Determination of Time Intervals

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Apart from quantifying changes in amplitude, the examination of signal morphology also permits the calculation of time intervals which describe the de- and repo-larization activities of the myocardium. ECG-determined QT interval prolongation has been associated with an increased risk of malignant arrhythmias in patients after MI (Schwartz and Wolf, 1978). The predictive value could be improved by the calculation of QT dispersion (QTd), measured as the difference between the maximum and minimum QT duration in the 12-lead ECG. QTd is assumed to reflect inhomogeneity of ventricular repolarization in different regions of the myocardium. The fact that ventricular arrhythmias may be due not only to myocardial scar but also to ischemia indicates that the latter is an important factor influencing electrophysiological processes (Higham et al., 1995). Thus, it was consistent to seek further dependency between CAD and QT duration in the setting of acute coronary syndromes and stable angina.

With regard to the determination of QTd, MCG has an advantage over 12-lead ECG as generally more than 36 registration sites covering a large part of the precordial thorax are available for investigation; this permits a more detailed examination of the spatial aspects of QT duration disparity. Indeed, QTd has allowed better separation between CAD patients without MI and healthy subjects on the basis of multichannel MCG compared to 12-lead ECG (Hailer et al., 1999a,b; Shabalin et al., 2002; Van Leeuwen et al., 2003a). However, consideration of the spatial distribution of QTd improved its sensitivity in the detection of CAD. Increased QT variability

Fig. 2.39. Precordial spatial distribution of QT interval duration based on acquisitions using a 37-channel biomagnetometer in four subjects. Healthy subject (N), patient with coronary artery disease without myocardial infarction (CAD); post-MI patient without ventricular tachycardia (MI); and post-MI

patient with ventricular tachycardia (VT). Isochron step = 20 ms; Z-axis: minimum QT value set to 0 ms, QTch = QT values for the channels. Note the progressively higher values, changes in local variability and changes in overall pattern in the patients. (From Van Leeuwen et al., 2003a, with permission).

Fig. 2.39. Precordial spatial distribution of QT interval duration based on acquisitions using a 37-channel biomagnetometer in four subjects. Healthy subject (N), patient with coronary artery disease without myocardial infarction (CAD); post-MI patient without ventricular tachycardia (MI); and post-MI

patient with ventricular tachycardia (VT). Isochron step = 20 ms; Z-axis: minimum QT value set to 0 ms, QTch = QT values for the channels. Note the progressively higher values, changes in local variability and changes in overall pattern in the patients. (From Van Leeuwen et al., 2003a, with permission).

between neighboring sites and an alteration in the global repolarization pattern compared to healthy subjects could be displayed in CAD patients not only under stress (Hailer et al., 1999a) but also in a resting condition (Hailer et al., 1999b; Van Leeuwen et al., 2003a) (Fig. 2.39).

The spatial patterns involved have been quantified using two smoothness indexes: SI, which quantifies the mean dispersion at each registration site; and SIn, which normalizes SI by taking the overall spatial dispersion pattern into account (Van Leeuwen et al., 1996). Local irregularity in QT duration as expressed by SI was increased in patients with CAD but no wall motion irregularity at rest, indicating more regional heterogeneity of repolarization in contrast to healthy subjects. Furthermore, SIn as a marker for the global pattern of QT dispersion was significantly higher in patients with CAD, which indicates a greater deviation from the pattern found in healthy subjects (Hailer et al., 1999b).

With regards to methodology, the manual determination of QT interval duration is cumbersome, particularly in multichannel measurements. The computed automatic calculation in MCG data has been shown to produce reliable results in several studies (Oikarinen et al., 1998; Smith et al., 2003; Van Leeuwen et al., 2003b). The effects of coverage area comparing precordial and prethoracic recordings have also been examined (Klein et al., 2002). Furthermore, the MCG-derived QTd has been shown to be robust with regard to rapid changes in autonomic tone, but it is affected by respiration and left ventricular loading (Haapalahti et al., 2000).

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