There could be a remedy in the near future that could drastically improve cardiomyopathic individuals' heart muscle and that is stem therapy. There has been some success in that area recently.
kenkeith gave a good answer. It's not that the doctors are going to look at one test and then put your son on the transplant list if the results from that one test aren't good enough. It's that right now, your son's heart isn't pumping effectively enough to sustain his life longterm, and if it doesn't improve, then something drastic will have to be done to help your son and to extend his life.
Again, I'm sure it is a shock to hear the doctors talking about a transplant. I can think of one way in which that is a good thing, though. It means that the doctors are willing to go to any lengths to save your son's life. They are not going to write him off, if by chance he doesn't respond to initial treatment. Actually, there have been people who wanted a transplant and didn't get it, but it sounds like your son's doctor's aren't ruling out any treatment that is available in medical science. If it were me who was the patient in your son's situation, I would want my doctors to have that attitude.
Maybe at one point, kenkeith's doctors were thinking transplant, too. Kenkeith is a good example that where there's life, there's hope. He went through something terrible, and he's still here to talk about it.
Six years ago my EF was 13%-29% (cath, echo), and today the formerly enlarged heart is normal and EF is 59%. My condition was due to ischemia (occluded blood vessels), and I was treated with ACE inhibitor and beta blocker medication...that reduced the heart's workload and provided enough relief for the heart to positively respond and heal.
Normal EF is 50-70% and 29% and below is considered heart failure range meaning medically the heart hasn't enough contractility to pump an adequate amount of blood into circulation with each heartbeat to meet oxygenated blood demand.
It is true there are flucuations of the EF, heart rate, respiration rate, and blood pressure, etc.. The interaction of these factors help maintain a balance of left and right side blood flow in balance. The heart slightly dilates (left ventricle) and with the dilation is an increase in contractility (increase of EF), etc. When the heart is overworked due to high resistance it pumps against high blood pressure, etc. and the left ventricle abnormally dilates to a size that reduces EF.
When a doctor by differential analysis has excluded all posible or known conditions to cause the problem it is characterized as an idiopathic (cause unknown) condition.
Myocarditis is an elusive illness to study, diagnose, and treat because the clinical presentation may range from nearly asymptomatic to overt heart failure requiring transplantation; a myriad of causes exist, and it is occasionally the unrecognized culprit in cases of sudden death. Myocarditis is defined as inflammatory changes in the heart muscle and is characterized by myocyte necrosis.
The upcoming echo will be able to visualize the heart wall movement with ultrasound and measure the contracttility of the left ventricle...also heart chamber sizes. Favorable results with medication and etc. may change to a known diagnoses and successfully treated.
Hope this helps give you some insight.
I really can't address anything about myocarditis. But I just want to say I'm sorry for the situation. Also, if the doctors want to put him on the transplant list, I would let them do it, because there usually is a significant length of waiting time for a donor heart for any individual who is on the list. The doctors can continue to treat him aggressively while he is on the waiting list, and if he then improves enough that he no longer needs to consider a transplant, he can always be taken off the list. I know it must be terrible to even hear the words, "heart transplant." It has to be a terrible shock. But I would not delay getting on the list if the doctors think that is what he needs. You don't want your son's donor heart to pass him by, if it should turn out that he does need it. Good luck.