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Advanced optical methods for monitoring and controlling the cardiac electrical activity.
Synchronous cardiac contraction is guaranteed by a sophisticated conductive system that allows the action potential, originated in the pacemaker cells, to rapidly recruit each cardiac myocyte of the heart. In addition, every cell is activated in its entirety thanks to a complex network of membrane invaginations, named the T-tubular system. T-tubules allow the electrical trigger to reach every portion of cardiac cells and eventually producing a coherent contraction of the cell. Both these two systems are needed to maintain the proper function of the heart. Here, we developed a random access multi-photon (RAMP) microscope that, in combination with a custom synthesized voltage-sensitive dye, is used to simultaneously measure action potentials and intracellular calcium transients at multiple sites within cardiac cells with submillisecond temporal and submicrometer spatial resolution. Action potential propagation across the whole heart was also monitored using an ultra-fast wild field mesoscope operating at 2 kHz. In this configuration, control of the electrical activity was achieved by employing transgenic mouse hearts expressing Channel Rhodopsin-2 (ChR2). In order to draw arbitrarily-chosen ChR2 stimulation patterns with sub-millisecond temporal resolution, the mesoscope was implemented with an optical solution based on a digital micromirror device. We employed these to study the mechanistic features of ventricular tachycardia and we designed mechanistically-based cardioversion/defibrillation patterns exploiting the transient refractoriness of myocardium produced by the ChR2 stimulation. Multiple regions of conduction block revealed to efficiently defibrillate arrhythmic hearts but with lower energy requirements as compared to whole ventricle interventions. In conclusion, this work demonstrates the capability of optical microscopy in dissecting cardiac electrophysiology at the single cell level as well as across the whole organ.