Your vet was wise to suggest a cardiac consult. I belong to a facebook group which is run mostly by veterianarians and veterinary nutritionists....this group addresses the DCM issues caused by nutrition issues. The group of veterinarians, cardiologists and nutritionists suggest choosing foods which are made by companies which follow WSAVA guidelines for manufacturing foods, which employ full time veterinary nutritionists to formulate foods and which do long term feeding trials to ensure that dogs are healthy while eating these diets.
The companies which are recommended are Purina, Royal Canin, Iams, Eukanuba,and Hills Science Diet. As long as you chose a grain inclusive diet from one of these companies the food will be both nutritious and safe and healthy to feed.
I feed my Poppy Purina Pro Plan Sport and chose it because it is formulated for sporting, working and very active dogs. The one she eats is chicken based, however there is a Pro Plan Sport Sensitive Skin and Coat formula which is salmon based, and it has absolutely no chicken products in it. A tremendous number of people whose dogs have food allergy issues have had great results feeding this formula.
Poppy had no issues with transitioning to her Pro Plan and LOVES IT. She has been eating it for 7 months and is fit as well as has an insanely thick and lush coat.
I have found that Petsmart carries this as well as Chewy. Chewy has the best prices and if you have an order of $49 it ships for free in 2 days. Petsmart will match Chewy prices. A 37.5 lb bag lasts Poppy 94 days and at 2 cups per day, she is 45 lbs, the cost is $ .48 per day to feed!
Pro Plan Focus also has a Salmon Sensitive Skin and Coat formula which contains no chicken and it is a lower protein percentage than the Sport version. You might look at this one and compare it with the Sport version. It would depend on your girl's activity level. Poppy is lean and fit and maintained her weight when we switched food to PPP Sport....she neither gained nor lost weight.
I hope this is helpful....feel free to ask questions.
I wish you well with the cardiology tests and hope your girl recovers quickly.
While Poppy does not have DCM or any other cardiology problems I made the food change out of an abundance of caution. I was comfortable changing to Purina because we started feeding our family dogs Purina in 1954 and fed Purina until 2001. All of our dogs had long and healthy lives without any major illness or injuries. I strayed away to grain free foods in 2001 and now am back feeding Purina and grain inclusive foods. I am comfortable with how they manufacture and test their formulas.
Many dogs who have been diagnosed early with DCM have had successful recoveries with change of diet and supportive medications, as long as the damage to the heart is not too severe. DCM which is inherited in some breeds, not standard poodles, do NOT recover with change of diet, however medications can make them more comfortable for the duration of their lives. You might want to join the facebook group Taurine-Deficient (Nutritional) Dialated Cardiomyopathy. It is run by veterinarians, is science and fact based and is very helpful and supportive. It can be overwhelming to read all the information posted in the group, however, if your girl does have this the group will be an invaluable resource.