Directional variability of cardiac fibres stimulation threshold. Relationship with orthogonal defibrillation shocks

The marked improvement recently obtained in defibrillation using separated pulses delivered in orthogonal directions suggests the possible dependence of heart-fiber threshold on longitudinal or transversal stimulation. The aim of this work was to identify this phenomenon on isolated cardiomyocytes. These cells, placed in a culture box, appeared spontaneously arranged along multiple directions. A 2-ms, 1.5-Hz stimulation was delivered between two parallel electrodes. Intensity was progressively increased until the cell contracted. The most longitudinal myocytes were always stimulated first (8 to 10 V). A progressive recruitment was then observed, and an 18- to 20-V intensity was required to provoke the contraction of the most perpendicularly stimulated myocytes. This variability, in a ratio 1 to 2, of stimulation threshold appears to be a fundamental property of cardiomyocytes. These results clearly show that, after delivery of two separated orthogonal defibrillation shocks, heart fibers were totally depolarized using the lowest energy required