My Percheron has a heart murmur and it hasn't affected her in the slightest. However her murmur use fairly minor, so always check with your vet first.
Hi Rachel! It is not uncommon at all that horses have a heart murmur, in fact many do. In the general case, it has no incidence on their performances or daily life and can be caused by mitral regurgitation. A study tested 750 race horses and found out that 320 had a heart murmur, and it diminished the performances of only 1/3 of them. If your vet is not alarmed, your mare's heart murmur should not be a matter. You will have to be attentive that she does not push too hard on effort when it is very hot for example (but as she must be retired it should not be a problem), and check on if she starts to cough regularly. You can also have an echography of her heart made, and check it yearly for example or if she shows any abnormal sign (coughing, tiredness...). I hope this helps! Your vet will know exactly what to explain, you can ask him more details :)