A. Heart murmurs don't always mean that there is heart disease present. I typically recommend survey chest films to evaluate heart size and the lung fields. As long as these look good then there is no reason why you should have to limit exercise.
To answer your other question, there are prescription diets that have low sodium and therefore are good for heart health. Ask your vet about Hills H/D or Hills K/D as options or he/she may have other low protein diets to recommend.
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