Effects of myocardial fiber orientation on the electrical induction of ventricular fibrillation

Although many studies have documented the importance of myocardial fiber orientation in the generation and maintenance of reentrant arrhythmia, its role in the induction of reentry by a single strong premature stimulus is held to be minimal. To study the importance of fiber orientation on the mechanisms of ventricular vulnerability to single strong premature electrical stimulation, computerized mapping studies using 56 closely (2.5-5 mm) spaced epicardial bipolar electrodes were performed on six open-chest dogs to determine the patterns of activation after the application of a single strong premature stimulus. In an additional three dogs, both epicardial and endocardial mapping studies were performed with closely spaced electrodes. The baseline driving stimulus (S₁) and the premature stimulus (S₂) were given to the same or different sites on the right ventricular epicardium. When the line connecting the S₁ and S₂ (the electrical axis of stimulation) was roughly parallel to the fiber orientation, the vast majority of the early sites were found to lie between the S₁ and S₂ sites, with none of the early sites occurring on the side of the S₂ site opposite to the S₁ site. At the onset of ventricular fibrillation, figure-eight reentry was easily demonstrated. In comparison, when the S₁ and S₂ were given to the same site, or when the electrical axis of stimulation was roughly perpendicular to the fiber orientation, the early sites could occur on all sides of the S₂ site. In these instances, a figure-eight reentry was not present at the onset of ventricular fibrillation, although this pattern could develop after the first few beats. These findings indicate that myocardial fiber orientation is an important factor in determining the patterns of activation during the electrical induction of ventricular fibrillation.