When someone is in afib, the top two chambers of the heart just wiggle around like jello, rather than contracting nice and forcefully to push blood down into the bottom two chambers of the heart (which then contract to send blood out to the body). Since the top chambers of the heart are just wiggling around and aren't contracting nicely, they aren't pushing as much blood down into the bottom chambers of the heart.
Most of the blood moves down into the bottom chambers because of a difference in the pressure between the top of the heart and the bottom of the heart (blood flows from the area of higher pressure (the top chambers of the heart) to the area of lower pressure (the bottom chambers of the heart).
When you're in the "normal" rhythm (called sinus rhythm), after the blood flows from the top of the heart to the bottom b/c of the pressure difference, the top chambers then contract to push the rest of the blood out. In afib, you don't get that extra squeeze of blood. We call this extra squeeze atrial kick. About 80% of the blood in the top part of the heart flows to the bottom b/c of the pressure difference, while the contraction of the top chambers gives that extra 20%.
It's like if you have a squeeze bottle and you turn it upside down. Most of the stuff in the bottle comes out and then you squeeze the bottle to get the last but of stuff out.
That loss of atrial kick can affect people many different ways, and feeling short of breath is one of them. Some people go back and forth (or in and out) of normal rhythm and afib. We call this paroxysmal atrial fibrillation (PAF). The very rapid heart rate that is often associated with PAF also doesn't help the situation. The heart is beating very fast and it's not filling as well as it should.
Of note, when someone is in afib and the top chambers of the heart aren't pumping blood smoothly, you can run into problems if a blood clot forms in there. If a clot does form, and then travels out of the heart and up to the brain, you can have a stroke. This is the main reason people with afib (either chronic afib or PAF) need to take blood thinners.
From your description I'm not sure if you have afib all the time or PAF. You may benefit from wearing a heart monitor for a couple of days. It would continuously record your heart rhythm (like an EKG but for an extended time). This would determine if you have afib all the time or go in and out of it.
The heart ultrasound (echocardiogram) will look at the structure of your heart (the valves, how well the walls are moving, etc). It can tell if you've had a heart attack in the past (dead heart muscle doesn't move and they can see that on the echo).
If you indeed have afib all the time, you would probably need rate control therapy (ie the atenolol your already on) and likely a blood thinner. If you have PAF, there are meds that can be taken to try to keep you from going in and out of afib (ie Amiodarone, Sotalol, etc). Some people also do the "pill in the pocket": when they go into AF, they take a does of medication (usually a beta blocker) to try and get back into normal rhythm.
As far as the Maze procedure, I'm not really sure how they tell if a person is a candidate for it. You would have to discuss that with a cardiologist.
Anyway...I hope they get you all straightened out and feeling better. Let me know if you have any other questions.
