There is provided a light emitting device which includes a light emitting element having a main emission peak in the wavelength region of greater than 420 nm and equal to or less than 500 nm, and a phosphor layer formed on the light emitting element. Water management and reactant distribution may be improved by increasing pressure gradients between adjacent channels. In one embodiment, the pacing rate is decreased by a set amount after pacing a set number of cardiac cycles.