I got the pacemaker january 2018. when I left the hospital they told me,that I had many atrium fdibrillations. but this is not uncommon, I should not worry about it. my medications were bisoprolol 7.5 mg/d + a blood thinner Eliquis .November 2018 I had a pacemaker check. the cardiologist told me, that I had 94% Fibrillations in the atrium, that means that the pacemaker electrode in the atrium was useless.
I depended more or less on the electrode for the right chamber. a couple of weeks later I noticed loss of energy, in the beginning I had to leave my bicycle in the garage, 2 weeks later I didn't even manage the walk to the bus. my weight went high, but I did not see any odemas in the legs. but apparently I got oedema in the lung, so difficulties with breathing (dyspnoe). I discussed the situation with my family doctor. she gave me a diureticum and she took care of an appointment with the cardiologist. (in Germany the patient has to wait a long time) . Februar 2019 I saw the cardiologist, the echocardiography confirmed my bad condition.: all (!!) the heartvalves were insufficient, even my mitralvalve ( I had a minimal invasive mitralvalve reconstruction 3 years ago) was highly insufficient, which was a great shock for me. . of course I did recherche in pubmed. and I knew, that in such a case they deactivate the electrode in the atrium. but that didn't work, because, like he said, the problem is located in the tissue of the right chamber. so he suggested digitoxin – 0.07 ng/ml.
to answer your question, I started the digitoxin only in februar this year. 10 days later I was fully recovered. 6 weeks later they measured the blood concentration of digitoxin (very important!!).. this was too high, so I started the lower dose (0.05 ng/ml).
you asked about side effects, I have none, but it is very important to choose the right dosage, depending on the weight of the patient.