Understanding the Loss of Capture during Mechanical Pacing
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Pacemakers are a corner-stone of modern heart rhythm management. However, restrictions and complications motivate exploration of alternative pacing modes. Mechanical stimulation can cause excitation, but pacing capture with repetitive stimulation is unsustainable. Various causes for a loss of pacing capture have been hypothesized, including changes in tissue mechanics and rundown of stretch-activated currents. Our goal is to understand why mechanical pacing fails. We performed ventricular mechanical pacing experiments in isolated rabbit hearts. Results demonstrated that the sustainability of mechanical pacing depends on the degree of tissue indentation and that capture is reversibly lost in a stimulation frequency-dependent manner. Increased tissue stiffness, elimination of active tension development and reduced background heart rate did not affect sustainability. Optical mapping revealed an increasing delay between mechanical stimulation and excitation with each paced beat, which suggests that a continuous reduction in depolarising current may account for the loss of pacing capture.