Short heart palpitations despite beta blockers in week 18 of pregnancy?

Hello, I had heart surgery a year ago (sinus venosus defect plus incorrect junction of the venous vein) in Munich using a cardiac catheter, and as a result I had two stents put in. Since then I have had frequent heart palpitations. Now I am pregnant and the heart palpitations are more frequent than before. When the heart palpitations started I went to my GP so that he could write me an ECG. From there I was taken to the hospital and they could tell that there is a blockage in the sinus. I now have to take 23 mg of metoprolol until I give birth. All well and good for a few days, but today I could feel my heart palpitations again for 1/2 second when I breathe in. Is that normal?