It can be hard to tell sometimes. My cardiologist said that the electrode pads have to be positioned just right to get a good diagnosis. Enjoy electrophysiology procedure is the sure way to tell. I am currently experiencing the same thing 6 years after my ablation for SVT (AVRT). I'm up to 150mg per day. After being at 75mg since the ablation in 2010. Years ago, I was up to 200mg per day. I was like a zombie at that dose. It really prevented my heart from responding to any exertion. 2 weeks ago, I had a stress test. I was throwing PVC's, PAC's, and brief salvos of eithe SVT or VT. The cardiologist was there the whole time, and like yours, wasn't really concerned. And I trust my cardiologist. If you haven't already had one, a 30 day monitor is good for capturing what's going on, and an electrophysiology procedure should pinpoint the location, and exactly what it is. I can see my direction is headed this way.....
By the way... at levels of 200mg, you may get a gradual excessive buildup of the drug in your blood. You should talk to your prescriber about this. When I felt particularly washed out, I skip a day to reset things. I wouldn't do this though without first talking to your physician.
Keep thinking of additional comments. Metoprolol did squat for my SVT, and ectopics.
What type of svt are you having do you know? If you have a reentry type they may be able to fix them. Pvcs are a bit harder to fix, the same with pacs from the atria. It can be a bit hard to tell the difference sometimes. Your runs may actually be runs of pacs and not necessarily pvcs but even so, even if they are pvcs the current thinking in the cardiology field is that they aren't a danger unless the heart is damaged and unhealthy. Even then it isn't a sure bet that one would go into VF unless you had a VT that sustained in a reentry pattern and does not stop. What feels like vt may be just a bunch of single pvcs in a row. meaning the signal isn't caught in a loop rather there is a spot that keeps firing. That spot can spontaneously stop at anytime while a loop situation can be indefinite and therefore lead to cardiac death. VT loops are usually caused by scar tissue. I am not sure your atrial septal defect would contribute to this. Things like leaky valves might. Or heart attacks. So odds are you are not in any sort of immediate health danger.
If you have a fixable svt it may help to get it corrected because svt can irritate the heart and contribute to more premature beats. They kind of feed off each other. This said, if you are having runs of pacs, which can also sometimes be called svt, those are harder to ablate so maybe get some clarity on your svt and see what can be done about it because beta blockers really don't stop either svt or premature beats. They can lessen the amount of premature beats and slow the heart down if it goes into svt but nothing really stops either totally except ablation. They likely would not do an ablation for the premature beats unless you are having an amount that poses a danger which is well into the 10,000 plus a day range. Premature beats can be hard to ablate and can come back so the best bet is to try and avoid the main triggers. Things that rile up the stomach or cause acid reflux. Sugars and carbohydrates. I get them every time I eat chocolate but I am not giving up chocolate. ;) Caffeine gets them going every time for me as well too. Becoming dehydrated. And finally stress and anxiety. As well I do have to be mindful to not push my workouts too hard or I can get them going. Other than that I just live with them.
But I had svt my whole life. A type called avnrt that was causing a loop situation around my avnode. As I got older the episodes got more frequent. My heart would race at rates well into the 200s. Most episodes were short until later in life and then I had trouble getting them to stop so I did the ablation almost 5 years ago and have not had an svt episode since. I do still get premature beats, pacs and pvcs and little runs of pacs but no more svt so my heart feels stronger in general just by ridding myself of the svt. So maybe contact your cardiologist and ask about the svt for more clarity and whether or not you are a good candidate for at least ablating that. Getting rid of one issue may help alleviate the premature beats. But again it has to be a kind that is easily ablatable so you need clarity on what is exactly going on.
I am always around if you have any more questions but do keep in mind that I am not a doctor and all I have said comes from my own experience and research. To know for sure you really need to ask your doctor. You have a right to ask question even after an appointment. You can call the office or some now have email capabilities. But you have a right to know. Take care and hope you can get a handle on things.