My wife has a pacemaker and it was actually pretty challenging to carry a backpack before we figured out a way to do it so that there was no pressure on the leads or battery (she is thin so can feel the leads and the battery just below her clavicle). She ended up not carrying her backpack on our first Camino and did a lot of research before our second one.
She used the waist straps to take most of the load but it was still difficult to do up the thin strap across her chest that attaches to the shoulder straps. She contact Osprey and their team made a modification to the strap pattern so that there was a small strap attached to front of each of the shoulder straps - these criss-crossed across the front of her body in a way that missed the area under her clavicle and therefore the leads and the battery.
In addition to this, she bought small sponges at a dollar store in Spain (they were dense cleaning sponges that were covered with terry cloth) to raise the straps further off her shoulders and this also helped to keep the straps away from the pacemaker battery and leads.
So, in our experience, it wasn’t as simple as getting a backpack that fit well. Osprey was amazing in helping us to figure out a way to adapt the straps so that her backpack worked for her. I believe there is also a company out of Australia that makes a pack that has this sort of strap pattern but it‘s been years since we looked into it and I can’t remember the name of the company.
Good luck!