Instantaneous vector mapping of ventricular tachycardia in canine epicardium during the healing phase of myocardial infarction.

Abstract

The purpose of the proposed research was the implementation of a vector mapping (VM) technique in order to characterize abnormal conduction in infarcted areas of the canine heart. Twenty-five mongrel dogs underwent programmed stimulation 4-6 days after total occlusion of the left anterior descending coronary artery. Activation was recorded from the epicardium using an 8 x 14 unipolar electrode plaque with a 2.5 mm inter-electrode distance. VM was performed by summing computer generated orthogonal bipolar electrograms to create a vector loop at each site. Isochronal mapping and electrogram (EG) morphology were also examined. Slow conduction was suggested by VM whenever vectors were oriented transverse to lines of apparent block. Results: Eight dogs (25%) had inducible monomorphic VT, 5 of which had epicardial reentrant circuits in the form of single loops (4 animals) or double loops (1 animal). In all 5 dogs, reentry occurred around lines of block 0.5 to 2.5 cm long and parallel to fiber orientation. Six different regions of suspected block were identified. In 3 of 6 regions, isocrones were perpendicular to the arcs of block and EGs were characterized by "double potentials". VM confirmed the conduction block by failing to show any vectors transverse to fiber orientation. In 1 of 6 regions, isochrones were parallel to the arc of suspected block and VM confirmed slow conduction. EGs were not fractionated in that region. Isochrones and VM were discordant in the remaining 2 regions, where isochrones suggested slow conduction and VM suggested block. In one of those regions, histology and electrogram data supported the presence of anatomic block. In the 2nd region, neither slow conduction nor block could be established with certainty but EGs showed double potentials. Conclusion: (1) Lines of block parallel to fiber orientation that form during sustained VT in the post infarct canine model may be lines of slow conduction in some cases; (2) Slow conduction across those lines is not always associated with fractionated EGs; and (3) VM may provide a more reliable differentiation between block and slow conduction than standard techniques.