[Korzun]

Мнения и книги - это увлекательно, но противоречиво, а документ - это попытка договориться и унифицировать.
Приведу полностью текст документа про QT:

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The QT Interval

Measurement of the QT interval and its adjustment for rate, gender, and QRS prolongation represent 2 of the major challenges in electrocardiography. They are matters of great importance to physicians, drug manufacturers, and regulatory agencies because of the relationship between prolongation of the QT interval and potentially lethal ventricular arrhythmias. The document released in October 2005 by the Food and Drug Administration (FDA) provides guidance for the design, conduct, analysis, and interpretation of clinical studies for evaluation of QT-interval prolongation (33).

QT and ST-T patterns vary a great deal in various genotypes of the LQTS. Zhang et al (34) described 10 different ST-T patterns in first 3 genotypes of the syndrome (4 in LQT1, 4 in LQT2, and 2 in LQT3), and these patterns were present in the majority of genotyped LQTS patients.

The QT interval is defined as the interval from the onset of the QRS complex, that is, the earliest indication of ventricular depolarization, to the end of the T wave, that is, the latest indication of ventricular repolarization. The problems associated with this measurement include the following: 1) recognizing the onset of the QRS complex and the end of the T wave, 2) determining the appropriate lead(s) in which to measure the QT interval, and 3) adjusting the QT interval for increases in QRS duration, gender, and rate.

When the majority of ECGs were recorded on single-channel analog machines, various leads were recorded sequentially, and the QT interval was measured manually in the individual leads. Determination of the end of the T wave was often difficult and sometimes impossible, and the onset of the QRS complex and the end of the T wave varied in different leads, appearing shorter when the axis of an individual lead was more perpendicular to the spatial vector of the onset of the QRS complex or the end of the T wave. The onset of the QRS complex tends to occur up to 20 ms earlier in V2and V3than in the limb leads (35). Some regard differences of up to 50 ms in QT intervals measured in the various leads in normal subjects as being normal (36); others have suggested that differences of up to 65 ms were still within the limit of normal (37). This value is reported to be less in women than in men (38).

When the QT interval is measured in individual leads, the lead showing the longest QT should be used (39). This is usually V2 or V3. However, if this measurement differs by more than 40 ms from that in other leads, the measurement may be in error, and measurements from adjacent leads should be considered. If the T wave and U wave are superimposed or cannot be separated, it is recommended that the QT be measured in the leads not showing U waves, often aVR and aVL (39), or that the downslope of the T wave be extended by drawing a tangent to the steepest proportion of the downslope until it crosses the TP segment. It should be recognized that defining the end of the T wave in these ways might underestimate the QT interval.

As detailed in the section on ECG technology (1), most currently used automated digital machines record all leads simultaneously. This technique permits their temporal alignment and superimposition, which facilitates a more accurate assessment of the beginning of the QRS complex, the end of the T wave, and the separation of the U wave from the T wave. As a result, the automatically measured QT interval is often longer than the QT interval as measured in any individual lead, and the values currently regarded as normal, which were established with single-channel sequential recordings, may no longer be valid. Most automated systems do not routinely display the superimposed tracings or the points used to derive the QT interval.

In view of the clinical importance of the QT-interval prolongation, it is essential to visually validate QT-interval prolongation reported by a computer algorithm.

In addition to administration of QT-prolonging cardioactive drugs, a number of conditions can induce QT prolongation. It is often possible to identify a specific cause of QT prolongation when appropriate clinical information is available; for instance, both hypokalemia and hypocalcemia can prolong phase 2 and phase 3 of the action potential and prolong the QT interval. It is not feasible here to compile a comprehensive list of all possible causes of QT prolongation. It is sufficient to emphasize that its presence in an ECG report should call for a careful clinical evaluation of possible causes.
Recommendation

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. In view of the clinical importance of QT-interval prolongation, it is essential to visually validate QT-interval prolongation reported by a computer algorithm.
QT Correction for Rate

Many formulas have been proposed to adjust the QT interval for rate (40, 41). The most widely used is the formula derived by Bazett (42) in 1920 from a graphic plot of measured QT intervals in 39 young subjects. This adjustment procedure divides the measured QT by the square root of the RR interval to derive the rate-adjusted value. The formula introduced by Fridericia (43), also in 1920, uses the cube root of RR. Bazett's formula leaves a strong positive residual correlation (r=0.32) and Fridericia's formula leaves a negative correlation (r=−0.26 to −0.32) with heart rate (44, 45), and the adjusted QT values may be substantially in error, particularly when the heart rate is high. More recently introduced formulas for QT adjustment as a linear or power function of RR or heart rate for adults (44, 45, 46, 47, 48) and for children (49) effectively remove the rate dependence of the adjusted QT, and they are clearly preferable to both Bazett's and Fridericia's formulas. Some investigators have introduced separate normal limits or rate correction factors for each heart rate subinterval using the so-called “bin method” (46, 50).
Recommendation

It is recommended that linear regression functions rather than the Bazett's formula be used for QT-rate correction and that the method used for rate correction be identified in ECG analysis reports. In addition, rate correction of the QT interval should not be attempted when RR interval variability is large, as often occurs with atrial fibrillation, or when identification of the end of the T wave is unreliable.

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