Characteristics of the Temporal and Spatial Excitable Gap in Anisotropic Reentrant Circuits Causing Sustained Ventricular Tachycardia

Abstract

The excitable gap of a reentrant circuit has both temporal (time during the cycle length that the circuit is excitable) and spatial (length of the circuit that is excitable at a given time) properties. We determined the temporal and spatial properties of the excitable gap in reentrant circuits caused by nonuniform anisotropy. Myocardial infarction was produced in canine hearts by ligation of the left anterior descending coronary artery. Four days later, reentrant circuits were mapped in the epicardial border zone of the infarcts with a multielectrode array during sustained ventricular tachycardia induced by programmed stimulation. During tachycardia, premature impulses were initiated by stimulation at sites around and in the reentrant circuits, and their conduction characteristics in the circuit were mapped. All circuits had a temporal excitable gap in at least part of the circuit, which allowed premature impulses to enter the circuit. Completely and partially excitable segments of the temporal gap were identified by measuring conduction velocity of the premature impulses; conduction was equal to the native reentrant wave front in completely excitable regions and slower than the reentrant wave front in partially excitable regions. In some circuits, a temporal gap existed throughout the circuit, permitting the entire circuit to be reset over a range of premature coupling intervals, although the size of the gap varied at different sites. In other circuits, the gap became so small at local sites that even though premature impulses could enter the circuit, the circuit could not be reset. Premature impulses could terminate reentry in circuits that could be reset or not. We also found a significant spatial gap, which was identified by determining the distance between the head of the circulating wave front, which could be located on the activation map, and its tail, which was the site most distal from the head as located by the site of entry of the premature wave front into the circuit. The spatial gap could also vary in different parts of the circuit. Therefore, nonuniform anisotropic reentrant circuits have both a temporal and spatial excitable gap with fully and partially excitable components that change in different parts of the circuit.

Heterogeneous gap junction remodeling stabilizes reentrant circuits in the epicardial border zone of the healing canine infarct: a computational study

“…The excitable gap is not constant for any given cycle; there are cycles with a large variation (from 1% to 25%) and cycles with a smaller (5%-15%) variation in excitable gap. The values of the excitable gap (and their variation within the cycle) in the computer simulations are similar to experimental measurements of the spatial excitable gap in the canine infarcted model (12). Drift in the x direction occurs during cycles with a large variation in the excitable gap (Fig.…”

“…6). The cycle lengths of experimental VTs in the EBZ range from 200 to 278 ms (mean 230 ms) (12). The length of the line of block in the model is ϳ5 cm (Fig.…”

Static Relationship of Cycle Length to Reentrant Circuit Geometry

“…One end of the wave front, for example, might cross the isthmus exit while the other edge lagged behind at the isthmus entrance. This could act to destabilize the excitable gap, 17 which depends in part on synchronicity of conduction along symmetrical portions of the circuit to be maintained, and therefore act to destabilize the functional arcs of block that bound the reentry isthmus. The relationship between isthmus width and narrowest width (Equation 5) suggests that block lines bounding the isthmus are often tapered inward by a constant proportion, regardless of the magnitude of isthmus width.…”

Turbulent electrical activity at sharp-edged inexcitable obstacles in a model for human cardiac tissue

“…We relate the critical period for spiral wave formation via pacing to the period of the spiral wave rotating in the same tissue. However, the period of spiral wave rotation, by itself, is determined by the refractory properties of cardiac tissue and an excitable gap, which is again determined by the curvature (source-sink mismatch) effects (14,46,51). In highly excitable tissue, the refractoriness is expected to be the main determinant of the spiral wave period (46).…”