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 (|∇V|) of the potential field and the zero crossing of the time course of the Laplacian (∇²V) 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±1.8 msec (R²=0.98) and 0.6±2.0 (R²=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 (R²=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