It could have be something it was born with or because of conditions right after hatching. When babies hatch and there is not enough humidity and instead hot dry air, it could affect their kidney development, they are so young and they are basically being desiccated when they need moist air and need to be soaked ... and that bad start in life can eventually weaken their heart. I am sure that is what I read. Very sorry. It's heart breaking, we know. Be strong. You loved your little. He knows that.