Theodore J. Dustman

Spatial Methods of Epicardial Activation Time Determination in Normal Hearts

AbstractThe purpose of this study was to demonstrate errors in activation time maps created using the time derivative method on fractionated unipolar electrograms, to characterize the epicardial distribution of those fractionated electrograms, and to investigate spatial methods of activation time determination. Electrograms (EGs) were recorded using uniform grids of electrodes (1 or 2 mm spacing) on the epicardial surface of six normal canine hearts. Activation times were estimated using the time of the minimum time derivative, maximum spatial gradient, and zero Laplacian and compared with the time of arrival of the activation wave front as assessed from a time series of potential maps as the standard. When comparing activation times from the time derivative for the case of epicardial pacing, spatial gradient and Laplacian methods with the standard for EGs without fractionation, correlations were high (R2=0.98, 0.98, 0.97, respectively). Similar comparisons using results from only fractionated EGs (R2=0.85,0.97,0.95) showed a lower correlation between times from the time derivative method and the standard. The results suggest an advantage of spatial methods over the time derivative method only for the case of epicardial pacing where large numbers of fractionated electrograms are found.

Determination of local myocardial electrical activation for activation sequence mapping. A statistical approach.

Electrical activation sequence mapping requires accurate identification of local activation, but because extracellular recordings do not exclusively reflect local events, complex electrograms may be difficult to interpret. In such cases, the assignment of local activation is subject to error that could affect interpretation of the resulting activation maps. The purpose of this investigation was to develop an approach that would provide quantitative indexes of error in the determination of local activation. An electrode array with 64 closely spaced unipolar electrodes was used to record from the left ventricular surface during open heart surgery. Electrograms with multiple deflections were recorded from four patients with scarred myocardium; two other patients with normal myocardial function served as controls. Each of 784 deflections was scored on the basis of three features: evidence for propagation, the configuration of the bipolar signal, and the effect of changing from the chest to an average reference. Local activation was considered probable if evidence for all three features was present and improbable if none of the three features was present. Deflections that were ambiguous with respect to this standard were excluded. Of over 30 test variables analyzed, the three with the greatest power to discriminate signals due to local activation from those due to distant activity were 1) a linear combination of the extracellular potential plus the ratio of the second derivative and the extracellular potential, 2) the second derivative, and 3) the minimum (greatest negative) first derivative. For each of these variables, the threshold value providing the greatest performance was identified by the maximum quality of efficiency, an index of agreement. This statistical approach provides an objective basis for determining local activation and provides a quantitative assessment of error that could enhance interpretation of electrical activation sequence maps.

Mechanisms of the Spatial Distribution of QT Intervals on the Epicardial and Body Surfaces

Spatial Distribution of the QT Interval. Introduction: The role of QT dispersion as a predictor of arrhythmia vulnerability has not been consistently confirmed in the literature. Therefore, it is important to identify the electrophysiologic mechanisms that affect QT duration and distribution. We compared the spatial distributions of QT intervals (QTI) with potential distributions on cardiac and body surfaces and with recovery times on the cardiac surface. We hypothesized that the measure of QTI is affected by the presence of the zero potential line in the potential distribution, as well as the sequence of recovery. We also investigated use of the STT area as a possible indicator of recovery times on the cardiac surface.

Optimal lead selection for detection of ST segment shifts.

A comparison was made to determine the ability of optimal sets of 2–6 unipolar leads and a normal Holter lead set to estimate ST potential distributions changes induced by balloon inflation during angioplasty. The performance of these lead nets was compared to measurements observed in recorded 32‐lead body surface maps. Unipolar lead potentials were estimated using a linear, least mean squared error estimator of the total body surface map. The correlation between maximum ST potential change in the body surface map and that predicted by the unipolar lead sets ranged from 0.84–0.93. The correlation between maximum ST segment change measured from the body surface map and measured from the Holter leads was 0.29. Therefore, shifts in ST segment potentials can accurately be estimated from a small number of unipolar leads. In contrast, current bipolar ambulatory recording techniques may introduce significant bias to such estimates.

Alternative methods of excitation time determination on the epicardial surface

The presence of fractionated electrograms (EGs) observed on the ventricular surface near an epicardial pacing site can lead to apparent errors in the assessment of excitation times when using the minimum time derivative method. This study used two spatial methods of excitation time determination in attempt to improve assessment of excitation times in areas where the time derivative of the unipolar EG has multiple minima. The maximum magnitude of the spatial gradient (|/spl nabla/V|) of the potential field and the zero crossing of the time course of the Laplacian (/spl nabla//sup 2/V) of the potentials were used as alternate means of excitation time determination. Epicardial potentials were recorded from electrode arrays of 525 and 744 sites at 1 mm and 2 mm uniform spacing during epicardial, ventricular pacing. Isochronal maps were generated using all 3 methods. The average difference in excitation times when comparing the time derivative method with the spatial gradient was 0.7/spl plusmn/1.8 msec (R/sup 2/=0.98) and 0.6/spl plusmn/2.0 (R/sup 2/=0.97) when compared to the Laplacian method for all leads where no fractionation was observed (n=1190). For leads where multiple-peaked time derivatives were observed the differences in the excitation times between the temporal and the spatial methods ranged from -11 to 16 ms (R/sup 2/=0.89). In conclusion, the spatial gradient and Laplacian approaches to excitation time determination are highly correlated with the temporal method and for cases of multi-peaked time derivatives, can provide results that agree with those from isopotential maps and conduction velocity measurements.

Experimental study and removal of the drift of the reference potential from the unipolar electrogram

Unipolar electrograms contain an electrical component produced by changes in the reference potential as a function of time during ventricular excitation. The purpose of this work was to evaluate and remove this component from experimental data. Successful removal of this component resulted in signal morphology that is consistent with the potential distributions.

Wavelet Analysis Of The Signal Averaged Electrocardiogram

Wavelet analysis of signal averaged electrocardiograms is compared to the more usual analytical technique of bidirectional high pass filtering. Wavelet analysis is shown to be as good a predictor of ventricular tachycardia as bidirectional filtering. In addition, wavelet analysis shows details of the electrocardiogram that are not visible in the bidirectional filtered waveform.

Optimal lead selection for detection of ischemia

A comparison of the sensitivity to ST segment changes between a normal Holter lead set and an optimized four lead unipolar lead set was made. Both lead sets were compared with a 32 lead body surface map to see which could most accurately determine peak changes in the ST segment during coronary angioplasty. The unipolar four lead predictions were made by using a linear estimator of the entire 32 lead set. The correlation between m e maximum change in the 32 lead set and that predicted by the 4 lead set was 0.96. The correlation between the 32 lead set and the Holter leads