Poor boy. I have no experience with this myself. Has the vet given you any advice for this condition? Do you know how severe his diagnosis is? If it has only been diagnosed through a physical, I suggest further methods diagnosis to confirm and find out the severity of the condition. This can be done with a radiograph, electrocardiogram or the test of choice (according to Mitral valve dysplasia | Canine Inherited Disorders Database | University of Prince Edward Island), an echocardiography.
According to some research, surgery can be an option and it is the only option that can completely cure it. Medication can be used for management but will eventually become ineffective. Heart surgery, in which the defective mitral valve can be replaced, is expensive and risky. The prognosis and recovery for this is easier and higher in a healthy, young dog with their renal system and other vital organs in ideal functioning condition. For those who survive, there may be cured completely or the condition of mitral valve dysplasia (MVD) is stopped at that stage and stabilised with medication.
Meanwhile, I found these articles for you to read through:
Mitral Valve Dysplasia - Treatment Keywords: Mitral Valve Dysplasia, Mitral Valve Disease, Mitral Valve Disorder, Mitral Valve Malformation,
Mitral Valve Disease - Management Keywords: Mitral Valve Dysplasia, Mitral Valve Disease, Mitral Valve Disorder, Mitral Valve Malformation
https://www.cardiospecialist.co.uk/Mi...eDysplasia.pdf
and
Heart Failure: Heart Disease and Heart Failure: Merck Veterinary Manual.
To begin, you need to know the stage of the diagnosis. If at Stage A (identified as high risk but no structural disorders) or Stage B1 (asymptomatic patients who have been diagnosed with the condition, typically through a "murmur of mitral valve regurgitation," but "...have no radiographic or echocardiographic evidence of cardiac remodelling in response to CVHD" do not require medical treatment. Apparently, CVHD stands for "chronic valvular heart disease" and is also known as mitral regurgitation.
I suspect your Lab is at Stage B1 but I am by no means a trained professional nor know of the condition. This is all found via quick research. Have you considered finding a specialist?
If at Stage B1, a veterinary cardiologist may prescribe medication aimed to reduce the stress on the heart by slowing heart rate, relaxing the arteries and decreasing blood pressure. Management involves keeping up with dental care to avoid bacteria entering the bloodstream and affecting the heart, low salt and fat diets to reduce water retention, avoiding extreme exertion and adding the supplements: Vitamin C, Vitamin E Tocopherol, CoQ130, fish oils high in Omegas 3 and 6.
At Stage B2 (asymptomatic but have "significant valve regurgitation, as evidenced by radiographic or echocardiographic findings of left-sided heart enlargement"), medical treatment is necessary. Diuretics are drugs that work to force the kidneys to remove fluid from lungs and increase fluid excretion above what is normally required. There are significant side effects to this though which can be seen in the link. A daily mashed banana is recommended to help restore potassium and phosphorus levels.
ACE inhibitors slow down the progression of heart failure by "relax[ing] blood vessels, lower[ing] blood pressure and increas[ing] the supply of blood and oxygen to the heart..." Side effects come with this as well.
The above management strategies are also for this stage. Exercise is also crucial to remain healthy, but of course, care is needed to avoid extreme exertion. Breathlessness and excessive panting can indicate too much exercise. In addition, to provide comfort from fluid-filled abdomens, a comfortable bed can be used to "support the upper half of the body and relieve some of the pressure off the stomach." Furthermore, it is also important to avoid exposure to extreme humidity and temperatures.
A good idea is to keep a journal to keep track of your the condition's progression. Take notes of your dog's behaviour and resting respiratory rates. If the resting respiratory rate is above 40 breaths per minute, contact your vet.
I don't believe your Lab is at Stage C (have past or current symptoms of heart failure associated by the condition which may require hospitalisation and aggressive therapy) so I won't go into it here. You can read about it in the linked articles though.
Hopefully this has been of use despite myself, not having any experience with management of this condition. I don't know how much of it is known information to you (I bet you've been doing lots of research) so I hope it didn't feel like a waste of time reading it.
Good on you for persevering. Cherish each day with him. Shower him with love and care. Don't panic or despair. If it's only been diagnosed with a physical, there's a good chance he's only at Stage B (at best Stage B1). This is considered a mild stage and many live relatively normal lives. Furthermore, your Lab might just surprise you!
By the way, what's his name? We love pictures too.
Sending you good thoughts.
ETA: I do strongly suggest seeking a veterinary cardiologist though.