Hey @WATTILA,
Welcome to the forum! 
This is a great question. In addition to what @mcneese.chad found, here are a couple of articles about cardiac drift.
In general, I think the best way to prevent this from happening for as long as possible is to stay hydrated throughout your ride.
In terms of which metric to follow (power vs. HR), metabolically speaking, I think keeping your HR in zone is most impactful, though keeping your power on target might be beneficial for other training purposes.
I’d be curious to hear what @The_Cog has to say about this topic if he hasn’t spoken on it already.