Left Bundle Branch Area Pacing or Biventricular Pacing in AF and Left Ventricular Dysfunction

This study is comparing two types of heart pacing devices in people with a permanent irregular heart rhythm (atrial fibrillation) and a weakened heart. Researchers want to know whether a newer pacing technique called left bundle branch area pacing (LBBAP) works as well as or better than the traditional biventricular pacing (CRT) for improving heart function. **You may be eligible if...** - You are 18 or older - You have permanent atrial fibrillation and a heart pumping function below 50% - You need a pacemaker due to a slow heart rate, or you qualify for cardiac resynchronization therapy - Your heart rate pacing is expected to exceed 40% of the time - You have been on heart failure medications for at least 3 months - You already have a device but need an upgrade to a resynchronization device **You may NOT be eligible if...** - You are under 18 - You are pregnant or trying to become pregnant - A lead cannot be safely implanted in the required heart chambers - You had heart valve surgery or a heart attack within the past 6 months - You have the most severe class of heart failure (NYHA class IV), a heart assist device, or are awaiting a heart transplant Talk to your doctor to see if this trial is right for you.