This is a very complicated problem. The length and orientation of the leads, type of pacemaker etc are all factors. And one mistake is one too many. This applies to a growing list of implants. Spinal cord stimulators, sacral nerve stimulators, deep brain stimulators, implantable defibs, cochlear implants, some hearing aids. Its a very long list. The possible complications are not just possible induced currents, but also heating. Many of the problems are covered here (although with MRI in mind) http://www.mrisafety.com. It's a great resource but is absolutely terrible to use and everything takes 20x longer than it should to find.