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kBSTRACT A custom-made probe, consisting of four electrodes arranged so that two orthogonal )ipolar electrograms could be recorded from a single site, was used to record epicardial activity during itrial and ventricular pacing in five normal and five anesthetized open-chest mongrel dogs with nyocardial infarction. Unfiltered bipolar electrograms recorded with a 2 mm interelectrode distance iveraged 36 + 15 mV in amplitude and 16 5 msec in duration in normal areas and 14 ± 11 mV and 13 ± 12 msec in infarcted areas (p < .01 infarct vs normal). The bipolar electrograms were vector ,ummed so that a vector loop could be generated at each site. The direction of epicardial impulse )ropagation as determined by multipoint isochronal activation mapping was compared with that ndicated by maximum x,y deflection of the vector loop. At 203 sites (141 normal and 62 infarcted) :here was a median error of only 13 degrees and an excellent correlation by linear regression (r2 = .95).[n normal myocardium vector loops were straight (60%), open (21%), or hooked (19%). In infarcted Tnyocardium, notched and irregular loops were occasionally seen. However, a clear maximum x,y leflection was still obtained from 98% of infarcted sites. During ventricular pacing in normal dogs, iniform epicardial conduction was observed for up to 4 cm longitudinal to fiber orientation but only 1 -m transverse to it. At selected sites longitudinal to fiber orientation conduction velocity was 0.618 n/sec, electrogram duration 12 msec, and vector amplitude 76 mV compared with 0.304 m/sec, 18 nsec, and 38 mV during conduction transverse to fiber orientation (p < .05 for all comparisons). Vector mapping of epicardial activation was performed during ventricular tachycardia induced by )rogrammed stimulation in two of five 2-week-old canine myocardial infarcts. Aside from minor .rregularities 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 f earliest activation. We conclude that vector loops generated by summing orthogonal local bipolar -lectrograms accurately represent the direction of epicardial activation in both normal and infarcted Tnyocardium. Such loops may prove useful in mapping tachycardias and in clarifying details about :ardiac activation processes. Circulation 74, No. 3, 603-615, 1986. MAPPING of the sequence of ventricular activation with single or multiple epicardial, transmural, or endocardial electrodes has provided basic information about the mechanism and origin of both experimental and clinical arrhythmias.'-1 However, a limitation of these studies is that they either require sophisticated recording techniques with expensive computerized analysis systems3' 6 or large numbers of recording sites using a single probe electrode to define areas such as the site of origin of ventricular tachycardia (VT

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(Fig 3). The patient's condition showed no progression while under observation and the man was discharged.