Kligfield Paul, Tyl Benoit, Maarek Martine, Maison-Blanche Pierre
Department of Medicine, Division of Cardiology, Weill Medical College of Cornell University, New York, NY 10021, USA.
Ann Noninvasive Electrocardiol. 2007 Apr;12(2):145-52. doi: 10.1111/j.1542-474X.2007.00153.x.
The global QT interval, emerging as a standard measurement provided by digital electrocardiographs, is defined by the earliest QRS onset and latest T-wave offset that occur in any of the standard leads. Differences between global ECG measurements and those from individual ECG leads have implications for the redefinition of normal values, for recognition of disease, and for drug safety. This study sought to quantify the differences between global QT intervals measured from 12 superimposed ECG leads with QT intervals and from single lead complexes, to examine the separate effects of QRS onset and T-wave offset on these differences, and to examine the reproducibility of these measurements.
QTo intervals (Q onset to T offset) from 50 digitized ECGs sampled at 500 Hz were examined by computer assisted derivation of representative complexes from standard leads II, V(2), and V(3), by both baseline and tangent methods. Global QTo intervals were measured from superimposition of the representative complexes of all 12 leads. A time-coherent matrix of waveform onset and offset points allowed direct comparison of the components of the differences.
Global QTo and Bazett-adjusted global QTc were greater than each of the baseline and tangent measurements in representative leads II, V(2), and V(3), with mean differences ranging from 8 to 18 ms. QRS onset was earlier in the global complex than in each of the representative leads, with mean differences of 3-5 ms, whereas T-wave offset was significantly later in the global complex than in each of the representative leads, with mean differences of 5-11 ms. Remeasurement of all ECGs after an interval of 6 months confirmed the relative magnitudes of the global and individual lead QTo durations and small mean differences between pairs (-0.9 to 2.7 ms). Although global QTo had the largest mean difference (only 2.7 ms), it had the smallest standard deviation of the mean difference and lowest coefficient of variability (1.58%) of all measurements.
Global QT measurements are systematically larger than measurements from representative complexes of individual leads. These differences result from the combined effects of earlier QRS onset and later T-wave offset in the global complex, with T-wave offset the more dominant component of the difference.
整体QT间期作为数字心电图提供的一项标准测量指标，由任何标准导联中最早的QRS波起始点和最晚的T波终点来定义。整体心电图测量值与各个心电图导联测量值之间的差异对正常值的重新定义、疾病的识别以及药物安全性均有影响。本研究旨在量化从12个叠加心电图导联测量的整体QT间期与单个导联复合波的QT间期之间的差异，研究QRS波起始点和T波终点对这些差异的单独影响，并检验这些测量值的可重复性。
通过计算机辅助从标准导联II、V(2)和V(3)中推导代表性复合波，采用基线法和切线法，对50份以500Hz采样的数字化心电图的QTo间期（从Q波起始到T波终点）进行检测。整体QTo间期通过叠加所有12个导联的代表性复合波来测量。波形起始点和终点的时间相干矩阵允许直接比较差异的各个组成部分。
整体QTo间期和Bazett校正的整体QTc间期均大于代表性导联II、V(2)和V(3)中的基线测量值和切线测量值，平均差异范围为8至18毫秒。整体复合波中的QRS波起始点比每个代表性导联中的更早，平均差异为3 - 5毫秒，而整体复合波中的T波终点比每个代表性导联中的明显更晚，平均差异为5 - 11毫秒。6个月后对所有心电图进行重新测量，证实了整体和单个导联QTo间期的相对大小以及各对测量值之间较小的平均差异（-0.9至2.7毫秒）。尽管整体QTo间期的平均差异最大（仅2.7毫秒），但其平均差异的标准差最小，且在所有测量值中变异系数最低（1.58%）。
整体QT测量值系统性地大于单个导联代表性复合波的测量值。这些差异是由整体复合波中更早出现QRS波起始点和更晚出现T波终点的综合作用导致的，其中T波终点是差异中更主要的组成部分。
