This could be a complicated situation, so I’ll try to make some helpful points without writing a book.
First of all – a heart murmur is only a sound, and it is only a symptom. It is NOT a disease or a diagnosis. The important thing to determine is what heart disease Bentley has and what is the stage of his disease. (Stage of heart disease is not the same thing as grade of murmur. This misconception is widespread.)
Normally we assume that middle-aged and senior Cavaliers have degenerative valvular disease (endocardiosis or MVD). However, you say that Bentley most likely has Cushings. Untreated Cushings can cause heart failure. Blood clots in the lungs can also result from Cushings, and this can cause severe pulmonary hypertension (from a pulmonary thromboembolism). PH symptoms (difficulty breathing) are similar to the symptoms of heart failure, but the treatment is very different. Lasix can make PH worse. Cushings also frequently causes general hypertension. It is really optimal to have a specialist treat dogs with heart disease and Cushings.
The first thing that I would recommend is that you have appropriate tests for Cushings done asap so that you have an accurate diagnosis and can decide on treatment for Cushings.
http://www.vetmed.wsu.edu/cliented/cushings.aspx
http://www.kateconnick.com/library/cushingsdisease.html
It may be that Bentley has concurrent Cushings and MVD. Cushings can make MVD worse, so you really want to control and treat both diseases. I would probably want a board certified internal medicine specialist (cardiologist would be my second choice) rather than a GP vet to give a diagnosis and treatment plan for the best possible outcome. Determining the best treatment plan for Cushings, esp. with concurrent heart disease, can be tricky as there are several medication options. I’d have chest x-rays, full blood chemistry, a blood pressure test, and possibly an echocardiogram done after the tests for Cushings.
I presume that Bentley has symptoms since your vet has already prescribed Lasix. A short course of Lasix alone is sometimes done to see if the symptoms resolve (called treating empirically) in order to try to confirm a diagnosis, but Lasix alone is never appropriate treatment for heart failure. If there are symptoms of difficult breathing that do not improve with Lasix, I’d be very concerned about pulmonary hypertension.
Frankly, I wouldn’t worry at all about supplements at this point as you have more important things to determine and on which to spend your money. You need to determine appropriate drug treatment first without worrying about adding other things like supplements into the mix. If the tests for Cushings are negative, you can focus sooner on the MVD with supplements, etc.
http://veterinarycalendar.dvm360.com/avhc/article/articleDetail.jsp?id=724749
Copied from above link: Hypertension is relatively common (50% or greater) though with the lack of blood pressure monitoring devices in many practices it often goes undiagnosed. Pulmonary thromboembolism, recurrent (often asymptomatic) urinary tract infections, proteinuria, pancreatitis, pulmonary mineralization, and calcium oxalate urolithiasis are also often frequently seen with Cushing's disease. Recently it has been noted that many dogs with hyperadrenocortism are hypoxic, whether or not they have mineralization of the lung parenchyma. This can be a serious consequence, leading to distress as well as excess strain on the right side of the heart. Some of the clinical problems caused by Cushing's disease are more bothersome than dangerous. Other clinical problems are life threatening such as thromboembolism or pancreatitis. Still other clinical problems can aggravate other disorders that the patient may have, such as is the case if hypertension is present in a dog with underlying heart disease. This is not an uncommon scenario since older dogs tend to have Cushing's as well as valvular heart disease. Hypertension in a dog with valvular problems can be a factor that leads to more rapid progression of the heart problem as well as difficulties in treating heart failure if it occurs.
Pat