editted to add I just realized the orgional post was from last October,(after I posted my answer) hopefully you already have your answers about your husband
When I googled Sluggish heart, it seem like dpepression can affect the heart function and make it sluggish (of course there are also studies saying there is no effect) since it seems to be a new finding years after his surgery, but when you are going thru the worst thing a parent can go thru, I wonder if depression is a factor
http://psy.psychiatryonline.org/cgi/reprint/45/2/129.pdf
This article is about heart attack patients,(NOT valve patients) but was the first thing I read linking depression to low HVR
http://www.nurses.com/article.mvc/Sluggish-heart-responses-may-tie-depression-t-0001?VNETCOOKIE=NO Sluggish heart responses may tie depression to heart attack deaths
October 29, 2001
..HRV measures cardiac autonomic function, which includes the sympathetic and parasympathetic nervous systems. These help control blood vessel size, blood pressure, the heart's electrical activity and its ability to contract...
A couple of the articles say, treating the depression may increase heart function, IF it is related to depression, but Honestly I just posted about these incase you might want to ask his doctors if they think depression might be playing a role in him and if so any recomendations.