Lanoxin is the brand name of a medicine called Digoxin. It is indicated in the treatment of cardiac failure accompanied by atrial fibrillation.
Prolomet XL is the brand name of a medicine called propranolol (also sold under the brand name of Inderal), which is primarily used in the treatment of high blood pressure and angina and uncommonly for prevention of migraine and essential tremors. It can also be used in certain specific types of cardiac arrhythmias due to (a) thyroid overactivity, (b) anxiety-induced or (c) drug-induced. One cannot just replace Lanoxin with Prolomet XL, since they contain entirely different molecules.