Clen may effect the heart. Forget the science and take a long trip around the 40+ forums devoted to bodybuilding searching specifically for users of clen and heart palpitations & abnormal heartbeats occuring well AFTER the user has ceased using.
What you will find will be at least a dozen to twenty or so who have these problems, months, even a year after a clen cycle. Some doses were very low and the cycle length very short.
The vast majority of users on the forums do not post that they have had these sorts of problems, but I have seen enough to know that some do.
So yes of course you should ALWAYS be worried about heart health no matter what you do be it an ECA stack or whatever. Some compounds however merit more concern than others. For instance phera-plex may thicken one side of the heart lining...probably reversible...cabergoline has been shown to bring on leaky heart valves...
Anyway Clen just happens to be one of those compounds that merits a high level of concern when it comes to heart health.