Vector mapping of myocardial activation

A custom-made probe, consisting of four electrodes arranged so that two orthogonal bipolar electrograms could be recorded from a single site, was used to record epicardial activity during atrial and ventricular pacing in five normal and five anesthetized open-chest mongrel dogs with myocardial infarction. Unfiltered bipolar electrograms recorded with a 2 mm interelectrode distance averaged 36 +/- 15 mV in amplitude and 16 +/- 5 msec in duration in normal areas and 14 +/- 11 mV and 23 +/- 12 msec in infarcted areas (p less than .01 infarct vs normal). The bipolar electrograms were vector summed so that a vector loop could be generated at each site. The direction of epicardial impulse propagation as determined by multipoint isochronal activation mapping was compared with that indicated by maximum x,y deflection of the vector loop. At 203 sites (141 normal and 62 infarcted) there was a median error of only 13 degrees and an excellent correlation by linear regression (r2 = .95). In normal myocardium vector loops were straight (60%), open (21%), or hooked (19%). In infarcted myocardium, notched and irregular loops were occasionally seen. However, a clear maximum x,y deflection was still obtained from 98% of infarcted sites. During ventricular pacing in normal dogs, uniform epicardial conduction was observed for up to 4 cm longitudinal to fiber orientation but only 1 cm transverse to it. At selected sites longitudinal to fiber orientation conduction velocity was 0.618 m/sec, electrogram duration 12 msec, and vector amplitude 76 mV compared with 0.304 m/sec, 18 msec, and 38 mV during conduction transverse to fiber orientation (p less than .05 for all comparisons). Vector mapping of epicardial activation was performed during ventricular tachycardia induced by programmed stimulation in two of five 2-week-old canine myocardial infarcts. Aside from minor irregularities caused by impulse spread around areas of block, vector loops indicated when impulses were spreading away from the area of early epicardial activity and thus directed mapping to the region of earliest activation. We conclude that vector loops generated by summing orthogonal local bipolar electrograms accurately represent the direction of epicardial activation in both normal and infarcted myocardium. Such loops may prove useful in mapping tachycardias and in clarifying details about cardiac activation processes.