Thank you for your query.
I am sorry to hear about the problems that your nephew is undergoing. I think that there is some communication problem as there is nothing as mitochondrial disease
. It can be cardiomyopathy
which can explain the low pulse and the grave prognosis. I suggest that you discuss with the doctor and understand the whole problem.
I hope I have answered your query to your satisfaction.
Wishing your nephew an early recovery.