Yeah, I have a left bundle branch block. Showed up on an EKG about 3.5 years ago. Here's what happened:
Fortunately, I take my medical early in the month. When the left (worse than the right or so my Doc says) bundle branch block was detected, my flight Doc got right on it. The first thing the cardiologist wanted was a stress test. I figured that meant the treadmill exercise test. Wrong. For a left bundle branch block the exercise test can sometimes give a false positive. So they used the chemical stress test.
It was just about the worst thing I've ever had. They inject something to speed your heart up. I was on the table feeling my heart racing, sweating and feeling like crap. Then they give you something else to bring you down and immediately give you a test like an MRI. It looks at something like 180 sections of your heart to determine if there is any damage.
There are three results for each of the sections of the test...damage, no damage or can't tell. I had ONE that came up Can't Tell. Guess what...no dice.
So the next step is an angiogram. A week or so later, you're scheduled into the hospital. They sedate you and go up through the big artery in your leg and do a visual check of your heart to see if you're damaged. In my case, I was clean as a whistle.
Once that test it completed, they send a letter to your FAA district office saying you're OK. You get a letter back acknowledging the fact that you have a bundle branch block and that you must immediately report ANY heart related problems to the FAA.
Once that is in your record the Doc will sign you off. In my case, it took a good three weeks to get all the ducks in a row.
For the next 3 years you have to report your cardiologist visit on your FAA medical. Other than that a bundle branch block is no big deal. Yeah, the ticker is working differently, but its working.
I've talked with guys who have the same thing and they haven't had any issues for 10 to 15 years.
Its a pain in the ass to get it evaluated and cleared, but if you have a Flight Surgeon and Cardiologist who cooperate, you can get it all done and all the hoops jumped through and not lose a day of flying.
Good luck, but don't worry. Its no big deal unless you've had some kind of heart failure in addition to the bundle branch failure.