Lead fields associated to optimal linear discriminant functions

Selected Abstracts from the Seventeenth international longed fQRSd region could be noted in the left precordium in 14 out of 15 patients with anterior Ml and in the right lower chest in 24 out of 25 patients with inferior Ml. Appearance of these prolonged fQRSd regions was constant in 5 patients, unstable in 33 patients, and absent in 2 patients throughout the observation period. The prolonged fQRSd region became clearly identifiable between the third day and first week in 24 patients, I2 of whom experienced documented VT, in contrast with 3 out of the 16 remaining patients. Dent formation at the site corresponding to infarcted myocardium within the prolonged fQRSd region was seen 3-4 weeks after onset in I7 patients, I2 of whom experienced documented VT, in contrast with 3 of 23 patients without dent formation (p < 0.01). In a patient who developed ventricular aneurysm, the prominent region of fQRSd proIongation appeared in the left precordium 3-4 weeks after onset and disappeared after aneurysmectomy. In conclusion, the region showing a prolonged fQRSd can be seen on the anterior chest in acute MI and is considered to be related to impaired ventricular conduction, which can possibly become a substrate for VT.

Lead Fields Associated to Optimal Linear Discriminant Functions G. Y. Kozmann. Z. S. Cserjes, A. Szoke, Central Research Institute for Physics, Budapest, Hungary Body surface potential maps (BSPMs) of different dichotomies (normals vs. myocardial infarctions with different localizations) can be separated with a low false classification rate by the use of optimally designed linear discriminants (LDs) represented in the form of weighting coefficient maps (WCMs). To reveal the physical meaning of these LDs, lead fields (LFs) associated with WCMs were evaluated. LDs separating normal subjects and anterior or inferior myocardial infarction (AMI and IMI) patients were estimated earlier by the use of a BSPM data bank collected at the University of Utah CVRTI, Salt Lake City. LD computations were carried out in the space of Karhunen-Loeve (K-L) eigenvectors, and finally, LDs were transformed back into the space of the 192-lead BSPM measurements and were referred to as WCMs. Lead fields associated with each WCM were computed by the use of the authors’ forward program package. The numerical chest model of the forward program had a real thorax geometry, while volume conductor conductivity was simplified to be homogeneous. Lead fields at epicardial level were estimated by the use of simulated body surface potentials corresponding to unitstrength normally oriented dipoles, which were evenly distributed over the epicardial surface of a realistically shaped heart model. The strength of LF in an arbitrary epicardial dipole location equals the dot product of the 192-element WCM vector and the 192-element body surface potential field vector of the dipole considered. Epicardial LF distribution was represented in contour line map form. LF patterns indicated that WCMs separating normal and AMI or IMI groups can be considered as optimal spatial filters re-

Congress on Electrocardiology

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sponding dominantly to cardiac regions where MIS were localized, while suppressing source contributions from elsewhere.

Erroneous Conclusions About Spatial Dispersion of QT Intervals Due to Overlap of the U and Terminal T Wave Potential Distributions Syamal K. Dey, Pentti M. Rautaharju, Harry P. Calhoun, Gerhard Stroink, Fred Kornreich, Terrence J. Montague, Dalhousie University, Free University of Brussels, and University of Alberta The purpose of this project was to improve detection of regional differences in “true” QT intervals by separating U wave potentials from terminal T wave distributions. This was achieved by fitting a monoexponential curve to the “visible” T wave from its inflection point following T peak and extrapolating this curve from U onset to U-P baseline in 120-lead body surface ECGs of 3 1 normal adult subjects. The end of T was defined as the point where the exponential curve reached a 20 pV threshold value. The results revealed pronounced repolarization activity persisting in all subjects past T wave offset by conventional definitions, with values of potential maxima ranging from 35 FV to 108 pV at time point of U wave onset. These maxima were clustered tightly in a region between leads VJ and Vq, with corresponding broad minima in upper right parasternal or right paravertebral regions in the back. QT isochrone maps derived from this late repolarization activity indicated that conventional global QT duration estimates permitting U wave interference are underestimates ranging from a minimum of 20 ms to values approaching the whole global U wave duration. Errors in estimating minimal regional QT durations can also be considerable. It is concluded that drastic errors are induced on conventional QT estimates in body surface ECGs in regions with prominent U wave potentials as well as in regions where U and T overlap is difficult to detect by conventional methods. U wave separation from the rest of ventricular repolarization activity seems essential in order to improve the reliability of estimation of the dispersion of QT intervals in body surface ECGs.

Detection of the Number of Independent Components in Multilead Electrocardiograms G. J. H. Uijen, A. van Oosterom, Departments of Cardiology and Medical Physics and Biophysics, University of Nijmegen, The Netherlands In the past, the number of independent signals in a multilead ECG (body surface maps) have been estimated by using principal components analysis. As many principal