Toprol is used for multiple things. Most people adjust to it well. I don't think your doctor would have prescribed it if he/she didn't feel it was safe.
Do you know what is causing your left atrium enlargement? I have the same. Mine is caused by mitral valve regurgitation. I was put on Toprol XL as well and I did not have high blood pressure (although it was not as low as yours). It does lower my blood pressure slightly but I don't seem to have any issues from it.
i take 25 mg of toprol xl for pvc's/pacs and i have low blood pressure too. i had trouble with toprol when my dosage was increased. its probably best to stay on a low dosage like 25 mg. for me its been effective without too many bad side effects.