The opening statement of a journal article reads
"Parkinson’s disease patients frequently have symptoms and signs of autonomic nervous dysfunction that are the source of considerable disability. Recent studies have revealed that most patients with Parkinson’s disease, and all with Parkinson’s disease‐associated orthostatic hypotension, have a loss of cardiac sympathetic innervation."

http://brain.oxfordjournals.org/content/127/4/768.short

So i take this to mean a loss of nerve supply to the heart, add the potential effect of dopaminergic treatments and then to that add non parkinson cardiac problems.....

I must do a search, perhaps this has been discussed before.