Results of a previous study suggested the possible dependence of heart-fiber stimulation threshold on the stimulus electric axis orientation. The aim of this work has been to demonstrate this phenomenon on isolated cardiomyocytes. The cells were placed in a culture box where they appeared spontaneously arranged along multiple directions. Stimulation pulses were delivered between two parallel electrodes, increasing progressively in intensity until the cells contracted. This contraction was always obtained for the most longitudinally stimulated myocytes first. Progressive recruitment was then observed by increasing the intensity, the most perpendicular myocyte contraction being obtained for a value approximately double that necessary to stimulate the longitudinal ones. This clearly shows that orthogonal shocks depolarize all heart fibers using the lowest energy required.
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