Worried about Chihuahua - heart issuess

My 8 year old female chihuhua had a heart scan on Monday and they couldn't find anything bar pericaridal effusion (which I know is fluid round the heart). They said they had been expecting valve issues or similar but nothing was there. They said they will scan again for free on 3rd April. Is pericardial effusion serious? They were dismissive of it and told my parents sometimes it can happen for no reason but when I google heart tumours and various other types of cancer come up as most likely cause but they never mentioned anything like this and my parents think I'm over reacting and thats she fine (they took her to the vet for the scan as I was at work). The scan was prompted as my chihuahua was quite violently sick and had diarrhoea in the middle of last week, she’s always had a delicate stomach so my parents who were looking after her whilst I was away didn’t think too much of it until she started struggling to breathe. The vet gave her some injections that day (anti sickness was one) and said bring her back the next day. They took her back the next day and the vet listened to her heart and it was fast and recommended the scan she had on Monday. Last week they put her on a heart tablet straight away (it states Zelys on the box) and a diuretic, Libeo. They stated after the scan to finish the course of both but that it will be likely they take her off the heart tablet and just leave her on the diuretic and that, that would help the pericardial effusion but I haven't read that diuretics help this... Should I be worried about pericardial effusion, has anyone had it just go away in their dog? They seemed to be suggesting since they couldn't see anything wrong with the heart bar the pericardial effusion that the fluid could be just one of those things or due to the stress of being sick but so many websites go on about tumours... She seems a bit slower than usual now but ok other than this, she is eating, going on walks, hasn't been sick again etc but just so worried, this all came out of nowhere, so hope I am worrying about nothing....

Hello and thankyou for this interesting question.   It might help to think of pericardial effusion as a symptom, like limping or stomach ache.  Limping can be caused by bone cancer, but it can also be caused by a pulled muscle or a broken leg, or by something in the foot or by a ligament strain.  Some of these options can easily be ruled out (eg by turning the foot over and looking for foreign bodies) but sometimes the cause can be difficult to spot and requires imaging (in that case, x-rays).  And inevitably there will be some cases where the patient limps, no-one's sure why - common causes have been ruled out - but they stop again and that's the end of it.  These are known as 'idiopathic.'  That said, there will be cases where the cause isnt immediately obvious but develops or grows and then becomes more obvious further down the line.   Obviously your dog does not have a limp but a pericardial effusion.  The same thing, however, applies; it sounds as though no one can be sure what is causing the pericardial effusion in your dog at this stage; the cause sounds not to be obvious and the vet has a plan to scan again in case of developing diseases.   I dont know what the % chance of it spontaneously resolving is, but I hear that knowing this figure may help you to assess the risks in your mind - and the cardiologist might be aware of it.  The chance may vary depending on the breed or the age of the patient.   Unfortunately, when there is only one patient ij front of you, the % chance doesn't always help very much; what you want to know is what is going on in this specific individual.   I, like your vet, hope that time will bring good news - but it sounds as though its too early to know.