Well that is good that is stops that quickly. That means that whatever is happening, your heart is able to correct itself without issue. I know how scary it is but if your heart is healthy then what is going on is generally not a danger. If you have clogged arteries or scaring on the heart that can cause what you are feeling to deteriorate into a dangerous situation which may be why the doctor ordered the other tests. A lot of times the arrhythmia are caused for unknown reasons and knowing that the heart is totally healthy gives the doctor confidence that no further treatment is needed. I will say thought that a 24 hour holter is likely not going to catch something that transient. You may need a 30 day monitor to catch exactly what it is. Considering that it doesn't happen everyday I am a little surprised the cardio didn't order one but you can ask him about it when you see him. I personally would like to know what it is but odds are whatever it is, is not a danger to you at this time. It's probably a good idea to try to be heart healthy moving forward but sounds like you are tolerating what is going on without it impacting your ability to function which is one of the biggest indicators for cardiologist whether intervention is necessary or not. So bottom line, I suspect the doctor wants to be good and sure your heart is totally healthy so he need not worry about how your heart behaves occasionally. So that said, I would go for the tests.
I did forget to mention that I went to see a Cardio bc I get very scary heart beats on occasion. My heart will suddenly flutter/quiver for a few seconds, beat super fast (but seems like normal rythm, for about 10 beats and then just go back to normal. All of this happens out of nowhere, maybe once a month, if that. I have no other symptoms...no pain, dizziness, sob. It just scares me! I have tried to take my pulse when it happens & when I get to it this is what it feels like. But a lot of times it is so quick, I cant even get to my pulse before it stops!
So if everything is normal why are you seeing a cardiologist in the first place? What prompted you to seek one out? Since it isn't that far off I would agree to wait and or call the office and ask. Doctors in general want to make sure all the basis are covered before signing off on a clean bill of health. An echo is a pretty common test for people who complain of heart rhythm issues though a nuclear test maybe not. I didn't get one when I went to get my heart checked for an svt. In any event, I suspect the doctor just wants to make sure you were thoroughly looked at. Try not to stress, the tests are ordered a lot of times even for people who have healthy hearts if symptoms are present just to be sure.