In the month prior to getting my pacemaker I heard warnings about the potential for rubbing and/or pressure due to pack straps over the pacemaker site. Because I didn't want to sacrifice my mountain time I asked my surgeon to place the pacer so it wouldn't be right under a pack strap. The nurses in the pre-operation room chuckled as I stood there in my hospital gown and put on my day pack followed by a large internal frame pack and finally my beloved Kelty external frame. The surgeon eyeballed the straps, marked my chest and used that info to place the pacer so it is NOT under any of my shoulder straps. I haven't had any problems with the straps and have not needed to limit my hiking, XC skiing or backpacking.
If you are considering getting a pacemaker I encourage you to work with your surgeon to ensure that you'll still be able to carry a pack without having the strap rub on your pacer site. Happy trails!