Session S55.2

The Safety Factor Approach in the Analysis of Reentrant Patterns of Activation in the Ischemic Virtual Heart

L Romero*, B Trenor, JM Ferrero, J Sáiz,
G Moltó, JM Alonso

Universidad Politécnica de Valencia
Valencia, Spain

During the acute phase of ischemia, ventricular arrhythmias sustained by reentrant patterns of activation can lead to ventricular fibrillation (VF). Unidirectional conduction block (UDB), resulting from heterogeneities in excitability within ventricular tissue, is essential for the initiation of reentry. Traditionally, VF has been directly related to dispersion in refractoriness. However, further theoretical studies relating refractoriness and reentry generation would be helpful to understand the implication of excitability and its dispersion. Recently, the safety factor for conduction (SF) has been proposed as a useful parameter to analyze propagation of action potential (AP) and conduction block. The aim of the present work was to analyze the relative role of refractoriness and the source-sink relationship in conduction block leading to reentrant patterns of activation during regional ischemia. For this purpose, the electrical activity of a 5.5 x 5.5 cm ventricular tissue subject to regional acute ischemia has been simulated using a modified version of the 2000 Luo-Rudy AP model. Ischemia was reproduced by means of its three main components: hyperkalemia, hypoxia and acidosis. The model of regional ischemia also included a realistically dimensioned border zone (BZ) for its three main components. The stimulation protocol consisted on the delivery of two longitudinal and rectangular pulses (S1-S2) with different coupling intervals (CIs). Our results show that premature stimuli applied at CIs comprised between 170 ms and 204 ms lead to reentry. We calculated the SF during reentry and the patterns were very similar for all the conducted simulations. The wavefront started propagating in the normal zone with a SF of 1.6. Then, the SF increased slightly in the BZ, which is consistent with the supernormal conduction phenomenon, and finally dropped below unity when propagation block ensued. At this stage, two wavefronts surrounded the zone of block, the central zone (CZ), and collided in the distal CZ, where the SF increased to 2.7 approximately, due to the bigger depolarizing current. Then, the wavefront penetrated in the CZ, already recovered from refractoriness, where the SF decreased to 1.2 because of the reduced excitability. Another relevant result of our work is the analysis of the arch of block. We observed that the premature stimuli (applied at different CIs of the VW) were blocked at sites where cells whose excitability was completely recovered in one third of the reentrant. In the other two thirds, the block ensued in sites where cells were still refractory. In summary, the pattern of SF for reentrant electrical activity during regional ischemia has been characterized and UDB may not only be due to refractoriness but also to a mismatch in the source-sink ratio.

(Abstract Control Number: 135)