Metoprolol is a beta blocker (beta adrenergic) that is used for both incipient hypertension and as a rate control medication. It affects rate AND force. It not only helps to keep a lid on a tachyarrhythmia, which both AVT and AF are (AF not always is above 100 BPM, so it is only classified as a tachyarrhythmia if it presents above 100 BPM)), but it can make the heart less 'cooperative' during high demand activity such as stair climbing, trail running in hills, etc. I complained to my cardiologist that I simply could not, for the life of me, run with the same energy or speed as prior to my diagnosis and his prescriptions (apixaban and metoprolol, plus he feared ischemia and wanted me on a statin right away). He replied that I cannot blame metoprolol for my reduced capacity. I know better, but in Canada the medical system is state-run and they get to call the shots.