsibility for damages resulting from the use of the information contained herein. All liability claims are invalid. The readers are advised to check the accuracy of all product-related information.
9. Literature
[1] von Olshausen, Klaus (). EKG-Information: Vom Anfänger zum Profi. 8., überarbeitete und erweiterte Auflage. Darmstadt: Steinkopff, S. 80.
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[8] DIN EN 60601-2-25 (VDE 0750-2-25):2016-08, S.57.
10. List of abbreviations:
| P | P wave (atrial excitation) |
| Q | first negative deflection |
| R | first positive deflection |
| S | negative deflection after R |
| T | T wave (repolarisation of ventricles) |
| U | U wave (possible as post-fluctuation of repolarisation) |
| ms | millisecond |
11. Table of figures
Figure 1 Progress of a cardiac excitation
Figure 2 Definition and determination of global intervals
12. Tables:
Table 1 QTc limit values
13. Annex
Annex 1: Settings in custo diagnostic 4.x
Servicecenter → Einstellungen → Ruhe-EKG → Befund → "AdultFemaleQTcLimit", bzw. "AdultMaleQTcLimit"
Annex 2: Settings in custo diagnostic 5.x
