I spent years dealing with human ECGs so not sure how relevant this is but
In young humans it is quite common for the hear rate to be affected by respiration. The heart and lungs are so close that asking someone to breath in and hold their breath gets rid of the arrhythmia on the ECG.
Obviously you cannot do that with a kitten, but it might just be that it is perfectly normal. I hope it is.
As a child we had a pedigree dog, mum wasn't sure if she wanted to breed her so she had a contraceptive injection for the first 2 years and was then neutered once mum decided not to breed her.
That's awful and you can only really go by what your vet tells you. But really I think that, even is there is a small risk, then you still have to have her done as not to do it would make life so difficult. Even if you were able to keep her inside and so avoid the endless kittens, unneutered females that are unable to mate end up becoming completely derranged and often end up attacking their owners.
If the vet decides that the risk is very large indeed then you could ask about oral contraceptives. I know these are (or maybe were) bizarrely the most common form of preventing unwanted kittens in Spain. I met a Spanish lady who had never even heard of the concept of neutering but gave these to her cats, so that is something you could ask about. It is of course a terrible idea generally as it is so easy for it to go wrong and any cats that get lost will then be producing feral colonies, but as a last resort for you it would be worth trying.