Hello,
I had the same situation a few weeks ago. I am asking that patients mom to contribute to this question too.
My first piece of advice is to ask for an echocardiogram exam. You don’t mention the degree or severity of the heart murmur (usually graded on a scale of 1-6) and it is important to say that a low grade doesn’t always indicate it is safe or irrelevant.
From here you can better assess anesthetic risk.
In general I tell clients to take out the spleen before it becomes a problem. A bleeding spleen can kill a dog quickly. So quickly you may not even have time for successful surgical intervention.
You have done an amazing job with diagnostics and treatment options. Don’t stop here.
Please let us know what happens.
Best of luck

this is the answer from my client, Trish, who’s dog had a very similar issue (heart murmur and incidental splenic mass found via ultrasound).

“Wow, what you are going through sounds quite sillier to what I just experienced with my oldest dog, who is almost 12. He is a Lab mix weighing about 54 lbs. A few months back he had is senior checkup, including a full blood panel and all looked good except trace amounts of blood in his urine and he has had a heart murmur his whole life and had never been on heart medication. He ate carpet a few weeks ago and while doing X-rays and ultrasounds to be sure it wasn’t causing a bowel obstruction, we found he had an enlarged heart and two masses on his spleen, with no indication of metastasis. He was acting himself, except for the belly issues from eating the carpet. Once we knew the carpet issue was under control, I got a second opinion about the spleen and had long talks with my vet about the situation. Both veterinarians suggested a splenectomy, and also to consider a cardiology evaluation to be sure his heart was strong enough for surgery. I was so nervous about what to do, he is older, acting fine and his quality of life is great. The thought of putting him through what I felt was major surgery was so frightening, especially since he is older, but I was also getting nervous about internal bleeding and ending up in emergency surgery and that being life threatening. I made the decision to see a cardiologist and they cleared him for surgery, and started him on heart medication; he then had the splenectomy the next day. The recovery was quick and he actually wasn’t much bothered by the surgery, maybe a day or two of lethargy. By the end of the first week I was having to keep him from playing and running, he was back to himself. Now, a few weeks later, I am so thankful I made the decision I did, he is just so happy, playful and back to being his energetic (in the old man style) self and even starting play sessions with his siblings. It was a roller coater ride for the few weeks after getting the diagnosis, getting a second opinion and making the decision to go forward with the surgery, but it was all worth it to have him back and not sit here and worry if he might be bleeding internally. I learned your mind can play those games with you when you know there is a potential unseen problem with your dog. One thing that made the decision a little easier was my vet telling me the surgery wasn’t much more on him then say a spay for a girl, it really wasn’t as major as I thought it would be, and based on the experience I went through, I’d agree it really didn’t take much out of him at all. I wish you all the best in making your decision, and I’d be happy to help answer any other questions you may have.
Trish”

hope this helps.. i am going to ask Trish to add Reggie’s story to our storylines section asap!