Here's a very good article on this subject.
Cardiac pacemakers are electronic devices with sensing circuits which detect small electrical signals from inside the heart. Pacemakers may detect extraneous electrical signals from other sources. The pacemaker can incorrectly interpret these signals as heart activity, which may inhibit the pacemaker. The result could be no output pulse or asynchronous pacing. Asynchronous pacing means that there is no coordination between the heart and the pacemaker.
If you have a St. Jude Medical pacemaker and use or are in close proximity to an electric welder, you shouldn?t have any problems. However, this doesn?t mean that there is a total absence of the effects of welding interference on pacemakers. Any problems caused by radiated interference will end when the arcing ends.