Trifexis

First let me say how sorry I am to hear about the sudden death of your dog. 

Here's a link on the clinical studies conducted on Trifexis.  Page 31 has a table with the adverse reactions experienced during the clinical trials.  I've been giving my dogs Trifexis since it came out and feel it is one of the safest products on the market.  I have not seen any definitive, confirmed evidence that Trifexis can cause any serious side effects.  One dog experienced a seizure during the trials and the drug is therefore not recommended for dogs with seizure disorders. 

http://www.fda.gov/downloads/AnimalVeterinary/Products/ApprovedAnimalDrugProducts/FOIADrugSummaries/UCM252248.pdf

Were you present when his heart failed?

As Jackie says it's not recommended for dogs who suffer seizures, however if your dog was one of those dogs where seizures may have been impending (they know little about that, but it does happen & I have a pug who has what my vet calls a "pre-seizurey condition";) then it may have triggered a seizure which could have triggered a heart attack.

That's no comfort, but as Jackie says it's regarded as one of the safest options out there.  I'm more than typically aware of the dangers b/c of my pug (and pugs are prone to seizures as a breed-related health issue anyway).

I am so sorry for your loss.  That's a whole lot for you to deal with.