IDENTIFICATION OF CONDUCTION BLOCK IN CARDIAC-MUSCLE - IN-VITRO OBSERVATIONS IN CANINE EPICARDIUM

Objective: The aim was to examine the ability of several previously pr oposed criteria and of ''vector mapping'' to distinguish slow conducti on from conduction block in canine myocardium. Methods: Two different in vitro preparations of canine epicardium were used. In 10 tissues, a n anatomical barrier was simulated by a cut (five longitudinal and fiv e transverse to fibre orientation). Eleven tissues removed two weeks a fter occlusion-reperfusion infarction were also studied. Isochronal ac tivation maps were constructed from extracellular and intracellular re cordings and vector loops formed by summing two orthogonally recorded bipolar electrograms were used to indicate the direction of cardiac ac tivation. Results: In the cut model, electrograms recorded from over t he anatomical barrier were usually normal or showed double potentials but could occasionally be fractionated. Isochronal activation maps obt ained from extracellular recordings were able to identify five of five anatomical barriers transverse to, but only one of five barriers long itudinal to, fibre orientation. The direction of cardiac activation in dicated by vector loops identified conduction block in all 10 preparat ions. In 11 tissues removed from canine ventricles with experimental m yocardial infarction, microelectrode recordings were used to character ise regions as either slow conduction or conduction block. Isochronal activation patterns obtained from extracellular recordings generally s howed impulses proceeding through zones of conduction and around zones of conduction block but disagreed with the results of microelectrode impalements in two of 11 cases. Electrogram morphology was also not al ways able to distinguish slow conduction from block. The direction of cardiac activation determined by vector mapping accurately characteris ed all regions of tissue as showing either slow conduction or conducti on block. Conclusions: Limited regions of conduction block or slow con duction are frequently present in epicardial tissues removed from expe rimental myocardial infarction. The morphology of extracellular electr ograms and isochronal activation mapping performed from extracellular recordings is often but not always able to distinguish slow conduction from conduction block. Vector mapping is useful in distinguishing slo w conduction from conduction block in these situations and may help ev aluate myocardial conduction patterns.