[Korzun]

Продлжение.

QT Correction for Gender and the Limits for Prolonged and Short QT Interval

Although Bazett's and Fridericia's formulas make no adjustment for gender, many studies have demonstrated that the QT interval is longer in young and middle-aged females than in males. The gender difference is potentially important because women are generally considered to be more prone to malignant arrhythmias in LQTS than men. The gender difference appears during adolescence (51), when the rate-adjusted QT shortens in boys, possibly as a testosterone effect, but undergoes little change in girls.

The reported gender difference in various studies varies from 6 to 10 ms in older age groups and from 12 to 15 ms in younger adults. Overall, the gender difference in rate-adjusted QT interval becomes small after 40 years of age and practically disappears in older men and women. Separate gender- and age-specific QT-adjustment formulas have been proposed to accommodate these differences (21, 44, 46, 47). Normal limits proposed in different studies vary to a certain extent, depending on the characteristics of the study population and particularly on the type of QT-adjustment function used. It is important to recognize that normal limits established using the upper and lower limits of actual percentile distributions of the rate-adjusted QT are preferable to those as mean values ±2×SD because these distributions are strongly skewed (44).

Normal standards for thresholds for abnormal QT from large subgroups of community-based populations are available (21, 44, 45, 46, 47, 48). These limits are relatively uniform in reports that have appropriately used linear regression functions with QT-rate adjustment as a linear or power function of RR or heart rate (21, 44, 46, 47, 48). The following normal limits are suggested as a practical compromise for the evaluation of QT-interval prolongation and shortening in adult men and women: prolonged QT: women, 460 ms or longer; men, longer than 450 ms; and short QT: women and men, 390 ms or shorter.

FDA guidelines for industry recommend that 3 severity levels for rate-corrected QT be reported when considering possible QT-prolonging effects of drugs: longer than 350 ms, longer than 480 ms, and longer than 500 ms (32).

Although the upper normal limits for QT adjusted for rate as a linear function of RR in small groups of children stratified by age, gender, and heart rate have been published (52), the limits for prolonged and short QT established in reasonably large groups of children have been reported only for Bazett's formula (53). In that report, the 98th percentile limit for rate-adjusted QT was approximately 450 ms in children younger than 12 years of age. The gender difference of 8 ms appeared in the 12- to 16-year age group. It should be noted that QT adjusted by Bazett's formula may produce false QT prolongations (45).
Recommendation

It is recommended that, in addition to rate, an adjustment for gender and age be incorporated into QT adjustment. As practical clinical limits for considering the QT interval as abnormal, it is recommended that the adjusted QT of 460 ms or longer in women and 450 ms or longer in men be considered a prolonged QT interval and that QT 390 ms and shorter be considered a short QT interval.
Correction for QRS Duration

The QT interval prolongs in ventricular conduction defects, and an adjustment for QRS duration becomes necessary (54, 55, 56). This can be accomplished best by incorporating QRS duration and RR interval as covariates into the QT-adjustment formula or by using the JT interval (QT duration–QRS duration) (55). If the JT interval is chosen, normal standards established specifically for the JT interval should be used (55).
Recommendation

QT- and JT-adjustment formulas have recently been introduced for use in the setting of prolonged ventricular conduction. With confirmation, they may be incorporated into automated algorithms to provide appropriate correction factors.
Evaluation of QT Intervals in Sequential Tracings

Evaluation of QT intervals from sequential ECG recordings is essential for identification of QT prolongation induced by pharmacological agents in drug studies. It is also helpful as an adjunct to ST analysis in determining the presence of active ischemia in patients suspected of having myocardial infarction.

When sequential tracings from patients recorded at different times of day and at different locations are compared, it is important to recognize that the time of day can influence the QT interval (57, 58), that differences may exist between the different recording systems and between the programs used for QT measurement, and that different formulas for QT-rate adjustment may have been used. Moreover, there is a significant interreader variability in the measurement of QT interval (59). Rigorous standardization of the recording and evaluation procedures should be followed when serial comparison of QT intervals is undertaken.

The FDA guide for industry suggests that in the evaluation of possible QT prolongation induced by drugs, 2 levels of change in serial ECGs in the rate-corrected QT from the baseline be reported: an increase of greater than 30 ms and an increase of greater than 60 ms (33).
Recommendation

It is recommended that for serial comparisons ECG recorders meet specific performance standards and, if possible, be of the same type; that uniform, carefully standardized ECG acquisition and QT-measurement procedures be used; and that, whenever possible, a single reader be responsible for overreading sequential tracings of an individual patient or research subject.
QT Dispersion

Increased heterogeneity of myocardial repolarization may predispose patients to the development of malignant ventricular arrhythmias (60). As indicated above, significant differences exist in the duration of the QT interval when measured in the individual leads. Visualization of these differences is facilitated by the display of a suitable subset of temporally aligned simultaneous leads with a slight separation on the amplitude scale.

The difference between the longest and shortest QT intervals is referred to as QT dispersion. This concept was introduced in 1990 for risk identification in patients with LQTS (61). Since its introduction, QT dispersion has been one of the most popular topics in ECG research. In November 2006, a PubMed search cited 670 publications with QT dispersion in the title, and a Google search under “QT dispersion measurement” revealed 171 000 communications.

The QT-dispersion concept has led to the expectation that QT dispersion is a measure of regional or localized heterogeneity of myocardial repolarization. Numerous studies have suggested an increased risk of morbidity and mortality for an increase in QT dispersion. However, substantial methodological problems with the QT-dispersion measurement have been identified that have raised fundamental questions about the validity of the concept (62, 63, 64, 65). In principle, the expectation that QT dispersion is a measure of the regional or localized heterogeneity of myocardial repolarization implies that the leads with the shortest and longest QT contain signal information at the terminal part of the T wave that is not present in the first 3 orthogonal components of the 12-lead ECG (or the composite global T wave) (65).

Until adequately validated data in specific clinical conditions are presented showing that QT dispersion on the body surface ECG is the counterpart of localized dispersion of myocardial repolarization and conveys adequately strong nondipolar signal information that cannot be extracted from the X,Y,Z components, it seems unwise to include it as a part of the routine ECG report.
Recommendation

It is recommended that QT dispersion not be included in routine ECG reports. However, because of the fundamental importance of the heterogeneity of myocardial repolarization in the genesis of malignant ventricular arrhythmias, continued research into the identification of markers of increased dispersion of myocardial repolarization on the body surface ECG is encouraged.

Т.е. данные документа (договоренностей и унификации) противоречат приведенной Вами, Игорь Петрович, ссылке. [К сожалению. И это только подчеркивает отсутствие единства и сложность унификации.]
На практике QT в большом проценте измеряется без проблем (если измеряется ), а при проблемах я лично буду руководствоваться пусть и не истиной последней инстанции, но унифицированным вариантом договоренностей большинства экспертов.

И про окончание Т в этом документе предлагается неплохой приемчик, который возможен в нашу компьютерную эру. Повторю часть цитаты, т.к. в длинном тексте могла затеряться:
It is recommended that selective subsets of temporally aligned superimposed ECG leads be made available as an optional display to facilitate QT measurement and to validate the onset and end points of the QT interval.

Т.е. усредненная (наложение компьютером отведений друг на друга) ЭКГ пусть лежит рядом при оценке начала и окончания QT.