I am very sorry you have lost your kitten. This is a rare disorder and one
of the cardiomyopathy type diseases and is found in Siamese and Burmese
cats as a genetic disorder. There is little research but there is evidence it
is genetic unfortunately so since it usually kills at a young age it is
likely a carried gene and both parents of your kittens carried it. Very very
few cats with this disease would reach breeding age.
Unfortunately the prognosis is not good. Oxygen therapy if the kitten is
struggling to breathe and long term treatment can help but you need to see
a cardiologist immediately to find which drugs will help the most, if it is
possible. There is, I believe, the possibility of surgery but research into
this disease is in its infancy. I have found you some useful links for the
disease in cats.
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