I'm sorry you and Herman have to go through this. I have no personal experience with treating cardiovascular issues, but they are relatively common - and so is the prescription of Lasix. Cardiac enlargement and pulmonary oedema are usually seen in those cases, I know of several people who've went through congestive heart failure with their hogs. Unfortunately long-term prognosis is poor.
The next step would indeed be a echocardiogram. If it's a heart issue, a complete blood count and biochemical profile can be done to monitor the effects of treatment (if you decide to go with treatment).