Effect of Purkinje-Myocyte Junctions on Transmural Action Potential Duration Profiles

Action potential characteristics change across the wall of the ventricles. It is generally accepted that epicardial action potential duration (APD) is shorter than that on the endocardium, although this difference diminishes with pacing frequency. Some studies have reported midmyocardial islands of increased APD and suggested a third cell type to account for this. Furthermore, human studies report this profile changes during heart failure. We postulate Purkinje system coupling may account for increased subendocardial APD prolongation. This study used computer simulations to model the effect of a purkinje-myocyte junction (PMJ) on the cut surface of a ventricular wedge preparation measuring 0.5 x 0.5 x 1 cm. Transmurally, the 1 cm was divided into epicardium (3 mm) and endocardium (7 mm) and a human ventricular ionic model with appropriate adjustments for each zone was used. A single 3 mm long Purkinje strand was inserted 2 mm into the wedge. To model Purkinje cells, the ventricular ionic model was modified to increase resting level and APD. Coupling parameters (resistance, junction size, conduction asymmetry) were varied to assess their effects. Pacing was performed by transmural current stimulation to a region on the endocardium. The PMJ significantly affected APD as measured on the cut surface. This affect was more pronounced the closer the PMJ was to the cut surface and the larger the PMJ was.We conclude that a third type of cell is not needed to produce islands of increased APD. Intrinsically longer APDs of the Purkinje system increase local myocardial APD in the vicinity of the junction. This complicates interpretation of myocardial APD data as the density and proximity of PMJs to the recording surface will affect surface APD.